Heredity and Evolution

Chapter Twelve
Heredity in organisms and evolution

Definition: Chromatin is a genetic material comprising of DNA, RNA, and proteins
which result in the formation of chromosomes within the nucleus of eukaryotic
organisms.
Location in the cell: This chromatin is located within the cell nucleus.
Functions: The main functions of this genetic material include:
1. Preventing DNA damage.
2. Tightly packing of the DNA to fit into the cell.
3. Control the DNA replication and gene expression.
4. Support the DNA molecule to permit the process of cell cycle – meiosis and
mitosis.
Chromatin

Structure of Chromatin:
• The structure of chromatin resembles the arrangement of string on beads.
• The primary protein components of chromatin are histones that help to
organize DNA into “bead-like” structures called nucleosomes by providing a
base on which the DNA can be wrapped around.
• The presence of histone proteins helps in supporting the chromatin structure.
• Chromatin makes it possible for a number of cell processes to occur including
DNA replication, transcription, DNA repair, genetic recombination, and cell
division.

Definition: Chromosomes are thread-like structures present in the nucleus, which
carries genetic information from one generation to another.
Position in the cell: It is spread in the nucleoplasm of a nucleus as a thread like
structure named chromatin.
Discovery of chromosomes: The scientist Strasburger in 1875 first discovered
chromosome.
Chemical Composition: Chromosomes is made up of highly organized DNA
molecules with histone proteins supporting its structure.
Size of chromosome: In length a chromosome may be form 3.5- 30.00 micron
(1micron=1/100mm) and in width 0.2 to 2.0 micron.
Chromosomes

 The main heredity material is chromosome.
 Chromosome means ‘coloured body’, that refers to its staining ability by certain dyes.
Number of chromosome:
 In different species the number of diploid set of chromosome may be 2 to 1600.
 A nematode species contains only 2 chromosomes in a cell, whereas a protozoan species
contains as much as 1600 chromosomes in the cell.
 A human cell contains total 23 pair of of which 22 are autosomes and 1 sex
chromosome.
 Each cell has a pair of each kind of chromosome known as a homologous
chromosome.
 The number of chromosomes in any species is constant for all the cells.

Haploid chromosome:
 It represents half of the somatic chromosome number of a species and is denoted by n.
 Since haploid chromosome number is usually found in the gametes, it is also known as
gametic number.
 The number of chromosomes in gametes (e.g. sperms, egg) is half of the somatic cell and
known as a haploid set of chromosomes, which is the result of meiosis during sexual
reproduction.
Diploid chromosome:
 It refers to somatic chromosome number of a species and is represented by 2n.
 Since diploid chromosome number is found in zygotic or somatic cells it is also referred
to as zygotic or somatic number.
 Chromosome number is preserved in the mitotic division of somatic cells, which is
required for an organism to grow, repair and regenerate.

Chromosome Structure: Main parts of chromosomes are:
Chromatin:
 During cell division, the chromatin fibres condense and chromosomes are visible
with distinct features.
 Chromosome is made up of chromatin. Chromatin is made up of DNA and proteins.
 Nucleosomes are the basic unit of chromatin.
Chromatid:
 During cell division, a chromosome is divided into 2 identical half strands joined by a
centromere. Each of the chromosome has two symmetrical structures called chromatids
or sister chromatids.
 Both chromatids are attached to each other by the centromere.
 At the anaphase of mitotic cell division, sister chromatids separate and migrate to
opposite poles

Centromere:
 Sister chromatids are joined by the centromere.
 Centromere divides the chromosome into two parts, the shorter arm is
known as ‘p’ arm and the longer arm is known as ‘q’ arm.
 Spindle fibres during cell division are attached at the centromere.
Satellite: It is an elongated segment that is sometimes present on a
chromosome. The chromosomes with satellite are known as sat-
chromosome.

Types of Chromosome based on the position of the centromere:
a. Metacentric: V-shaped chromosomes with centromere in the middle giving rise
to two equal arms.
b. Sub-metacentric: L-shaped with centromere near the centre of the chromosome
giving rise to two unequal arms.
c. Acrocentric: J-shaped centromere present at one end giving rise to one very
short arm and an exceptionally long arm.
d. Telocentric: Rod- shaped or T-shaped chromosome with centromere present on
the proximal end. No ‘p’ arm (short arm) present. Telocentric chromosomes are not
found in humans

Functions of Chromosomes:
1. The function of chromosomes is to carry genes (which control the
characteristics of organisms) to the offspring from parents. Colours of human
eyes, nature of hair, compositions of skin etc. continue intact through the
flow of heredity carried by chromosomes. This is why chromosomes are
designated as the physical basis of heredity.
2. They play a vital role in cell division, heredity, variation, mutation,
repair and regeneration.
3. Chromosomes protect the DNA from getting tangled and damaged.
4. Histone and non-histone proteins help in the regulation of gene expression.

