This document discusses various topics related to cell structure and life processes in living organisms. It begins by defining metabolism as the totality of biochemical reactions that maintain living cells. There are two main types of metabolic processes - catabolism which breaks down molecules and releases energy, and anabolism which builds molecules and requires energy. Nutrition provides the necessary energy and materials for these processes. Carbohydrates, proteins, and other organic and inorganic nutrients are discussed. The document also summarizes asexual and sexual reproduction, and describes various modes of reproduction in plants including pollination and seed formation. It provides examples of gymnosperms and angiosperms, distinguishing their characteristics.

1. Cell Structure
Living Organisms: Growth, reproduction, ability to sense environment and mount a suitable
response come to our mind immediately as unique features of living organisms.
Other features like metabolism, ability to self-replicate, self-organise.
All living organisms grow.
Reproduction, like wise, is a characteristic of living organisms.
Another characteristic of life is metabolism.
Hence, cellular organisation of the body is the defining feature of life forms.
Consciousness therefore, becomes the defining property of living organisms.
An organism is considered as living when it performs the different life processes in one form
of another.
The occurrence of life processes can differentiate between living organisms and non-living
objects.
Life processes performed by living organisms are nutrition, movement, growth, reproduction
and respiration, as well as sensitivity and excretion.
Metabolism:
Metabolism refers to a series of chemical reactions that occur in a living organism to sustain
life. Metabolism is the total amount of the biochemical reactions involved in maintaining the
living condition of the cells in an organism. All living organisms require energy for different
essential processes and for producing new organic substances.
The metabolic processes help in growth and reproduction and help in maintaining the
structures of living organisms. The organisms respond to the surrounding environment due
to metabolic activities. All the chemical reactions occurring in the living organisms from
digestion to transportation of substances from cell to cell require energy.
Metabolic process there are two types of metabolic process:
● Catabolism
● Anabolism
Catabolism – This process is mainly involved in breaking down larger organic molecules into
smaller molecules. This metabolic process releases energy.
Anabolism – This process is mainly involved in building up or synthesizing compounds from
simpler substances required by the cells. This metabolic process requires and stores energy.
Metabolism is related to nutrition and the existence of nutrients. Bioenergetics describes the
metabolism as the biochemical pathway through which the cells obtain energy. One of the
major aspects is the energy formation.

2. Nutrition and Energy:
The processes of metabolism depend on the nutrients that get digested to produce energy.
This energy is necessary to synthesize nucleic acids, proteins and other biomolecules in our
body.
Encompassed nutrients include various substances for the body requirements which are
either in the sufficient amount or insufficient, resulting in poor health, concerning metabolism.
Necessary nutrients help by supplying the required energy and other necessary chemicals
that the body cannot synthesize on its own. Food provides different substances that are
essential for the bodybuilding and repairing of tissues along with the proper functioning of
the body.
The diet requires both organic nutrients and inorganic chemical compounds.
Organic nutrients include fats, vitamins, carbohydrates, and proteins.
Inorganic chemical compounds include oxygen, water, and other dietary minerals.
Carbohydrates in Metabolism:
Carbohydrates are supplied in three forms:
● Starch
● Sugar
● Cellulose
Starch and sugar are the major forms of energy for humans. Metabolism of carbohydrates
and sugar helps in the production of glucose.
Proteins in Metabolism:
Proteins are important for building tissues. They help in maintaining the structure of the cells,
its functions, the formation of haemoglobin, and several other body functions. The amino
acids of proteins are beneficial for nutrition. Few amino acids are not synthesized by the
body and are taken in from the food we eat. These amino acids include:
Lysine
Tryptophan
Methionine
Isoleucine
Leucine
Phenylalanine
Valine
Threonine
How to Increase Metabolism?
Metabolism can be increased by:
To be fit and healthy, we need to avoid more calories intake and lose extra pounds. We eat
to deliver energy for our body to perform its functions. Eating too little quantities could slow

3. down our metabolism and body cannot provide essential minerals. As per the research,
extreme dieting leads to weight loss which is muscle mass and not fat mass.
Having proper breakfast, boost up the body’s metabolism and keeps us energetic throughout
the day. Skipping morning breakfast are more likely to have poor metabolic energy.
Caffeine stimulates the central nervous system and can activate our metabolism rate by 5 to
8 percent.
According to researchers, fibre can help in burning fat by 30 percent. People who include
more fibre in their diet remain fit and healthy.
Including more organic foods like peaches, bell peppers, celery, apples, lettuce, grapes can
boost up the metabolism rate in our body.
Reproduction:
Reproduction is the process of producing offspring that are biologically or genetically similar
to the parent organism.
What is Reproduction?
Reproduction is a biological process by which an organism reproduces an offspring that is
biologically similar to the organism. Reproduction enables and ensures the continuity of
species, generation after generation. It is the main feature of life on earth.
Let us have a detailed overview of reproduction, its types and the modes of reproduction in
plants and animals.
Types of Reproduction:
There are basically two types of reproduction:
1. Asexual Reproduction
2. Sexual Reproduction
Asexual Reproduction:
“Asexual reproduction refers to the type of reproduction in which only a single organism
gives rise to a new individual.”
Binary Fission (Asexual Reproduction):
Asexual reproduction does not involve the fusion of gametes, and therefore, the offsprings
produced are genetically identical to the parent. The organisms produced by asexual
reproduction are less diverse in nature. This type of reproduction is practised widely by
unicellular organisms.
The process involves rapid population growth and no mate is required for the process.
However, a lack of genetic diversity makes organisms more susceptible to diseases and
nutrition deficiencies.
Asexual reproduction is further divided into:
Binary Fission: In this, the cell splits into two each cell carrying a copy of the DNA from the
parent cell. For eg., amoeba.
Budding: In this, a small bud-like outgrowth gives rise to a new individual. The outgrowth
remains attached to the organism until it is fully grown. It detaches itself and lives as an
individual organism. For eg., hydra

4. Fragmentation: In this, the parent organism splits into several parts and each part grows
into a new individual. For eg., Planaria
Sporogenesis: In this type of reproduction, a new organism grows from the spores. These
can be created without fertilization and can spread through wind and animals.
Sexual Reproduction:
“Sexual reproduction is a type of reproduction that involves the production of an offspring by
the fusion of male and female gametes.”
In sexual reproduction, male and female gametes are formed to produce an offspring. These
gametes are either formed by the same individual or by different individuals of the opposite
sex.
This process is usually slow and complex compared to asexual reproduction. The organisms
so produced are genetically diverse. Thus, they can evolve along with the changing climatic
conditions. Humans and many multicellular organisms exhibit a sexual mode of
reproduction.
Reproduction in Plants (Asexual reproduction):
Plants reproduce by sexual and asexual means. Vegetative reproduction is the main mode
of plant reproduction. Roots such as a corm, stem tuber, rhizomes and stolon undergo
vegetative propagation.
Sexual reproduction in plants takes place through pollination in which the pollen grains from
the anther of a male flower transfer to the stigma of the female flower.
What is Pollination?
Pollination is a method where pollen grains are picked from an anther, which is the male part
of a flower and transferred to the flower’s female part called the stigma. To make the
pollination work successfully, the pollen grains must be transferred from the same species of
flower.
Process of Pollination
The process of pollination begins when the pollen grains from the respective flowers lands
on the stigma and form a pollen tube with the style length, which connects both the stigma
and ovary. After the completion of the pollen tube, the pollen grain starts transmitting sperm
cells from the grain to the ovary.
Later the process of fertilization in plants will take place when the sperm cells will reach the
ovary and egg cells. The seed is then released from the parent plant and making it able to
grow into a plant and continue the reproductive cycle with the use of the pollination method.
Types of Pollination:
All plants having flowers completely rely on pollination method for reproduction. There are 2
types of pollination –
● Self Pollination
● Cross-Pollination
Self Pollination:
It is referred to as the primary type of pollination as it includes a single flower. Self-pollination
occurs when pollen grains fall directly from anther into the stigma of the flower. This process

