The document discusses the structure and function of cells, cellular reproduction through mitosis and meiosis, human reproductive systems and organs, the process of human development from embryo to fetus during pregnancy, and some genetic abnormalities that can occur. The key points are: 1. Cells are the basic unit of life and come together to form tissues, organs and organ systems. Cellular reproduction occurs through mitosis, which produces identical cells, and meiosis, which produces gametes like sperm and egg cells. 2. The human reproductive systems include male organs that produce sperm and female organs involved in producing eggs and supporting pregnancy. Fertilization occurs when sperm meets egg. 3. A fertilized egg undergoes cell

1. THE LIVING CELL
1. The Living Cell. The cell is the building block of all living matter. Every plant and animal is
made up of a great number of cells. That is why they are called multicellular organisms. It is
the combination of cells that accounts for (a) the variety of living things that we see around us,
and (b) the different body parts of every organism.
2. A cell is the smallest living part of your body. It is so small that it cannot be seen by the naked
eye. You will need a microscope to see one. The powerful high technology microscopes, called
electron microscopes, make it now possible to see even the tiniest structures inside the cell.
3. Cells of a particular kind that are grouped together make up a tissue. Muscles, bones and
nerves are examples of tissues. A group of tissues make up an organ; group of organs makes
up a system.
4. The human reproductive system, for example, is made up of the male and female reproductive
organs. Each organ is made up of different tissues that are composed of the same kind of cells.
5. Structure of a Cell.
a. Cell Membrane. Every cell is surrounded by a cell membrane that keeps the outside out
and the inside in, except for things that a cell needs in order to grow and stay alive, such as
water, nutrients, oxygen and other things.
b. Nucleus. Inside the cell membrane is a nucleus, which is responsible for the three most
important functions of a cell: reproduction, differentiation and metabolism.
c. Cytoplasm. Surrounding the nucleus is a jellylike substance called the cytoplasm which
contains the mitochondria, lysosomes and vacuoles. Mitochondria extract energy from
food and convert it to a form that can be used by the cell. Lysosomes are bodies that store
digestive enzymes. Vacuoles are fluid-filled membranes for storing and expelling wastes.
Note: All these living materials in a cell are called protoplasm.

2. CHROMOSOMES
1. The processes of reproduction, metabolism, differentiation, and heredity can be traced to one
of the materials inside the nucleus of a cell – the chromosomes.
2. Chromosomes are thin, long and threadlike structures which are made up of long chains of
DNA (deoxyribonucleic acid), RNA (ribonucleic acid), and a number of proteins.
3. The DNA, consisting of fragments called genes, serves as the blueprint that enables the cells to
make an exact copy of themselves and to make another individual of the same species.
4. Every species has a characteristic number of chromosomes. The number is what differentiates
one species from another. The chromosomes are arranged in homologous pairs. Human
beings have 22 pairs of body chromosomes and 1 pair of sex chromosomes. The pair of sex
chromosomes is represented by the letters X and Y; XY for the male and XX for the female.
The 23rd
pair in male is not homologous; the X chromosome is very much bigger than the Y
chromosome.
5. Human beings have 23 pairs of chromosomes in all their cells except the sex cells (egg and
sperm), which only have 23 chromosomes.
6. In sexual reproduction, a sperm unites with an egg to form a new cell, called zygote, from
which a new individual is born. Both sperm and egg cell contribute their set of chromosomes
to the formation of the new individual.
7. If the sex cells were to have the same number of chromosomes as the body cells, the new
individual will not be anything like any human because it will have 92 chromosomes. The fact
that two human beings can sexually reproduce another human being only means that sex cells
have only half the number of chromosomes. Thus, when a sperm cell unites with an egg cell,
the new cell will have the correct 23 pairs of chromosomes for a human being.