Nucleic Acids
Discovery and Naming of Nucleic Acid:
 In 1868 Meischer isolated a substance from the nucleus of pus cells. By digesting pus cells
in HCl, he obtained a pure material and named it nuclein. Nuclein had strong acidic
properties and contained considerable amount of phosphorus.
 In 1889 Altmann coined the term nucleic acid.
Definition:
 Nucleic acids are long chain polymers of nucleotides; hence sometimes nucleic acids
are referred to as polynucleotides.
 Nucleic acids possess all the information needed for an organism’s cell structure, function,
development and reproduction.
 Nucleic acids include DNA and RNA. These molecules are composed of long strands of
nucleotides.

Composition of Nucleotides/ Nucleic Acid: Each nucleotide is composed of
three distinct parts: one molecule each of sugar, phosphoric acid and a
nitrogenous base.
1. Pentose (Five-Carbon) Sugar
2. Nitrogenous Base
3. Phosphate Group

1. Pentose (Five-Carbon) Sugar:
 All nucleotides contain a 5-carbon sugar (pentose).
 The pentose sugar in DNA is deoxyribose and in RNA it is ribose.
 Ribose + nucleic acid  Ribose-nucleic acid, as “ribonucleic
acid” (RNA).
 Deoxyribose + nucleic acid  de-oxy-ribose-nucleic acid, “deoxyribo
nucleic acid” (DNA).

2. Nitrogenous Bases:
 The bases in nucleic acids are heterocyclic compounds containing nitrogen
and carbon in their rings. They are bases because they contain an amino
group.
 The nitrogenous bases are of two types:
a. Purines.
b. Pyrimidines

a. Purines:
 Purines have two carbon-nitrogen rings.
 Adenine (A) and Guanine (G) are classified as purines.
 Both RNA and DNA contain the same two types of purines.
b. Pyrimidines:
 Pyrimidine ring is similar to the benzene ring
 There are three common pyrimidines: cytosine (C), thymine (T) and
uracil (U).
 Cytosine and Thymine are found in DNA, while Cytosine and Uracil occurs
in RNA.

Phosphoric Acid:
 Phosphoric acid (H3PO4) is attached to each sugar at the 3′ and 5′ C positions
to give rise to the sugar-phosphate backbone.
 Free nucleotides in the cell have 3 phosphate residues, generally attached to
the 5′ C of the pentose. During the phosphodiester bond formation, two
phosphate groups are removed from one of the two participating nucleotides.
 A polynucleotide may have thousands of such phosphodiester linkages.
 The nucleotides of the polymer are linked by phosphodiester bonds
connecting through the oxygen on the 5' carbon of one to the oxygen on the
3' carbon of another.

Nucleosides: The combination of a base and a pentose is termed as nucleoside.
Nucleotides: When a phosphoric acid molecule is attached to the pentose
residue of a nucleoside, it is called a nucleotide. Phosphoric acid may attach at
either 5’C or 3′ of the pentose.
Structure of Polynucleotide Chain:
Nitrogenous base + pentose sugar (via N-glycosidic linkage) = Nucleoside.
Nucleoside + phosphate group (via phosphoester linkage) = Nucleotide.
Nucleotide + Nucleotide (via 3′-5′ phosphodiester linkage) = Dinucleotide.
Many nucleotides linked together = Polynucleotide.

Types of Nucleic Acids: There are two types
of nucleic acids:
1.DNA (deoxyribonucleic acid)
2.RNA (Ribonucleic acid)

DNA:
Definition:
 DNA is a group of molecules that is responsible for carrying and transmitting the
hereditary materials or the genetic instructions from parents to offsprings.
 The main component of chromosome is deoxyribonucleic acid (DNA).
 DNA is a stable substance in a chromosome.
 It is usually a double stranded spiral structure of polynucleotides. A strand is
complementary to the other.
 In fact, DNA is a precise thread but in prokaryotic cell DNA is usually circular.
Location of DNA:
 Nuclear DNA is the DNA contained within the nucleus of every cell in a eukaryotic
organism.
 The DNA present in the mitochondria of the cell is termed as mitochondrial DNA.
 Plastids have their own DNA, and they play an essential role in photosynthesis.

Chemical Composition of DNA:
 The DNA molecule is composed of materials called nucleotides which is basic building
blocks of DNA, and each nucleotide is composed of three different components such as
five carbon sugars, nitrogen bases (adenine, guanine, cytosine, and thymine) inorganic
phosphate.
 Nitrogen bases are of two types, such as, purines and a pyrimidine. Adenine (A), guanine
(G) are purines and cytosine (C) and thymine (T) are pyrimidines.
 Adenine of a strand bonds with a thymine of another strand by two hydrogen bonds,
and guanine of a helix connects with a cytosine of another helix by three hydrogen
bonds. This bond is always developed in between a purine and pyrimidine. So a strand of
DNA is complementary to another strand but not just as the same.
 It is discovered that the number of nitrogenous bases in the DNA is 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.