5. is quite simple and fast, which leads to a reduction in genetic diversity as the sperm and egg
cells of the flower share some genetic information.
Advantages and Disadvantages of Self-pollination
Self- pollination ensures that recessive characters are eliminated.
The wastage of the pollen grain is very less compared to cross-pollination
In the process of self- pollination, the purity of the race is maintained, as there is no diversity
in the genes
In self- pollination, there is no involvement of external factors like wind, water, and other
pollinating agents.
Self-pollination ensures that even a smaller quantity of produced pollen grains from plants
have a good success rate in pollination.
Disadvantages
The major disadvantage of Self- pollination is there is no mixing up of genes. Due to which:
The vigour and vitality of the race are reduced
The immunity to diseases is reduced in the resultant offsprings.
Cross-Pollination:
It refers to a complex type of pollination that allows the transfer of pollen grains from the
anther of the flower into the stigma of another flower. This method leads to an increase in
genetic diversity as different flowers will share and combine their genetic information to
create unique offspring.
Types of Cross-Pollination
The process of cross-pollination requires the help of biotic and abiotic agents like animals,
birds, wind, insects, water and other agents as pollinators.
Pollination by Wind- Anemophily
There are only a few flowers that use wind pollination and their features are greenish, small
and odourless flowers. As these flowers do not attract the pollinators, their energy is not
used for making colourful petals. This type of pollination usually occurs when plants lack
flowers with nectar and other features including inconspicuous. The male parts of the
Anemophilous flowers tend to produce very large quantities of pollen and the stigma, the
female reproductive part of a flower are very large, sticky and feathery to extend completely
outside the flower. Thus the pollen is more likely to reach them.
Coconut, palm, maize, grasses and all gymnosperms are the best examples of
wind-pollinated plants.
You must have seen that your car is fully covered with yellow film in the spring, it is actually
the pollen that uses wind for pollination process.
Gymnosperms: Gymnosperms are a group of plants that produce seeds not enclosed
within the ovary or fruit.
The word “Gymnosperm” comes from the Greek words “gymnos”(naked) and
“sperma”(seed), hence known as “Naked seeds.” Gymnosperms are the seed-producing
plants, but unlike angiosperms, they produce seeds without fruits. These plants develop on
the surface of scales or leaves, or at the end of stalks forming a cone-like structure.

6. Gymnosperms belong to kingdom ‘Plantae‘ and sub-kingdom ‘Embryophyta’. The fossil
evidence suggested that they originated during the Paleozoic era, about 390 million years
ago.
Basically, gymnosperms are plants in which the ovules are not enclosed within the ovary
wall, unlike the angiosperms. It remains exposed before and after fertilisation and before
developing into a seed. The stem of gymnosperms can be branched or unbranched. The
thick cuticle, needle-like leaves, and sunken stomata reduce the rate of water loss in these
plants.
The family of gymnosperms consist of conifers, the cycads, the gnetophytes and the species
of Gynkgophyta division and Ginkgo biloba.
Let us have an overview of the characteristics, examples, classification and examples of
gymnosperms.
Characteristics of Gymnosperms
Following are the important characteristics of gymnosperms:
● They do not produce flowers.
● Seeds are not formed inside a fruit. They are naked.
● They are found in colder regions where snowfall occurs.
● They develop needle-like leaves.
● They are perennial or woody, forming trees or bushes.
● They are not differentiated into ovary, style and stigma.
● Since stigma is absent, they are pollinated directly by the wind.
● The male gametophytes produce two gametes, but only one of them is functional.
● They form cones with reproductive structures.
● The seeds contain endosperm that stores food for the growth and development of the
plant.
● These plants have vascular tissues which help in the transportation of nutrients and
water.
● Xylem does not have vessels and the phloem has no companion cells and sieve
tubes.
Classification of Gymnosperms:
Gymnosperms are classified into four types as given below –
Cycadophyta-
Cycads are dioecious (meaning: individual plants are either all male or female). Cycads are
seed-bearing plants where the majority of the members are now extinct. They had flourished
during the Jurassic and late Triassic era. Nowadays, the plants are considered as relics from
the past.
These plants usually have large compound leaves, thick trunks and small leaflets which are
attached to a single central stem. They range in height anywhere between a few centimetres
to several meters.
Cycads are usually found in the tropics and subtropics. Some members have adapted to dry
arid conditions and some also have adapted to oxygen-poor swampy environments.
Ginkgophyta-
Another class of Gymnosperms, Ginkgophyta, has only one living species. All other
members of this class are now extinct.

7. The Ginkgo trees are characterised by their large size and their fan-like leaves. Also, Ginkgo
trees have a large number of applications ranging from medicine to cooking. Ginkgo leaves
are ingested as a remedy for memory-related disorders like Alzheimer’s.
Ginkgo trees are also very resistant to pollution, and they are resilient against diseases and
insect infestations. In fact, they are so resilient that after the nuclear bombs fell on
Hiroshima, six Ginkgo trees were the only living things to survive within a kilometre or two of
the blast radius.
Gnetophyta-
Just like any other member of gymnosperms, Gnetophytes are also relics from the past.
Today, only three members of this genus exist.
Gnetophytes usually consist of tropical plants, trees, and shrubs. They are characterised by
flowery leaves that have a soft coating. This coating reveals an ancestral connection with the
angiosperms.
Gnetophytes differ from other members of this class as they possess vessel elements in
their xylem.
Coniferophyta-
These are the most commonly known species among the gymnosperm family. They are
evergreen; hence they do not shed their leaves in the winter. These are mainly characterised
by male and female cones which form needle-like structures.
Coniferous trees are usually found in temperate zones where the average temperature is 10
℃. Giant sequoia, pines, cedar and redwood are examples of Conifers.
Gymnosperms Examples
Following are some of the examples of gymnosperms:
Cycas
Pinus
Araucaria
Thuja
Cedrus
Picea
Abies
Juniperus
Larix
Gymnosperms Life Cycle
The life cycle of gymnosperms is both haploid and diploid, i.e., they reproduce through the
alternation of generations. They have a sporophyte-dominant cycle.
The gametophyte phase is relatively short. The reproductive organs are usually cones.
Male Cones– These have microsporophylls that contain microsporangia. Microsporangium
produces haploid microspores. A few microspores develop into male gametes called pollen
grains, and the rest degenerate.