3. CELLULAR REPRODUCTION
1. All cells grow and divide. The increase in the number of cells as you grow is due to cellular
reproduction, or the ability of a cell to produce an exact copy of itself by a process called cell
division. In this process, a cell divided and becomes two, two becomes four, and four becomes
eight, and so on.
2. Mitosis.
a. In mitosis, the nucleus of the cytoplasm divides. First a cell grows in size. Each
chromosome in the nucleus makes an exact copy of itself. Then the nucleus and cytoplasm
splits into two parts, each part having the same number of chromosomes as the original
cell.
b. Mitosis results in two new cells, called diploid, having the same number of chromosomes
and same characteristics as the original cell.
c. Cells become old and die. When they die, they are replaced by new ones through mitosis.
It is the primary process in the building and repair of body tissues.
3. Meiosis.
a. All body cells are reproduced by mitosis except the sex cells. If sex cells are reproduced by
mitosis, then every sperm or egg shall contain 46 chromosomes. And when an egg cell
unties with a sperm cell, the new cell will therefore possess 92 chromosomes. That is not a
human being.
b. The fact that this does not happen, means there is another method of cell division for sex
cells. This is called meiosis (from the Greek word which means “to make smaller”), which
produces new cells having only chromosomes, or half as many chromosomes as the old cell.
These new cells with only one of each pair of chromosomes are called haploid.
c. Meiosis occurs in two stages. In meiosis I each sex cells splits into two haploids. After
meiosis II each haploid then splits into two by a process similar to mitosis, forming four
haploid cells having half as many chromosomes as the diploid cells.
4. The difference between mitosis and meiosis comes from a manner by which the chromosomes
split into two.
a. In mitosis, each chromosome makes an exact copy of itself and then breaks into two. Thus
the daughter cells are exactly the same as the mother cell.
b. In meiosis, the chromosomes arrange themselves in pairs and then divide into two sets,
such that each set gets only one of each pair of chromosome.
5. Cellular Reproduction of Cells.
a. Egg cells are formed in the female’s reproductive organ, the ovary. Sperm cells are
developed in the male’s reproductive organ, the testes. In their first stage, they are called
germ cells and look like any other cells in the human body with the same number of
chromosomes and genetic information. In the ovary and the testes, germ cells reproduce
many times over by the process of mitosis. The transformation of an egg cell and sperm
cells from their respective germ cells is by the process of meiosis.
b. In the ovary, the germ cell undergoes meiosis I producing one big haploid cell and one small
haploid cell. After meiosis II, three small haploid cells and one big haploid cell are
produced. Only the big haploid cell will grow into an egg cell. The rest will disintegrate. At
birth, the ovaries already hold all the eggs the females will ever have. During her
reproductive years, one egg will mature each month. A mature egg is the largest cell in her
body.
c. In the testes of the male, the germ cell also undergoes cell division by meiosis I and II. But
unlike in the ovary, all the haploid cells are of the same size and become sperm cells. The

4. production of sperm begins at puberty and continues throughout his lifetime. The testes
can produce millions upon millions of sperm cells. The sperm mixes with a fluid produced
by the seminal vesicle, prostate and Cowper’s gland forming the semen.

5. HUMAN REPRODUCTIVE ORGANS
1. The male role in the human reproduction is to provide the sperm that fertilizes the egg, and
deposit the sperm into the vagina of the female. Sperm are produced within the testes. The
testes are two round glands, protected by a sac called scrotum. Each testicle contains small
twisted seminiferous tubules where the sperms are formed. From the testes, sperm travel
upward through the epididymis to a long tube, called vas deferens, both of which are ducts
that carry sperm to the prostate. The prostate is a gland that produces the fluid that, together
with the sperm, constitutes semen. From the prostate, sperm travel through the urethra of
the penis and, from there, into the vagina of the female during sexual intercourse. It takes
around two to three hours for sperm to reach the egg in the fallopian tube. Only one sperm
penetrates and fertilizes the egg.
2. The female’s initial role in human reproduction is to produce the egg cells, or ova, in the
ovaries. There are two ovaries in the female body. Although each ovary holds hundreds of
thousands of immature egg cells, only one of these ovaries produces and releases a mature egg
every 28 days, a process known as ovulation. It floats into the fallopian tube, where it may be
fertilized. The egg descends into the uterus where the embryo develops. The uterine wall
tissues constitute part of the placenta which serves to nourish the developing embryo. The
placenta provides the connection between the mother and the embryo. If the egg is not
fertilized, menstruation occurs.
3. Health and hygiene are very important factors in reproduction. Sexual organs should be kept
clean at all times, especially because bacteria multiply more rapidly on body parts that are not
exposed to air.

6. PUBERY AND ADOLESCENCE
1. Adolescence is the time between childhood and adulthood, which is often, referred to as the
teenage years. During this time, the teenager grows physically, intellectually, emotionally and
socially. The beginning of puberty generally marks the beginning of adolescence.
2. Puberty is the state of physical development marking the beginning of one’s reproductive life,
which means that he or she is capable of producing children.
3. In girls, puberty begins with the first menstrual period. This means that her body is able to
ovulate. This is also when a girl’s breasts grow larger, her hips become rounded, her pubic
hairs and her body take a more womanly appearance.
a. Menstruation comes from the
Latin word mensis which means
month. It is so-called because
it takes place about every 4
weeks of the month.
b. At ovulation, the thickness of
the mucous membrane in the
uterine membrane increases
and is rich with blood. If the
egg is not fertilized, a hormone
is released from the ovary to
signal the slowdown of blood
flow to the mucous lining. As a
result, the uterine membrane
shrinks and begins to lose fluid
and tissue, causing severe
injury to the cells. This injury causes the mucous lining to fall off, which causes the
bleeding, and menstruation begins. The blood does not clot because of the presence of a
protein called plasmin in the tissue.
4. In boys, the beginning of puberty is not signaled by anything as obvious as in girls. During
puberty, a boy’s voice deepens, his shoulders broaden, his hips narrow, and his pubic hairs
begin to grow.
5. The sex organs produce not only the sex cells but also the sex hormones. The testes in males
produce the testosterone. The ovaries in females produce the estrogen and the progesterone.
Sex hormones stimulate the appearance of secondary sex characteristics and promote
hormonal interactions that bring about the reproductive process.
6. Estrogen is released from the follicle, a tiny structure inside the ovary. The development of the
follicle is stimulated by another hormone called follicle-stimulating hormone (FSH) which is
secreted by the pituitary gland. It stimulates the lining of the uterus in order to increase cell
division.
7. When an egg matures and the follicle breaks, progesterone is secreted by the follicle together
with more estrogen. Both hormones stimulate the thickening of the uterine wall tissue. At the