Watson and Crick’s model of DNA:
American scientist Watson and British scientist Crick in 1953 described first the
double helical structure of DNA and for this contribution both of them won
Nobel Prize.
They proposed that DNA molecule takes the shape of a double helix, an elegantly
simple structure that resembles a gently twisted ladder. The rails of the ladder are
made of alternating units of phosphate and the sugar deoxyribose; the rungs are
each composed of a pair of purine/ pyrimidine bases.
They suggested that in a DNA molecule there are two such polynucleotide chains
arranged anti-parallel or in opposite directions i.e., one polynucleotide chain runs
in 5→3 direction, the other in 3→5 direction.

Important features of Watson and Crick’s model of DNA:
1. The DNA structure can be thought of like a twisted ladder. This structure is described
as a double-helix. The sides of the ladder are made sugar - deoxyribose and phosphate
molecules while the steps of the ladder are made up of a pair of nitrogen bases.
2. The double helix comprises of two polynucleotide chains.
3. The two strands (polynucleotide chains) of double helix are antiparallel, meaning that
the two strands of DNA run in opposite directions - one strand runs in a 5′ to 3′
direction, while the other strand runs in a 3′ to 5′direction. The two strands are wound
round each other to form a double helix.
4. Each polynucleotide chain has a sugar-phosphate ‘backbone’ with nitrogenous bases
directed inside the helix.
5. The two polynucleotides in a double helix are complementary. The sequence of
nitrogenous bases in one determines the sequence of the nitrogenous bases in the other.
Complementary base pairing is of fundamental importance in molecular genetics.

6. The nitrogenous bases of two antiparallel polynucleotide strands are
linked through hydrogen bonds. There are two hydrogen bonds between A
and T, and three between G and C. The hydrogen bonds are the only
attractive forces between the two polynucleotides of double helix. These serve
to hold the structure together. The order of the nitrogenous bases determines
the genetic code or the DNA’s instructions.
7. A complete twist in a helix is 34A0 long and in a complete twist, there
are ten nucleotides. So the length between the two adjacent nucleotides is
3.4A0.
8. The diameter of the twisted helical structure in everywhere is 20A0.

DNA Function:
 DNA is the genetic material which carries all the hereditary information which are coded in the arrangement
of its nitrogen bases.
 DNA is the true structure and carrier of the behavioural characters of organisms and it directly carries the
characters of parents to their offspring from generation to generation.
 Genes are the small segments of DNA, consisting mostly of 250 – 2 million base pairs, depending on the
gene. A gene code for a polypeptide molecule. Thus, the sequence of a gene can be used to make a
polypeptide, which then forms a protein. As every organism contains many genes in their DNA, different
types of proteins can be formed. Proteins are the main functional and structural molecules in most of the
organisms.
 Apart from storing genetic information, DNA function involves:
 Replication process: Transferring the genetic information from one cell to its daughters and from one
generation to the next
 Equal distribution of DNA during the cell division
 Mutations: The changes which occur during the DNA sequences
 Transcription
 Cellular Metabolism
 DNA Fingerprinting
 Gene Therapy

RNA
RNA or ribonucleic acid is a polymer of nucleotides which is made up of a
ribose sugar, a phosphate, and bases such as adenine, guanine, cytosine,
and uracil.
RNA Structure:
 RNA is a single-stranded helix.
 RNA is a long polymer consisting of ribonucleotides.
 The ribonucleotides are linked together by 3′  5′ phosphodiester bonds.
 The nitrogenous bases that compose the ribonucleotides include
adenine, cytosine, uracil, and guanine.

Types of RNA: There are three main types of RNA:
1. mRNA (messenger)
2. RNA (transfer)
3. rRNA (ribosomal)

Messenger RNA (mRNA):
The mRNA is synthesized in the nucleus which enters the cytoplasm to participate in
protein synthesis.
It carries the genetic code copied from the DNA during transcription.
Function:
 mRNA transcribes the genetic code from DNA into a form that can be read and used to
make proteins.
 mRNA carries genetic information from the nucleus to the cytoplasm of a cell.

Ribosomal RNA (rRNA):
The rRNA is the component of the ribosome and are located within the in
the cytoplasm of a cell.
Function:
In all living cells, the ribosomal RNA plays a fundamental role in the synthesis
and translation of mRNA into proteins.

Transfer RNA (tRNA):
tRNAs are an essential component of translation, where their main function
is the transfer of amino acids during protein synthesis. Therefore they are
called transfer RNAs.
Each of the 20 amino acids binds with it and transfers it to the growing
polypeptide chain.
Function:
 Its job is to translate the message within the nucleotide sequences of mRNA
into specific amino acid sequences.

Functions of RNA:
1. RNA is a nucleic acid messenger between DNA and ribosomes.
2. It serves as the genetic material in some organisms (viruses).
3. Some RNA molecules play an active role within cells by catalyzing
biological reactions, controlling gene expression, or sensing and
communicating responses to cellular signals.
4. Messenger RNA (mRNA) copies DNA in the nucleus and carries the info to
the ribosomes (in cytoplasm).
5. Ribosomal RNA (rRNA) makes up a large part of the ribosome; reads and
decodes mRNA.
6. Transfer RNA (tRNA) carries amino acids to the ribosome where they are
joined to form proteins.