8. Female Cones– The megasporophylls cluster together to form female cones. They possess
ovules containing megasporangium. It produces haploid megaspores and a megaspore
mother cell.
The pollen reaches the egg through wind or any other pollinating agent, and the pollen grain
releases a sperm. The nuclei of male and female gametophytes fuse together to form a
zygote. This is known as fertilisation.
The seed appears as scales which can be seen on the cones of the gymnosperm.
Angiosperms:
Angiosperms are vascular plants with stems, roots, and leaves. The seeds of the
angiosperm are found in a flower. These make up the majority of all plants on earth. The
seeds develop inside the plant organs and form fruit. Hence, they are also known as
flowering plants.
Angiosperms are the most advanced and beneficial group of plants. They can grow in
various habitats as trees, herbs, shrubs, and bushes.
Characteristics of Angiosperms
Angiosperms have diverse characteristics. The important characteristics of angiosperms are
mentioned below:
● All plants have flowers at some stage in their life. The flowers are the reproductive
organs for the plant, providing them with a means of exchanging genetic information.
● The sporophyte is differentiated into stems, roots, and leaves.
● The vascular system has true vessels in the xylem and companion cells in the
phloem.
● The stamens (microsporophyll) and the carpels (megasporophyll) are organized into
a structure called the flower.
● Each microsporophyll has four microsporangia.
● The ovules are enclosed in the ovary at the base of the megasporophyll.
● Angiosperms are heterosporous, i.e., produce two kinds of spores, microspore
(pollen grains) and megaspores.
● A single functional megaspore is permanently retained within the nucellus.
● The pollen grains transfer from the anther to stigma and reproduction takes place by
pollination. They are responsible for the transfer of genetic information from one
flower to the other. The pollen grains are much smaller than the gametophytes or
reproductive cells present in the non-flowering plants.
● The sporophytes are diploid.
● The root system is very complex and consists of cortex, xylem, phloem, and
epidermis.
● The flowers undergo double and triple fusion which leads to the formation of a diploid
zygote and triploid endosperm.
● Angiosperms can survive in a variety of habitats, including marine habitats.
● The process of fertilization is quicker in angiosperms. The seeds are also produced
quickly due to the smaller female reproductive parts.
● All angiosperms are comprised of stamens which are the reproductive structures of
the flowers. They produce the pollen grains that carry the hereditary information.
● The carpels enclose developing seeds that may turn into a fruit.

9. ● The production of the endosperm is one of the greatest advantages of angiosperms.
The endosperm is formed after fertilization and is a source of food for the developing
seed and seedling.
Classification of Angiosperms
The classification of angiosperms is explained below:
Monocotyledons
The seeds have a single cotyledon.
The leaves are simples and the veins are parallel.
This group contains adventitious roots.
Each floral whorl has three members.
It has closed vascular bundles and large in number.
For eg., banana, sugarcane, lilies, etc.
Dicotyledons
The seeds of these plants have two cotyledons.
They contain tap roots, instead of adventitious roots.
The leaves depict a reticulate venation.
The flowers are tetramerous or pentamerous and the vascular bundles are organized in
rings.
For eg., grapes, sunflower, tomatoes, etc.
The angiosperms originated about 250 million years ago and comprise 80% of earth’s plant
life. They are also a major source of food for humans and animals.
Sexual Reproduction in Plants:
A few plants produce seeds without fertilization and the process is called apomixis. Here, the
ovule or the ovary gives rise to new seeds.
Reproduction is one of the most fundamental processes carried out by living organisms.
However, there are differences in the way living organisms exhibit the process.
Modes Of Reproduction In Plants
In plants, reproduction is carried out via two modes:
Asexual Mode – New plants are obtained without producing seeds
Sexual Mode – New plants are obtained from seeds.
Asexual Reproduction In Plants:
In asexual reproduction in plants, plants are reproduced without the formation of seeds.
Following are a few ways in which plants reproduce asexually.
Vegetative Propagation-
As the name suggests, reproduction occurs through the vegetative parts of a plant such as
stems, leaves, buds, and roots. These plants take less time to grow and are exact replicas of
their parents as they are reproduced from a single parent.

10. Budding-
Small bulb-like projections arise from yeast cells, eventually detaching itself from the parent
cell. This then matures to grow into a new yeast cell. These, in turn, produce more buds and
the chain continues forming a number of new yeast cells within a short period of time.
Fragmentation&
Some organisms have the ability to break into two or more fragments, with the new fragment
becoming a new, independent individual. They multiply rapidly in a short period of time.
Spore Formation-
Spores are present in the air and are covered by a hard protective coat to bear low humidity
and high-temperature conditions. Spores germinate and develop into new organisms under
favourable conditions.
Micropropagation-
An explant is taken from a plant and allowed to grow in a nutrient medium under controlled
conditions in the laboratory. The cells divided rapidly and form an unorganised mass of cells.
This unorganised mass of cells is known as a callus. The callus is transferred to another
nutrient medium to facilitate the differentiation of different parts of the plant. The plantlets are
then transferred to the fields.
Advantages of Asexual Reproduction in Plants
● A large number of plants can be produced within a short period.
● The exact copies of the parent plant are produced.
● Many seedless varieties are obtained through the vegetative method.
● Less attention is required by the plants grown through asexual means than through
seeds.
● Sexual Reproduction In Plants
The reproductive parts of plants are flowers, Stamen being male reproductive part and pistil
being the female reproductive part. If one of these reproductive parts are present in a flower,
it is said to be a unisexual flower. Example: papaya. If both Stamen and Pistil are present in
flowers they are called bisexual flowers. Example: rose.
● Pollen grains form the male gametes. The pistil consists of style, stigma, and the
Ovary. The ovary consists of one or more ovules. Ovules are where female gametes
or the egg is formed. Female and male gametes fuse to form a zygote.
Pollination-
When pollen is transferred from the anther to the stigma of a flower through carriers such as
insects it is called pollination. It can be a case of self-pollination if pollen lands on the stigma
of the same flower or another flower of the same plant. If pollen grains land on the stigma of
a flower of a different plant, but of the same kind, it is called cross-pollination.
Fertilization-
A zygote is formed as a result of the fusion of gametes which later develops into the embryo.
Fruits and seeds are formed post-fertilization. Ripened ovary goes on to become a fruit.
Ovules give rise to seeds which contain the embryo in a protective covering.

11. Reproduction in Animals:
Animals reproduce sexually as well as asexually. Sexual reproduction involves the fusion of
male and female gametes. This process is known as fertilization. Fertilization can be
external or internal. External fertilization is the process in which the male sperm fertilizes the
female egg outside the female’s body. On the contrary, in internal fertilization, the fusion of
male and female gametes takes place inside the body of the female.
Asexual reproduction involves reproduction processes such as binary fission, budding,
fragmentation, etc. The organisms have no reproductive systems and therefore no formation
of male and female gametes takes place.
Thus, we see how beneficial reproduction is to continue life on earth.
Modes of Reproduction:
Depending on the number of parents involved, there are different modes of reproduction. In
animals is two types of reproduction:
● Sexual Reproduction.
● Asexual Reproduction.
Sexual Reproduction in Animals:
The process in which the male and female gametes fuse together to form a new individual is
called sexual reproduction. Let us have a brief account of the human reproductive organs
and their role in reproduction.
Sexual reproduction is a natural way of reproduction in humans, animals and the majority of
plants also choose to reproduce sexually. This type of reproduction is more complex and
lengthy as compared to asexual reproduction. Moreover, reproducing sexually gives the
benefit of variation and offsprings are unique. Sexual reproduction consists of a set of events
and can be divided into three stages: Pre-fertilization, Fertilization, and Post-fertilization.
Stages of Sexual Reproduction:
1. Pre-Fertilization-
This stage involves the events prior to fertilization. Gamete formation (gametogenesis) and
transfer of gamete are the two processes that take place during this stage. Gametes are sex
cells, which are haploid (23 chromosomes) in nature and are distinct in males and females.
The male gamete is called sperm whereas female gamete is called ovum or egg. In every
organism, these gametes are formed within special structures. Since female gamete is
immobile, male gametes need to be transferred for fertilization.
In plants, this is attained by pollination. Unisexual animals transfer gametes by sexual
intercourse.
2. Fertilization-
Once the haploid male and female gametes meet and fuse together to form a zygote, this is
known as fertilization or syngamy. This can occur either outside the body called external
fertilization or inside the body called internal fertilization.
Fertilization in most animals is similar to that in humans. Animals also produce gametes for
fusion. But the fusion of gametes may take place inside or outside the body. Based on this,
fertilization is of two types – internal and external fertilization.