7. same time the estrogen signals the pituitary gland to stop producing the FSH and start
secreting luteinizing hormone (LH) which causes ovulation and facilitates the movement of the
ovum in the fallopian tube.
8. Progesterone and estrogen stimulate the development of the female glands, although high
levels of these hormones suppress lactation. But upon the release of the placenta during birth,
the levels of these hormones drop dramatically and at the same time, hormones from the
pituitary glands stimulate lactation. The first flow of milk is colostrum, a milky yellowish fluid
that has the same composition as milk but has more protein and less fat. Milk follows after
two days.

8. HUMAN EMBRYO AND PREGNANCY
1. The fertilized egg, or zygote, undergoes cell division as it moves along the fallopian tube. By
the time it reaches the uterus, it has become a many-celled human embryo. The embryo
implants itself in the uterus where it continues to grow. It respires, removes its wastes, and
is nourished through the placenta. The embryo is enclosed in a sac called amnion, which is
filled with amniotic fluid. The fluid keeps the embryo moist and protects it from bumps and
jolts. The umbilical cord connects the embryo and the placenta. The cord contains blood
vessels which transports food to and gathers wastes from the embryo. Oxygen and
nutrients diffuse from the mother’s blood to the placental membrane to the embryo’s
blood.
2. The embryo begins with the rough shape of a hollow ball. All the cells in the embryo look
no different. From these cells, the differentiation of tissues begins to take place. After 5
weeks, the embryo is about 1 centimeter long and limbs begin to grow. After 8 weeks, the
embryo is about 2.5 centimeter long, most of the internal organs are formed, and so are its
face, eyes, ears, nose, mouth and the beginnings of sex organs. At this point the embryo is
called a fetus. From this point, differentiation and growth processes very rapidly. How the
complex process of differentiation is still not clearly understood. What is known is that the
process follows the information encoded in the genes of the fertilized egg, which tells what
tissue to produce, when to produce it, and when to stop producing it.
3. Pregnancy usually
takes nine months
from conception, or
fertilization. Early in a
woman’s pregnancy,
there is an increase in
the levels of the
hormones estrogen
and progesterone. The
first results of this
increase are the
tenderness and
fullness in the breasts
and “morning
sickness,” during which
the woman is
experiencing dizziness
and vomiting. The uterus increases in size as the fetus grows. By the 20th
week, the weight
of the uterus is about 20 times what it was before conception, and the movement of the
fetus can already be felt. By the 27th
week, the movement of the fetus can also be seen.
4. By the time the baby is ready to be born, its head is positioned outwards the opening of the
uterus into the vagina. Childbirth begins with labor, i.e. when the muscles of the uterus
repeatedly contract and relax. These contractions move the baby slowly toward the vagina.
The amniotic sac breaks, the fluid is released, and the muscle contractions follow more
frequently and stronger until the baby is born.
5. Immediately after birth, the baby is blue because it has stopped taking oxygen through the
umbilical cord. But once the baby takes its first breath, and its blood is refurnished with

9. oxygen, the baby turns pink. To initiate the baby’s first breath, the doctor or nurse may
hold the baby upside-down and give a slight tap at his back. The baby’s first cry is his fist
breath. Sometimes, if this does not happen, the doctor or nurse may have to suck out the
fluid from the baby’s nose.
The umbilical cord is tied and
cut off. What remains of it
will dry up and fall after a few
days.
6. Pregnancy should not be
taken lightly. The woman
should seek parental care
from an obstetrician in order
to ensure safe childbirth.
Blood and urine tests are
among the important medical
tests made. Ultra sound
imaging is also necessary to
“see” the position and
condition of the fetus.
7. Cellular reproduction may not
always proceed normally.
Sometimes accidents happen,
which result in one less or
more chromosome in the
nucleus. When this happens,
retardation and other
abnormalities may occur.
8. The type of abnormality depends on which particular gene is in excess or lacking. For
example, Down’s syndrome, or mongolism, occurs when an egg acquires both members of
the pair of chromosome 21. This can happen if the said pair did not separate during
meiosis I and the said egg is fertilized by a normal sperm cell. This condition is known to
occur in the pregnancy of women above 35 years old.
9. Other examples of abnormalities brought about by the failure of a pair of chromosome to
separate are harelip, polydactyl (having one more finger or toe), and defective eye
movement.

10. 10.Sometimes a piece of chromosome breaks off and gets lost. Losing chromosome 5, for
example, results in an individual with a round and moon-like face, and is physically and
mentally retarded. Leukemia results from the loss of one of chromosome 22.
11.The use of drugs and other medicines, such as antihistamines, tranquilizers, and
hallucinogens during pregnancy has been known to cause chromosome failures.