Character DNA RNA
Full form Deoxyribonucleic Acid Ribonucleic Acid
Location
DNA is found in the nucleus, with a
small amount of DNA also present
in mitochondria.
RNA forms in the nucleolus, and
then moves to cytoplasm depending
on the type of RNA formed.
Structure
DNA has twisted double stranded
helix structure.
RNA is generally single stranded.
Nitrogenous
bases
Each DNA contains nucleotide
nitrogenous bases such as adenine,
guanine, thymine and cytosine.
Each RNA contains the nitrogenous
base such as adenine, guanine, uracil
and cytosine
Chain of
Nucleotides
Long chain of nucleotides Relatively short chains
Sugar
DNA contains deoxyribose sugar in
its nucleotides.
RNA contains ribose sugar in its
nucleotides.

Character DNA RNA
Propagation DNA is self-replicating.
RNA is synthesized from DNA on an
as-needed basis.
Function
 Storing genetic information
 Directs protein synthesis
 Determines genetic coding
 Directly responsible for metabolic
activities, evolution, heredity, and
differentiation.
 Transferring genetic information
from the DNA to proteins
 Carrying it outside the nucleus
 Translating it to proteins
Role as Genetic
Material
In all organisms other than certain
viruses
Very rarely (in some viruses)

DNA Replication
 DNA replication is the process by which DNA makes a copy of itself during cell
division.
 DNA replication, also known as semi-conservative replication, is the process by which
DNA is essentially doubled.
 In 1956 Watson and Crick first successfully proposed the replication process of DNA.
 Replication is the basis of evolution of all morphologically complex forms of life.
 DNA replication, also known as semi-conservative replication. In semi-conservative
replication, each of the two parental DNA strands would act as a template for new DNA
strands to be synthesized, but after replication, each parental DNA strand would base pair
with the complementary newly-synthesized strand just synthesized, and both double-
stranded DNAs would include one parental or “old” strand and one daughter or
“new” strand.
 New DNA is made by enzymes called DNA polymerases.

Steps in DNA Replication:
1. DNA synthesis is initiated at particular points within the DNA strand known as
‘origins’. There are multiple origin sites.
2. A DNA double helix is always anti-parallel; in other words, one strand runs in the 5'
to 3' direction, while the other runs in the 3' to 5' direction. The first step in DNA
replication is to unzip the double helix structure of the DNA molecule and
expose each of the two strands. This is carried out by an enzyme called helicase
which breaks the hydrogen bonds holding the complementary bases of DNA
together (A with T, C with G).
3. The separation of the two single strands of DNA creates a ‘Y’ shape called a
replication fork. The two separated strands will act as templates for making the
new strands of DNA.

4. DNA Primase is an enzyme that is important in DNA replication. It synthesises a small
RNA primer. Once the RNA primer is in place, DNA polymerase gets started which is
ultimately responsible for the creation and expansion of the new strands of DNA by
combining ‘A’ (Adenine) with ‘T’ (Thymin), ‘T’ with ‘A’ , ‘C’ with ‘G’ and ‘G with ‘C’ with
complementary to the template strand. Thus one of the new stands remains and combines
with a new stand to make a complete DNA.
5. The process of expanding the new DNA strands continues until there is either no more
DNA template left to replicate (i.e. at the end of the chromosome), or two replication forks
meet and subsequently terminate. Finally, an enzyme called DNA ligase seals up the
sequence of DNA into two continuous double strands.
At the end, a new strand, combining with another old strand, forms the structure of a
molecule of DNA. As the new DNA emerges, it has a new and an old strand. This principle is
known as semi-conservative method.

Transcription
 Transcription is the process in which a gene's DNA sequence is copied
(transcribed) to make an RNA molecule.
 Transcription is the first step of gene expression where using the information
from genes in our DNA makes proteins.
 RNA polymerases are enzymes that transcribe DNA into RNA. Using a DNA
template, RNA polymerase builds a new RNA molecule through base pairing.
 RNA polymerase always builds a new RNA strand in the 5’ to 3’ direction.
That is, it can only add RNA nucleotides (A, U, C, or G) to the 3' end of the
strand.

Translation
 Translation is the process by which a protein is synthesized from the
information contained in a molecule of messenger RNA (mRNA).
 Translation occurs in a structure called the ribosome, which is a factory
for the synthesis of proteins.
 During translation, a cell “reads” the information in a messenger RNA
(mRNA) is read and used to build a protein.

Codon:
 In an mRNA, the instructions for building a polypeptide are RNA nucleotides read in
groups of three. These groups of three are called codons.
 A codon is a sequence of three nucleotides which together form a unit of genetic code
in a DNA or RNA molecule.
 The code determines the order in which amino acids are added to a polypeptide chain
during protein synthesis. Therefore, the genetic code dictates the sequence of amino acids
in a protein.
 The code is a triplet.

The genetic code:
 The genetic code is made up of codons, which are three-letter chains of
nucleotides. Each codon codes for one specific amino acid.
 The sequence of nucleotides on DNA or RNA which determines the
sequence of amino acids in a polypeptide chain is termed as Genetic
code.