12. Internal Fertilization:
In sexual reproduction, the male inserts the semen into the female reproductive tract to fuse
with the egg. If the fusion takes place within the female parent, it is called internal
fertilization. In humans and most animals like cats, lions, pigs, dogs, hens, etc., the fusion of
gametes takes place internally. In this type, a zygote is formed within the mother and gets its
nourishment from her.
External Fertilization:
When the fusion of sperm and egg takes place outside the female parent, it is called external
fertilization. Only a minority of organisms exhibit this type of gamete fusion. For example,
fish, frogs, etc. Here the female parent deposits her eggs in the external environment and
later, the male parent ejects his sperm over them, and then the fusion of the gametes takes
place in the external environment.
Gametes that fuse externally have to face many challenges. Since eggs and sperms are
deposited in the external environment, the chances of fusion are very less. Predators may
eat the eggs or the zygote that is formed. To compensate for this loss, organisms like fish
and frogs lay hundreds of eggs at a time.
3. Post-Fertilization-
Fertilization results in diploid zygote formation. Eventually, the zygote divides mitotically and
develops as an embryo. This process is called embryogenesis. During embryogenesis, cell
differentiates and modifies accordingly. Zygote development depends on the organism and
its life cycle.
Animals are classified into oviparous and viviparous based on whether the zygote develops
outside or inside the body respectively. In angiosperms, the zygote develops into the ovary
and ovary transforms into the fruit while ovules develop into seeds.
In the animal kingdom, external fertilization is a type of fertilization where the sperm-egg
fusion takes place externally, outside the female body. The embryo develops and matures in
the external environment.
While in internal fertilization, the sperm-egg fusion takes place inside the female body. But
the development of embryo may take place either internally or externally. Based on this,
animals are classified into two, namely, oviparous and viviparous animals.
Viviparous Animals:
Animals that give birth to offspring are called viviparous. In viviparous animals, both
fertilization, as well as the development of the embryo, takes place inside the female
reproductive system. Once the fetus development is complete, the mother delivers the baby.
This condition is referred to as matrotrophy where the embryo obtains the nutrients directly
from the mother and not the yolk.
Examples of Viviparous Animals:
Human beings, dogs, cats, elephants, etc are few examples of viviparous animals.
Oviparous Animals:
Animals that lay eggs are called oviparous. In oviparous animals, fertilization takes place
internally but embryo development takes place externally.

13. The eggs of birds such as hen and duck carry immature embryo in them. The hard shells of
eggs protect them from damage. Once the fetus is matured, the egg hatches. The trait of
egg-laying animals is known as oviparity.
Examples of Oviparous Animals:
All birds lay eggs with a typical hard calcium shell. Frogs are egg-laying amphibians which
have soft gelatinous eggs requiring constant hydration. Almost all fishes are oviparous.
Except for some species of snakes, all other reptiles are oviparous. In mammals, Echidna
and platypus are egg-laying.
Metamorphosis in Oviparous Animals:
Viviparous animals give birth to young ones. All organisms mature, grow, and eventually
become adults. But the process of “growing up” varies. Insects and most other invertebrates
undergo a sequential transformation from young ones to adult. This process of a drastic
change of a larva into an adult is called metamorphosis. This type of growth stages can be
observed in many insects like butterflies, silkworms, cockroach, etc.
The only animals with backbones that can undergo metamorphosis are amphibians. For
example, in frogs, there are three stages. Their appearance at each stage differs. They
begin as an egg, then become a larva (tadpole) and later become an adult frog.
Ovoviviparity In Ovoviviparous Animals:
Ovoviviparous animals lay eggs and develop the eggs inside the mother’s body. The eggs
are hatched inside the mother. Once the egg hatches, it remains inside the mother for a
period of time and is nurtured from within but not via a placental appendage. Ovoviviparous
animals are born live.
Some examples of ovoviviparous animals are sharks, rays, snakes, fishes, and insects.
Oviparity is different from ovoviviparity in a way that the eggs in oviparity may or may not
undergo internal fertilization but are laid and depend on the yolk sac to get nourished till the
time they hatch.
Ovoviviparity shows internal fertilization of eggs typically via copulation. For instance, a male
shark penetrates his clasper into the female to release sperms. Fertilization of eggs takes
place when they are in the oviducts and sustain to develop here, and are supplied by the
egg yolk in their egg. The female counterpart of guppies accumulates extra sperms which
they use to fertilize their eggs for a period up to eight months. The younger ones remain in
the oviducts when the eggs hatch and last there to grow and develop till they mature to be
given birth and sustain life.
These animals show no umbilical cord which is typically their physical attachment to the
mother for nutrient requirements and gas exchange. In such cases, nourishment is obtained
from the yolk of the egg. When this yolk is depleted, the mother provides additional nutrition
in the form of unfertilized eggs and uterine secretions.
One of the advantages ovoviviparous animals is that, after birth, the young are competent
enough to feed and defend on their own. This means that they can fend for themselves in
the wild and are capable of living without the need for their mother’s protection. For instance,
rattlesnakes are ovoviviparous and right after birth, they have fully developed venom glands
that are as potent as the adult rattlesnakes.

14. Development of Embryo:
As stated before, fertilization results in the formation of unicellular zygote. Zygote starts to
divide and multiply and eventually develops into an embryo. Embryo moves to the uterus
and attaches to uterus walls.
This is called implantation. Implanted embryo eventually develops different body organs
such as the heart, hands, legs, eyes, etc. A completely developed embryo is called a fetus.
The whole process takes place during the period of 8-9 months. This period or condition is
called pregnancy. Once the fetus is mature, the mother delivers the baby.
This is how an embryo develops in humans and animals but this may take place internally or
externally.
Embryo development refers to the different stages in the development of an embryo.
Embryonic development of plants and animals vary. Even in animals, every species
undergoes different stages during embryonic development.
Let us learn about human embryonic development and various stages.
After fertilization, the zygote is formed. The zygote divides mitotically to form 2, 4, 8, 16
celled stages. These cells are known as blastomeres.
Morula- Embryo having 8 to 16 blastomeres.
The morula continues dividing mitotically and gets transformed to blastocyst. The outer layer
of the blastocyst is called trophoblast and it gets attached to the uterine wall known as the
endometrium. The implantation starts in the first week but gets completed by 2nd week.
The inner cell mass of blastocyst forms embryo. Blastocyst differentiates further to
embryonic and extraembryonic tissues. The implantation completes at the 2nd week.
The interdigitated chronic villi of trophoblast and uterine cells form the placenta, which is the
connection between the mother and the growing foetus.
The placenta provides nourishment and oxygen to the embryo and helps in removing carbon
dioxide and waste produced by the embryo. It also acts as an endocrine gland and secretes
various hormones like hCG (Human Chorionic Gonadotropin), estrogen, progestogens, etc.
for maintenance of pregnancy.
Gastrulation starts in the 3rd week, the inner cell or embryo starts differentiating into three
germinal layers, i.e. ectoderm, endoderm and mesoderm. These cells transform and get
differentiated to all the tissues and organs, like nerve, blood, muscle, bone, digestive tract,
etc.
Ectoderm- nervous system, brain, spinal cord, epidermis, hair, nails, etc.
Mesoderm- connective tissue, muscles, circulatory system, notochord, bone, kidney, gonads
Endoderm- internal organs, stomach, liver, pancreas, bladder, lung, gut lining.
What is Disease?
“A disease is a condition that deteriorates the normal functioning of the cells, tissues, and
organs.”
Diseases are often thought of as medical conditions that are characterized by their signs and
symptoms.
The disease can also be defined as:
“Any dangerous divergence from a functional or normal state of an entity.”
When a person is inflicted with a disease, he exhibits a few symptoms and signs that range
from normal to severe depending upon the medical condition. Hence, in order to identify

15. different diseases, the normalcy of an entity needs to be studied and understood as a clear
demarcation between disease and disease-free is not always apparent.
The diseases are usually caused by many factors rather than a single cause. When we have
a disease, we eventually show some signs, such as headaches, cough, cold, or weakness.
These signs are referred to as “symptoms.” In almost all diseases, symptoms are shown
immediately after having been struck by the disease. However, it varies depending upon the
seriousness of the disease.
Today, there are various ways to classify diseases.
Types of Diseases:
Diseases can be of two types-
● Infectious diseases.
● Non-infectious diseases.
Infectious Diseases:
Diseases that spread from one person to another are called communicable diseases. They
are usually caused by microorganisms called pathogens (fungi, rickettsia, bacteria, viruses,
protozoans, and worms). When an infected person discharges bodily fluids, pathogens may
exit the host and infect a new person (sneezing, coughing etc). Examples include Cholera,
chickenpox, malaria etc.
Common Infectious Diseases:
The table below gives an idea about various common infectious diseases caused by
different pathogens.
List of Infectious Diseases:
Here is the list of a few infectious diseases:
Polio
Rabies
Mumps
Dengue
Plague
Malaria
Anthrax
Cholera
Measles
HIV/AIDS
Smallpox
Influenza
Meningitis
Diphtheria
Melioidosis
Hepatitis A
Hepatitis B
Hepatitis C
Tuberculosis
Yellow Fever
Typhoid Fever

16. Whooping cough
SARS-Severe Acute Respiratory Syndrome
COVID-19
Types of Infectious Diseases:
There are various types of infectious diseases caused by different pathogens. These
diseases are mentioned below:
Viral Infections-
There are millions of viruses existing in the world. They are the main cause of viral infections
such as common cold, influenza, etc.
The virus invades the body of a host and attaches itself to the cell where it releases its
genetic material. The cell replicates and the virus multiplies. The cell lysis and releases more
viruses that infect new cells.
Few viruses change the function of the cells instead of killing the cells. For eg., Human
Papillomavirus, Epstein-Barr Virus causes uncontrolled replication of cells that leads to
cancer.
Bacterial Infections-
Bacteria can survive in any environment from extreme heat to extreme cold and even
radioactive waste. There are numerous bacterial strains some of which cause diseases.
The bad bacteria cause diseases while good bacteria destroy bad bacteria and prevent
diseases. Cholera, tuberculosis, diphtheria, typhoid are some of the infectious diseases
caused by bacteria. They can be treated by antibiotics but some bacteria become
antibiotic-resistant and cannot be treated.
Fungal Infections-
A fungus decomposes and absorbs organic material with the help of an enzyme. Many
fungal infections appear in the upper layers of the skin while some penetrate to the deeper
layers. Fungal spores when inhaled can lead to fungal infections that affect the whole body.
Prion Diseases-
Prion is a protein without a genetic material. If the prion is folded abnormally, it affects the
structure of the normal proteins and causes deadly diseases such as Creutzfeldt-Jakob
Disease. Such diseases spread rapidly and are usually fatal. They do not replicate in the
host but stimulates abnormal behaviour in the body cells.
Other Infections-
Protozoa, Helminths, and Ectoparasites are also responsible for causing infectious diseases.
Protozoa are transferred by contact with faeces. Amoebic dysentery is caused by protozoa.
Helminths include flatworms and roundworms that cause infections in humans.
Ectoparasites such as mites, lice, ticks, etc. attach to the skin and cause infections.
Non-infectious Diseases:
These diseases are caused by pathogens, but other factors such as age, nutritional
deficiency, gender of an individual, and lifestyle also influence the disease. Examples include
hypertension, diabetes, and cancer. They do not spread to others and they restrain within a

17. person who has contracted them. Alzheimer’s disease, asthma, cataract and heart diseases
are other non-infectious diseases.
Cell Cycle:
The cell is the basic structural and functional unit of any living being. It is the fundamental
building block, which when combined with similar cells forms a tissue and organs. A cell
comprises several organelles:
Cytoplasm
Cytoskeleton
Endoplasmic reticulum (ER)
Golgi apparatus
Lysosomes and peroxisomes
Mitochondria
Nucleus
Plasma membrane
Ribosomes
The cell undergoes a series of events that result in the duplication of cell along with the
DNA. This is known as the cell cycle. Let us have a look at the events taking place in the
division of cell during a cell cycle.
Cell cycle refers to the series of events that take place in a cell, resulting in the
duplication of DNA and division of cytoplasm and organelles to produce two daughter cells.
The cell cycle was discovered by Prevost and Dumas (1824) while studying the cleavage of
zygote of Frog. It is a series of stages a cell passes through, to divide and produce new
cells.
This entire process where with the help of one single parent cell a new cell population grows
and develops is known as the cell cycle.
Phases of Cell Cycle
Cell cycle or cell division refers to the series of events that take place in a cell leading to its
maturity and subsequent division. These events include duplication of its genome and
synthesis of the cell organelles followed by division of the cytoplasm.
Human cells exhibit typical eukaryotic cell cycle and take around 24 hours to complete one
cycle of growth and division. The duration of the cycle, however, varies from organism to
organism and cell to cell.
A typical eukaryotic cell cycle is divided into two main phases:-
● Interphase-
Also known as the resting phase of the cell cycle; interphase is the time during which the cell
prepares for division by undergoing both cell growth and DNA replication. It occupies around
95% time of the overall cycle. The interphase is divided into three phases:-
● G1 phase (Gap 1) – G1 phase is the phase of the cell between mitosis and initiation
of replication of the genetic material of the cell. During this phase, the cell is
metabolically active and continues to grow without replicating its DNA.
S phase (Synthesis) – DNA replication takes place during this phase. If the initial quantity of
DNA in the cell is denoted as 2N, then after replication it becomes 4N. However the number