DNA fingerprinting/ Profiling
 DNA fingerprinting was invented in 1984 by Professor Sir Alec Jeffreys.
 DNA fingerprinting is a technique that shows the genetic makeup of living things. It
is a method of finding the difference between the satellite DNA regions in the
genome.
 DNA fingerprinting is a method used to identify an individual from a sample of
DNA by looking at unique patterns in their DNA.
 In DNA fingerprinting method, instead of looking at the whole sequence of a
person’s DNA, these techniques look at the presence or absence of common markers
that can be quickly and easily identified.
 Almost every cell in our body contains our DNA. On average, about 99.9 per cent of
the DNA between two humans is the same. The remaining 0.1% percentage is what
makes us unique.

Principal of DNA Fingerprinting:
DNA fingerprinting involves identifying differences in some specific regions in
DNA sequence called as repetitive DNA.
DNA fingerprinting typically relies on short tandem repeats (STRs), which are
unique to individuals. These sections of DNA can be compared between two
different samples. If they show the same pattern after gel electrophoresis, it
indicates that the samples are from the same source.
A DNA fingerprint looks something like the columns on the paper below. On
this paper, each dark band represents a fragment of VNTRs – and each column
is a different tissue sample. A match would be indicated by two columns whose
VNTRs patterns matched precisely.

Satellite DNA:
 Satellite DNA regions are stretches of repetitive DNA which do not code for any
specific protein. These non-coding sequences form a major chunk of the DNA profile
of humans.
 Repetitive DNA are separated from bulk genomic DNA as different peaks during
density gradient centrifugation. The bulk DNA forms a major peak and the other small
peaks are referred to as satellite DNA.
 Satellite DNA sequence show high degree of polymorphism and form the basis of
DNA fingerprinting.
 Satellite DNA is of two types based on base composition, length of segment, and
number of repetitive units
i. micro-satellites - STR
ii. mini-satellites - VNTR

VNTR:
 VNTR refers to a type of tandem repeats in which a short sequence of nucleotides (10-60
base pairs) are repeated a variable number of times in a particular locus. Therefore,
VNTR is also known as mini-satellites.
 Generally, the number of repeated units in a VNTR vary between individuals. Hence, the length
of the array formed by VNTRs also varies between individuals.
 VNTRs are the first types of polymorphisms used in DNA profiling to determine the DNA
characteristics of a particular person.
STR:
 STR refers to a type of tandem repeats in which a short sequence of nucleotides (2-6 base
pairs) are repeated a variable number of times in a particular locus. STRs are a type of
microsatellites, and they are also known as short sequence repeats (SSRs) in plant
genetics.
 STRs are the commonest type of genetic polymorphisms analysed currently in forensic
genetics.

Following are the steps involved in DNA fingerprinting:
Isolating the DNA.
↓
Digesting the DNA with the help of restriction endonuclease enzymes.
↓
Separating the digested fragments as per the fragment size by the process of
electrophoresis.
↓
Blotting the separated fragments onto synthetic membranes like nylon/ nitrocellulose
paper.
↓
Hybridising the fragments using labelled VNTR probes.
↓
Analysing the hybrid fragments using autoradiography

1 Extracting DNA from Cells:
 To perform DNA fingerprinting, we must first have a DNA sample.
 In order to procure this, the first requirement is an organic specimen. Bone of a person, teeth, hair,
blood, saliva, semen can serve as valuable organic specimens.
 These samples must be treated with a series of chemicals to break open cell membranes, expose the
DNA sample, and remove unwanted components – such as lipids and proteins – until relatively
pure DNA emerges.
2 Treatment with Restriction Enzymes:
 Scientists use repeat sequences – portions of DNA that have the same sequence so they can be
identified by the restriction enzymes, but which repeat a different number of times in different
people.
 Once sufficient DNA has been isolated, it must be cut with restriction enzymes to isolate the
VNTRs. Restriction enzymes are enzymes that attach to specific DNA sequences and create
breaks in the DNA strands.
 Once the DNA has been cut to isolate the VNTRs, it’s time to run the resulting DNA fragments on
a gel to see how long they are.

3 Gel Electrophoresis:
 Gel electrophoresis is a brilliant technology that separates molecules by size.
 The gel separates the different sizes of DNA fragment generated by cutting up the DNA.
 The gel is made from something called agarose and is just a pure firm jelly.
 By putting the liquid DNA fragments in the hole at one end and passing an electric current
through the gel, the DNA fragments move into the gel with the electric current. Small
fragments move faster than larger fragments, so the DNA fragments are separated as they
move in the gel. The effect of the gel is so precise that scientists can tell exactly how big a
molecule is by seeing how far it moves within a given gel in a set amount of time.
 It’s called “electrophoresis” because, to make the molecules move through the gel, an
electrical current is applied. Because the sugar-phosphate backbone of the DNA has a
negative electrical charge, the electrical current tugs the DNA along with it through the gel.

4 Transfer onto Southern Blot:
 Now that the DNA fragments have been separated by size, they must be transferred to a medium where
scientists can “read” and record the results of the electrophoresis.
 The gel-separated DNA fragments are then transferred to white nitrocellulose paper which fixes
them in place, so the paper now carries an exact replica of the DNA on the gel. This is called
“Southern blotting”.
5 Treatment with Radioactive Probe:
 Now that the DNA is fixed onto the blotting paper, it is treated with a special probe chemical that sticks to
the desired DNA fragments. This chemical is radioactive a small chunk of radioactive DNA of a particular
sequence of letters, which means that it will create a visible record when exposed to X-ray paper.
6 X-Ray Film Exposure:
• The last step of the process is to turn the information from the DNA fragments into a visible record. This is
done by exposing the blotting paper, with its radioactive DNA bands, to X-ray film.
• To ensure a clear imprint, scientists often leave the X-ray film exposed to the weakly radioactive Southern
blot paper for a day or more.
• Once the image has been developed and fixed to prevent further light exposure from changing the image,
this “fingerprint” can be used to determine if two DNA samples are the same or similar!

Polymerase Chain Reaction
• Nowadays, it has become possible to identify more accurately the suspected specimen
with little amount of it through the process of polymerase chain reaction (PCR)
method.
 Polymerase chain reaction (PCR) is a common laboratory technique used to
make many copies (millions or billions) of a particular region of DNA.
 Typically, the goal of PCR is to make enough of the target DNA region that it can be
analysed or used in some other way.
 PCR is used in many areas of biology and medicine, including molecular biology
research, medical diagnostics, and even some branches of ecology.
 PCR is used in molecular biology to make many copies of (amplify) small sections of
DNA or a gene. Using PCR it is possible to generate thousands to millions of copies
of a particular section of DNA from a very small amount of DNA.

DNA Fingerprinting Uses:
 To Identify Criminal:
This process is frequently used in criminal investigations to determine whether blood or tissue samples
found at crime scenes could belong to a given suspect. The small profile on the place of occurrence of crime
or profile from the specimen of victim of a crime is compared with the map obtained from the blood or
organic specimen of a suspected person.
To perform the DNA test, the first requirement is an organic specimen. Bone of a person, teeth, hair, blood,
saliva, semen can serve as valuable organic specimens.
In this process, first forms of specimen DNA should be isolated. By more than one small restriction
enzymes, DNA is then cut into many pieces. Through a special method called (electrophoresis agarose or
polyaxialamaid gel), DNA fragments are separated alongside their length.
On a special type of nitrocellulose paper, the radioactive isotope hybridized with DNA and keeping it on x-
ray film through the method of autoradiogrphy visible rows of bands are determined and with the specimen
from the place of occurrence and suspected specimen, it is compared. This method is called DNA finger
printing.
 This technology is also used in paternity tests, where comparison of DNA markers can show whether a
child could have inherited their markers from the suspected father.
 It is used to determine the gender of badly damaged bodies of accident victims or of archaeological
specimens.

Determination of human sex (gender):
 In the cells of human body, the number of chromosome is 46, i.e. 23 pairs. Among them 22
pairs or total number of 44 are autosomes and the remaining pair is sex chromosome.
 Autosomes play roles in physiological, embryonic and the formation of body of organisms which
have no part in determining sex.
 The two sex chromosomes are marked with X and Y. They play significant role in
determining sex.
In women, in the diploid cells both sex chromosomes are X i.e. XX.
But in case of men of the two chromosomes one is X and the other is Y.
Both the X chromosomes are long in structure, rod shaped but chromosome Y is little
shorter than the chromosome X.
At the time of development of egg, meiosis occurs, and every egg possesses a chromosome.
But in case of a man at the time of the formation of sperms, half number of sperms contain
X chromosome and other half number of sperms contain Y chromosome.

An egg can be fertilized with either one of the sperm type X or Y. So, zygote can be having both
the chromosomes as type X, or can be having one X and the other one Y.
The baby, which is born having both the chromosomes as X i.e. XX, will be a baby girl and the
baby, who is born with a chromosome of type X and the other one is of Y, will be a baby boy.
If the principles and orders of determining human sex are pondered over well, we will find that in
determining sex, a mother plays no role because a mother always produces eggs containing only X
chromosome.
The other side of the fact is that a father produces sperms contrarily, with having both the type X and
Y. At the time of conception, the type of sperm, that will fuse with the egg, determine the sex of the
offspring.
In male two types of gametes are produced. 50 per cent of the total sperm produced carry the X-
chromosome and the rest 50 per cent has Y-chromosome besides the autosomes. Females, however,
produce only one type of ovum with an X-chromosome. There is an equal probability of fertilisation
of the ovum with the sperm carrying either X or Y chromosome.

As in fertilization only one sperm is fused with an egg, this is why either X or Y
chromosome of a father successfully accomplishes the job of determining sex of offspring.
If the Y chromosome bearing sperm fuses with an egg, the zygote would bear
chromosomes of both the type X and Y. The chromosomes would be XY. The result
would be the offspring as a son.
Thus, it is evident that it is the genetic makeup of the sperm that determines the sex of
the child. It is also evident that in each pregnancy there is always 50 per cent
probability of either a male or a female child.
In most of the traditional societies, because of ignorance, mothers are held responsible for
giving birth to a baby girl. For these erroneous concepts, mothers have to endure many
mental and physical oppressions. Though it is an accidental coincidence from the scientific
point of view, responsibility of determining sex lies on the shoulder of the father, but
not on the mother.