18. of chromosomes does not vary, viz., if the number of chromosomes during G1 phase was
2n, it will remain 2n at the end of S phase. The centriole also divides into two centriole pairs
in the cells which contain centriole.
● G2­phase (Gap 2) – During this phase, the RNA, proteins, other macromolecules
required for multiplication of cell organelles, spindle formation, and cell growth are
produced as the cell prepares to go into the mitotic phase.
Some cells like cardiac cells in the adult animals do not exhibit division and some others only
divide to replace those cells which have been either damaged or lost due to cell death. Such
cells which do not divide further attain an inactive G0 phase also known as quiescent phase
after they exit the G1 phase. These cells remain metabolically active but do not divide unless
called upon to do so.
● M phase-
This is the mitotic phase or the phase of the equational division as the cell undergoes a
complete reorganization to give birth to a progeny that has the same number of
chromosomes as the parent cell. The other organelles are also divided equally by the
process of cytokinesis which is preceded by mitotic nuclear division. The mitotic phase is
divided into four overlapping stages:-
● Prophase,
● Metaphase,
● Anaphase, and
● Telophase.
Mitosis- The process by which a eukaryotic cell separates the nuclear DNA and
chromosomes and divides into two different but similar sets of nuclei is known as mitosis.
The chromosomes are pulled apart by a mitotic spindle, which is a specialized structure
consisting of microtubules.
Cytokinesis-
In this phase, the cytoplasm of the cell divides. It begins as soon as the mitosis ends. Plant
cells are much tougher than animal cells, as they have a rigid cell wall and high internal
pressure. Thus, cytokinesis occurs in plant and animal cells differently.
Cell Division:
Cell division happens when a parent cell divides into two or more cells called daughter cells.
Cell division usually occurs as part of a larger cell cycle. All cells reproduce by splitting into
two, where each parental cell gives rise to two daughter cells.
These newly formed daughter cells could themselves divide and grow, giving rise to a new
cell population that is formed by the division and growth of a single parental cell and its
descendant.
In other words, such cycles of growth and division allow a single cell to form a structure
consisting of millions of cells.
Explore the cell division notes to learn about the types and phases of cell division.
Types of Cell Division:
There are two distinct types of cell division out of which the first one is vegetative division,
wherein each daughter cell duplicates the parent cell called mitosis. The second one is
meiosis, which divides into four haploid daughter cells.

19. Mitosis: The process cells use to make exact replicas of themselves. Mitosis is observed in
almost all the body’s cells, including eyes, skin, hair, and muscle cells.
Meiosis: In this type of cell division, sperm or egg cells are produced instead of identical
daughter cells as in mitosis.
Binary Fission: Single-celled organisms like bacteria replicate themselves for reproduction.
Phases of the Cell Cycle
There are two primary phases in the cell cycle:
Interphase: This phase was thought to represent the resting stage between subsequent cell
divisions, but new research has shown that it is a very active phase.
M Phase (Mitosis phase): This is where the actual cell division occurs. There are two key
steps in this phase, namely cytokinesis and karyokinesis.
The interphase further comprises three phases:
G0 Phase (Resting Phase): The cell neither divides nor prepares itself for the division.
G1 Phase (Gap 1): The cell is metabolically active and grows continuously during this phase.
S phase (Synthesis): The DNA replication or synthesis occurs during this stage.
G2 phase (Gap 2): Protein synthesis happens in this phase.
Quiescent Stage (G0): The cells that do not undergo further division exits the G1 phase and
enters an inactive stage. This stage is known as the quiescent stage (G0) of the cell cycle.
There are four stages in the M Phase, namely:
Prophase.
Metaphase.
Anaphase.
Telophase.
Reproductive Health:
Components of Reproductive Health:
There are three essential components of sexual and reproductive health care-
Family planning – It has a significant impact on the well-being of families and especially
women. With better family planning and the use of contraceptives, one can avoid unwanted
pregnancies, and space births and also protect themselves from STDs.
Sexual health – It refers to a respectful and positive approach towards sexual relationships.
It is a very important prerequisite for good reproductive health.
Maternal health – It refers to the maintenance of a woman’s health during pregnancy and
after childbirth.
Importance of Reproductive Health:
It is very important for an adult and adolescents to be aware of sexual health, reproduction,
contraceptives, and STDs. This will help in maintaining good reproductive health, physically
as well as mentally. People can protect themselves from sexually transmitted infections and
diseases only if they are well informed about the same.
Women should be aware of their fitment for pregnancy. They must have access to proper
medical services when they are pregnant, have a safe delivery and deliver a healthy baby.

20. Nucleic acids are the organic materials present in all organisms in the form of DNA or RNA.
These nucleic acids are formed by the combination of nitrogenous bases, sugar molecules
and phosphate groups that are linked by different bonds in a series of sequences. The DNA
structure defines the basic genetic makeup of our body. In fact, it defines the genetic
makeup of nearly all life on earth.
What is DNA?
“DNA is a group of molecules that is responsible for carrying and transmitting the hereditary
materials or the genetic instructions from parents to offsprings.”
This is also true for viruses, as most of these entities have either RNA or DNA as their
genetic material. For instance, some viruses may have RNA as their genetic material, while
others have DNA as the genetic material. The Human Immunodeficiency Virus (HIV)
contains RNA, which is then converted into DNA after attaching itself to the host cell.
Apart from being responsible for the inheritance of genetic information in all living beings,
DNA also plays a crucial role in the production of proteins. Nuclear DNA is the DNA
contained within the nucleus of every cell in a eukaryotic organism. It codes for the majority
of the organism’s genomes while the mitochondrial DNA and plastid DNA handles the rest.
The DNA present in the mitochondria of the cell is termed mitochondrial DNA. It is inherited
from the mother to the child. In humans, there are approximately 16,000 base pairs of
mitochondrial DNA. Similarly, plastids have their own DNA, and they play an essential role in
photosynthesis.
Full-Form of DNA:
DNA is known as Deoxyribonucleic Acid. It is an organic compound that has a unique
molecular structure. It is found in all prokaryotic cells and eukaryotic cells.
DNA Types:
There are three different DNA types:
A-DNA: It is a right-handed double helix similar to the B-DNA form. Dehydrated DNA takes
an A form that protects the DNA during extreme conditions such as desiccation. Protein
binding also removes the solvent from DNA, and the DNA takes an A form.
B-DNA: This is the most common DNA conformation and is a right-handed helix. The
majority of DNA has a B type conformation under normal physiological conditions.
Z-DNA: Z-DNA is a left-handed DNA where the double helix winds to the left in a zig-zag
pattern. It was discovered by Andres Wang and Alexander Rich. It is found ahead of the start
site of a gene and hence, is believed to play some role in gene regulation.
Who Discovered DNA?
DNA was first recognized and identified by the Swiss biologist Johannes Friedrich Miescher
in 1869 during his research on white blood cells.
The double helix structure of a DNA molecule was later discovered through the experimental
data by James Watson and Francis Crick. Finally, it was proved that DNA is responsible for
storing genetic information in living organisms.
DNA Structure:

21. The DNA structure can be thought of as a twisted ladder. This structure is described as a
double-helix, as illustrated in the figure above. It is a nucleic acid, and all nucleic acids are
made up of nucleotides. The DNA molecule is composed of units called nucleotides, and
each nucleotide is composed of three different components such as sugar, phosphate
groups and nitrogen bases.
The basic building blocks of DNA are nucleotides, which are composed of a sugar group, a
phosphate group, and a nitrogen base. The sugar and phosphate groups link the nucleotides
together to form each strand of DNA. Adenine (A), Thymine (T), Guanine (G) and Cytosine
(C) are four types of nitrogen bases.
These 4 Nitrogenous bases pair together in the following way: A with T, and C with G. These
base pairs are essential for the DNA’s double helix structure, which resembles a twisted
ladder.
The order of the nitrogenous bases determines the genetic code or the DNA’s instructions.
Among the three components of DNA structure, sugar is the one which forms the backbone
of the DNA molecule. It is also called deoxyribose. The nitrogenous bases of the opposite
strands form hydrogen bonds, forming a ladder-like structure.
The DNA molecule consists of 4 nitrogen bases, namely adenine (A), thymine (T), cytosine
(C) and Guanine (G), which ultimately form the structure of a nucleotide. The A and G are
purines, and the C and T are pyrimidines.
The two strands of DNA run in opposite directions. These strands are held together by the
hydrogen bond that is present between the two complementary bases. The strands are
helically twisted, where each strand forms a right-handed coil, and ten nucleotides make up
a single turn.
The pitch of each helix is 3.4 nm. Hence, the distance between two consecutive base pairs
(i.e., hydrogen-bonded bases of the opposite strands) is 0.34 nm.
The DNA coils up, forming chromosomes, and each chromosome has a single molecule of
DNA in it. Overall, human beings have around twenty-three pairs of chromosomes in the
nucleus of cells. DNA also plays an essential role in the process of cell division.
DNA Packaging:
DNA packaging is the process of tightly packing up the DNA molecule to fit into the nucleus
of a cell.”
DNA is an organic, complex, molecular structure found in both prokaryotic and eukaryotic
cells and also in many viruses. It is a hereditary material which is found in the nucleus of the
cell and is mainly involved in carrying genetic information.
The DNA structure has the following characteristics:
The strands of the DNA are helically wounded, every single strand forms a right-handed coil.
The pitch of each helix is 3.32 nm, and about 10 nucleotides make up one turn.
The distance between two succeeding base pairs is 0.34 nm
The total length of a DNA is the distance between two succeeding base pairs and the
product of a total number of base pairs.
A typical DNA strand has a length of approximately 2.2 meters, which is much longer than a
nucleus.
Prokaryotic cells can be distinguished from eukaryotic cells by the absence of a well-defined
nucleus. However, their negatively charged DNA is arranged in a region called the nucleoid.
They appear as a loop wrapped around a protein molecule having a positive charge.

22. All eukaryotes have a well-defined nucleus that contains DNA. DNA is a negatively charged
polymer, packed compactly within the chromatin, engirdling the histone proteins, a ball of
positively charged proteins.
The octamer of histone proteins is wrapped with a DNA helix, giving rise to a structure called
nucleosomes. The nucleosomes are further coiled, which results in the formation of
chromatin fibres. Chromatin fibres are stained thread-like structures, whereas nucleosomes
are beads present over them. These chromatin fibres condense to form chromosomes
during mitosis.
Histones:
Histones are the proteins promoting the DNA packaging into chromatin fibres. Histone
proteins are positively charged, possessing several arginine and lysine amino acids binding
to the negatively charged DNA. There are two types of Histones:
● Core Histones
● Linker Histones
H2A, H2B, H3 and H4 are the core histones. Two H3 and H4 dimers and two H2A and H2B
dimers form an octamer.
Linker histones lock the DNA in place onto the nucleosome and can be removed for
transcription.
Histones can be modified to change the amount of packaging a DNA does. The addition of
the methyl group increases the hydrophobicity of histones. This results in tight DNA
packaging.
Acetylation and phosphorylation make the DNA more negatively charged and loosens the
DNA packaging.
Enzymes that add methyl groups to histones are called histone methyltransferases. The
enzymes that add acetyl groups to the histones are called histone acetyltransferase, while
the ones that remove the histones are called histone deacetylases.
Chargaff’s Rule:
Erwin Chargaff, a biochemist, discovered that the number of nitrogenous bases in the DNA
was present in equal quantities. The amount of A is equal to T, whereas the amount of C is
equal to G.
A=T; C=G
In other words, the DNA of any cell from any organism should have a 1:1 ratio of purine and
pyrimidine bases.
DNA Replication:
DNA replication is an important process that occurs during cell division. It is also known as
semi-conservative replication, during which DNA makes a copy of itself.

23. DNA replication takes place in three stages:
Step 1: Initiation
The replication of DNA begins at a point known as the origin of replication. The two DNA
strands are separated by the DNA helicase. This forms the replication fork.
Step 2: Elongation
DNA polymerase III reads the nucleotides on the template strand and makes a new strand
by adding complementary nucleotides one after the other. For eg., if it reads an Adenine on
the template strand, it will add a Thymine on the complementary strand.
While adding nucleotides to the lagging strand, gaps are formed between the strands. These
gaps are known as Okazaki fragments. These gaps or nicks are sealed by ligase.
Step 3: Termination
The termination sequence present opposite to the origin of replication terminates the
replication process. The TUS protein (terminus utilization substance) binds to terminator
sequence and halts DNA polymerase movement. It induces termination.
Genetic Material:
DNA is the main hereditary material, but some contain RNA too, e.g. retrovirus. RNA mostly
plays the role of a messenger in higher organisms. Though both are nucleic acids, why is
there a difference in functions? Scientists elucidate that, this is due to the difference
between the chemical structure of DNA and RNA. For a molecule to be considered as
genetic material, it must accomplish certain properties. They are as follows:
Replication- the ability to make its copies.
It should have chemical and structural stability.
Mutation- it should offer a chance for evolution.
It should possess a hereditary unit which follows “Mendelian inheritance”.
DNA vs RNA
The difference between DNA and RNA explains the reason why DNA serves as the genetic
material instead of RNA. By comparing DNA and RNA, it is evident that both the nucleic
acids are able to replicate. Stability is an important criterion for continuity. DNA fulfils this
criterion. This point was proved in Griffith’s experiment. The heat-killed S strain bacteria
retained their virulent property when appropriate conditions were provided. In the case of
RNA, 2‘-OH group present in them make them more reactive. Hence, DNA is proved to be
less reactive chemically and more stable structurally than RNA. Thymine makes DNA more
stable than RNA, where it is substituted by uracil.
The unstable nature of RNA makes it more vulnerable to mutation. This property help virus
with RNA as the genetic material evolves at a faster rate. At last, coming to the expression of
Mendelian characters, RNA is much faster than DNA. That is RNA can independently code
for protein synthesis while DNA coding for protein synthesis is mediated by RNA. RNA acts
as a messenger for DNA in the process of protein synthesis.
Though both the nucleic acids can act as genetic material, DNA is much more preferred.
DNA is stable both chemically and structurally which make it well-built genetic material. RNA
in humans does not act as a genetic material but play various other roles such as an