Sex-Linked Inheritance:
 Sex-Linked Inheritance is the inheritance of a trait that is determined by a
gene located on one of the sex chromosomes.
 Sex-linked genes are located on either the X or Y chromosome, though it
more commonly refers to genes located on the X-chromosome.
 The genes which occur exclusively on the X chromosome is called X-linked
genes while the genes which exclusively occur in Y chromosome are called
holandric genes.

X-linked inheritance:
 X-linked inheritance means that the gene causing the trait or the disorder is
located on the X chromosome.
 Females have two X chromosomes; males have one X and one Y.
 Genes on the X chromosome can be recessive or dominant. Their
expression in females and males is not the same. X-linked recessive genes
are expressed in females only if there are two copies of the gene (one on
each X chromosome). However, for males, there needs to be only one
copy of an X-linked recessive gene in order for the trait or disorder to be
expressed.

Genetic diseases:
 In some genetic diseases, a mutation occurs in gene of sex chromosomes. These types
of diseases are called sex-linked disorder. Sex-linked diseases are passed down
through families through one of the X or Y chromosomes.
 In humans, about 120 sex-linked genetic disorders are found.
 As the size of Y chromosome is comparatively short and there are less genes on it. In most
cases sex-linked disorders occur due to the mutation of the X chromosome. A single
recessive gene on that X chromosome will cause the disease. They include disorders
like Color-blindness and Haemophilia.
 The common sex-linked disorders that are mostly found in humans are mostly
recessive. X-linked recessive diseases most often occur in males. Males have only one
X chromosome.

 In female, there are two X chromosomes. Even if one X chromosome is
affected by mutation, symptoms of genetic disorders are never expressed as the
other X chromosome remains normal. Two X chromosomes in a female are
unlikely to have the same type of mutation. So a female is not usually affected
by sex-linked disorder, but merely acts as a carrier (the person who is not
affected by sex-linked disorder but carries the mutant gene of the sex-
linked disorder is called a carrier).
 In gens of male, there is only one X chromosome. So they do not act as
carrier, rather they show the symptom of sex-linked disorders directly if
only the X chromosome is affected.

Colour blind or colour blindness:
 Colour blindness is a condition when someone cannot properly identify
any colour between red, green or both the colours from other colours.
 To identify colour, we have pigments in our optical nerve cells. Being
colour blind, the patient is deficient of colour identifying pigments in their
optical nerve and suffers from colour blindness. If someone lacks a single
pigment then he would not be able to differentiate colour red and green.
It is the universal problem of colour blindness. For lacking of more than one
pigment besides red and green, the patient cannot differentiate the colour
blue and yellow.

 The colour is perceived in cone cells of the retina of the eye. This cone cell contains three
pigments which absorb a particular wavelength of light. That means one absorbs blue, second
absorbs red, and the third absorbs green light. Human eye detects only three colours- red,
green, and blue. But the brain mixes the signals coming from cone cells to develop the wide
spectrum of colours that we perceive. Each of the three pigments is encoded by a gene of the
separate locus. The locus for the blue pigment is found on Y and those for green and red
pigments lie on X-chromosomes. Human colour blindness is caused by defects of the red and
green pigments; called as red-green colour blindness. Mutations produce this defect. Color-
blindness is a recessive and X-linked trait. That is why it is recessive to normal vision
and only gets inherited through X-chromosome. Therefore, it shows criss-cross
inheritance.

 Genes for this character is located on the X-chromosome but not on Y-
chromosome. So, the trait is transferred from father (sufferer) to his daughters
and carrier mother to her sons.
 One man out of ten is seen colour blind. In comparison, very few numbers
of women suffer from this problem.
 Along with heredity, some medicines such as taking hydroxy chloroquinine
for the treatment of rheumatism triggers a side effect causing colour
blindness by disintegrating the colour pigment in the eyes. Advice from a
registered ophthalmologist can be a worthy solution to cope up with the
situation of colour blindness.

A cross between colour-blind man with a normal colour vision woman:

A cross between a colour-blind woman with a normal colour vision male:

Thalassemia:
 Thalassemia is the name of a disease of acquiring abnormal state of red blood cells.
 Thalassemias are inherited blood disorders characterized by decreased hemoglobin
production.
 Because of this disease, red blood cells are disintegrated. So, the patient suffers from
anaemia. This disease genetically passes from generation to generation.
 In Bangladesh context, Thalassemia is an important hereditary blood concerning problem.
It is guessed that every year 7000 babies are born with the problem Thalassemia and
at present the number of patient may be one lakh.
 It is an autosomal recessive disorder. i.e., when both father and mother are the carrier or
both are the patient of thalassemia, only then does it dominates in the offspring. When the
marriage is held between maternal and paternal cousins or between close (blood related)
relatives there is a higher probability of giving birth to a child with thalassemia.