24. adapter, enzyme, helps in protein synthesis, etc. RNA functions as a messenger for
information to be transferred.
Genetics:
Genetics is the branch of biology dealing with the principles and mechanism of inheritance
and variation.
Inheritance is the basis of heredity and by this process, traits are passed on from the parents
to the offsprings. Continuity of the gene pool is maintained by the process of inheritance.
Genes are the basic unit of inheritance and located on chromosomes.
Variation exists among individuals of one species. Variation is due to crossing over,
recombination, mutation and environmental effects on the expression of genes present on
chromosomes.
Mendel’s Laws of Inheritance:
Gregor Johann Mendel is called “Father of genetics”.
Mendel performed experiments on Garden pea. He took 14 true-breeding plants of pea
having seven distinguishable characters, which have two opposite traits.
He called genes as “factors”, which are passed from parents to offsprings.
Genes, that code for a pair of opposite traits are called “alleles”.
He gave three laws of inheritance based on his observation:
Law of Dominance: One of the alleles is dominant and gets expressed in the phenotype in
case of the heterozygote, e.g. When we cross homozygous tall (TT) and dwarf (tt) plants, in
the offsprings we get all the tall plants having the genotype Tt, so tallness is a dominant trait
over dwarfness.
Law of Segregation of genes: Each allele separates during meiosis at the time of gamete
formation. There is no blending and characters are passed to different gametes.
Homozygotes produce only one kind of gametes and heterozygotes produce different kinds
of gametes.
Law of Independent assortment: It states that alleles for different traits are inherited
independently. He showed that using a dihybrid cross.
Test Cross: It is to find out the genotype of the plant showing dominant trait, the given plant
is crossed with the recessive homozygote. The two observations are:
If the phenotype of offsprings shows only the dominant trait, then the parent plant was
homozygote to the dominant trait
If the offsprings produced are of both phenotypes, then the parent plant was heterozygote to
the dominant trait.
Incomplete Dominance:
When neither of the two alleles is dominant and the phenotype of the heterozygote does not
resemble any of the parents. The heterozygote expresses intermediate or a mixture of two
parents’ traits
Example: The flower colour inheritance of snapdragon (dog flower). On crossing true
breeding red (RR) and white flower (rr), we get all pink colour flowers in the F1 generation,
which on self-pollination give red: pink: white flowers in the ratio 1:2:1 in the F2 generation.

25. Co-dominance:
When both the alleles express themselves together in an individual, they are said to be
co-dominant
Example: The inheritance of the ABO blood group in humans is controlled by the gene I. The
gene I has three allelic forms, IA, IB and i. In a human being, any two out of three alleles are
presentIA and IB code for different kinds of sugar polymers present on the surface of RBC
and ‘i’ does not produce any sugarIA and IB are dominant over ‘i’, but IA and IB are
co-dominant and express themselves together.
Chromosomal Theory of Inheritance:
Sutton and Boveri supported Mendel’s observations and stated that chromosomes are the
carrier of genes
Chromosomes occur as a homologous pair and the two alleles of a gene are located on the
homologous pair of chromosomes at the same site
Homologous chromosomes separate during meiosis in the process of gamete formation
Chromosomes segregate and assort independently
During fertilization, gametes combine and produce the offsprings with the diploid no. of
chromosomes, that is similar to the parent
Morgan extensively worked on fruit flies, Drosophila melanogaster and provided
experimental evidence to support the chromosomal theory of inheritance
Linkage and Recombination
Physical association of genes located on a chromosome is known as linkage
In a dihybrid cross, if the two genes are tightly linked or present on the same chromosome,
the parental combination is more prevalent than non-parental combinations or recombinants
The linkage and recombination are directly dependent on the distance between a pair of
genes. More the distance, greater is the probability of recombination
Multiple alleles- When a trait is governed by more than two alleles, e.g. ABO blood group.
Polygenic Inheritance- When a trait is governed by multiple independent genes, that have a
similar or additive effect on the trait, it is known as polygenic inheritance, e.g. eye colour,
skin pigmentation, height, hair colour, etc.
Polygenic inheritance is also affected by environmental conditions.
Pleiotropy- When a single gene controls many phenotypic traits, it is known as a pleiotropic
gene. The different phenotypic expressions are mostly a result of the effect of a gene on
metabolic pathways.
E.g. a single gene mutation in the gene coding for the enzyme phenylalanine hydroxylase
results in the disease known as phenylketonuria, which is characterised by mental
retardation, reduced hair and skin pigmentation.
Sex Determination:
There are different systems of sex determination present in different organisms.
Henking first observed X chromosome and named it X body.
The chromosomes that determine the development of sexual characters are known as sex
chromosomes and the rest of the chromosomes are known as autosomes.
When the male produces two different kinds of gamete, it is known as male heterogamety,
e.g. humans, grasshoppers, drosophila, etc.

26. When the female produces two different kinds of gamete, it is known as female
heterogamety, e.g. birds.
Sex determination in the honey bee:
Haplo-diploid sex-determination system
Female (queen or worker) is formed by the fusion of an egg and sperm and have diploid (32)
no. of chromosomes
Male (drone) is formed from an unfertilized egg by parthenogenesis and have haploid (16)
no. of chromosomes. Sperms are produced by mitosis
Mutation:
Any changes in the sequence of DNA is called a mutation. Viable mutations get inherited
from one generation to another. A mutation changes the genotype as well as the phenotype
of an organism
It is linked to various diseases, but not all mutations are harmful
Changes like, deletion, insertion, duplication, substitution, etc. result in mutation. A mutation
is the major cause of cancer. There are many mutation inducing agents (mutagens) such as
UV rays
There are two types of genetic mutation:
Point mutation: There is a substitution in the single base pair of DNA, e.g. in the sickle cell
anaemia. The 6th codon of the gene coding for the 𝛃-globin chain of haemoglobin changes
from GAG to GUG, resulting in the substitution of glutamic acid by Valine.
Frameshift mutation: It results from the insertion or deletion of one or more pairs of bases in
DNA. it changes the reading frame of triplet codons, that code for certain amino acids of the
protein.
Genetic Disorders:
There are many disorders in the human being that are inherited and caused due to mutation
in the gene or alteration in chromosomes.
Pedigree Analysis helps in determining the risk of getting a genetic disorder in the offsprings
by studying the inheritance pattern of a particular trait present in various generations of an
individual.
Genetic disorders can be grouped into two types:
1. Mendelian Disorders-
These are disorders due to alteration in the single gene
It follows the same inheritance pattern, as per Mendel’s law
Pedigree analysis can help trace the inheritance pattern and also determine if the trait is
dominant or recessive
Chromosomal Disorders:
These are disorders due to excess, absence or abnormal arrangement of chromosomes
Chromosomal disorders are of two types:
(i) Aneuploidy- Gain or loss of one or more chromosomes. It is due to failure of segregation
of chromatids during anaphase of meiosis
(ii) Polyploidy- It is often found in plants. This happens due to an increase in the full set of
chromosomes. Failure of cytokinesis results in polyploidy
Some examples of chromosomal disorders:

27. Down’s syndrome- Trisomy of chromosome 21. Symptoms include mental retardation, short
stature, furrowed tongue, partially opened mouth
Klinefelter’s syndrome- Total 47 chromosomes with one extra X chromosome, i.e. XXY, They
are sterile, tall, overall masculine with feminine characteristics such as breast development
(gynecomastia)
Turner’s syndrome- Total 45 chromosomes. One X chromosome is missing, i.e. XO. females
are sterile, short stature and under-developed sexual characters
By: Mustafa Ahmed Barbhuiya.
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