 A red blood cell is composed of two types of protein 𝜶 globulin and 𝜷 globulin. Thalassemia is caused for
the disintegration of the two genes related to the above mentioned proteins. There are two types of thalassemia
due to two types of gene disintegration.
i. 𝜶 Thalassemia
ii. 𝜷 Thalassemia
Consequently, defective red blood cells are usually found 𝜶 Thalassemia is caused if the gene for 𝜶 globulin
production is absent or changed. This type of disease is found at large in the community of South East Asia,
Middle East, China and Africa.
As the same way, 𝜷 Thalassemia is caused when the gene for the production of the protein 𝜷 globulin is
disintegrated. Though this type of disease is more found in the people of Mediterranean region, some people
having Africans, Americans, Chinese origin may also suffer from the blood disease. 𝜷 Thalassemia is also
called Thalassemia of a khuli.
 On the basis of the inherited gene, Thalassemia is grouped into two categories.
In case of major Thalassemia, the victim baby obtains genes from both the parents and in case of minor
Thalassemia a baby obtains genes either from the father or from the mother. This type of body does not
express any sign of Thalassemia but functions as a carrier of Thalassemia genes.

Symptom:
 Due to severe thalassemia, the baby may die in the womb.
 Babies born with major Thalassemia may suffer from anemia just after birth, up to one year of age.
 Iron overload: People with thalassemia can get an overload of iron in their bodies, either from the disease
itself or from frequent blood transfusions. Too much iron can result in damage to the heart, liver, and
endocrine system, which includes glands that produce hormones that regulate processes throughout the body.
The damage is characterized by excessive deposits of iron. Without adequate iron chelation therapy, almost
all patients with beta-thalassemia accumulate potentially fatal iron levels.
 Infection: People with thalassemia have an increased risk of infection.
 Bone deformities: Thalassemia can make the bone marrow expand, which causes bones to widen. This can
result in abnormal bone structure, especially in the face and skull. Bone marrow expansion also makes bones
thin and brittle, increasing the risk of broken bones.
 Slowed growth rates: Anemia can cause a child's growth to slow. Puberty also may be delayed in children
with thalassemia.
 Heart problems: Diseases, such as congestive heart failure and abnormal heart rhythms, may be associated
with severe thalassemia.

Treatment:
Thalassemia is treated by transfusing blood at regular intervals and by providing
the patient with required medicines. The patient must not eat iron enriched fruits
and medicine, as they may accumulated and cause harm to other body organs. If
the liver is badly affected, other diseases or jaundice can be initiated. The patient
suffering from major thalassemia faces threats of life from age 20 to 30. Besides,
if the liver is badly affected other diseases or jaundice can be initiated.

Theories on evolution of organisms:
 In this world of diversity, we are familiar with many living organisms, among which only
1.3 million are animals. Besides this four hundred thousand species of plants are
identified.
 Once Man had an idea that the world is unchangeable i.e., the shape, volume of earth
remained unchanged from its inception, they thought that there was no difference
between the primitive and current (present) living kingdom.
 But in 500 BC a scientist named Xenophanes, first discovered fossils. He proved that
the living organisms are not unchangeable i.e., there are many changes happening
between the past and the present.
 In 400 BC Aristotle proved that in the living kingdom there are some species which
are more developed in comparison to other species. These developed species adapted
with changes of the environment, and came to their present form through evolution.
 Generally evolution is a slow process, and through this process complex organisms
originate from simple organisms. There are also very few examples of abrupt evolution.

Origin of Earth:
 The Universe is very old-almost 20 billion years ago. It contains huge galaxies. Galaxies
contain stars and clouds of gas and dust. The origin of universe is explained by Big Bang
theory. The Big Bang theory states that a huge explosion occurred, the universe expanded,
temperature came down and hydrogen and helium were formed later. The galaxies were
then formed due to condensation of gases under gravitation.
 According to the latest scientific data, 4.5 billion years ago, the earth was a heated gas
mass evolved from the sun in the solar system of the milkyway galaxy. This gas mass
radiated heat and gradually became denser to a liquid state. Then the mass solidified from
outer side to the inner side and released vapour, creating cloud surrounding the mass. Rain
produced from that cloud created the sea on the solid level of the earth. In the course of
time, life originated in the sea. These lives underwent gradual changes and resulted in
today's diverse world.

 After many hypothetical and research based experiments, modern people have attained
the concept that evolution is the basis of the origin of life.
 Evolution comes from the Greek world "Evolveri". The English philosopher and
educationist Herbert Spencer first used the world evolution.
 Evolution is a process that results in heritable changes in population spread over
many generations leading to diversity of organisms on earth.
 Once it was thought that through slow and continuous changes a complicated and
developed species was developed from a simple and unicellular organism. This
process is known as evolution.
But evolution does not always occur slowly, sometimes it occurrs fast with environmental
changes. Not only that, sometimes complex organisms turns into simpler ones due to
evolution (The Mexican catfish lost its eyesight when it shifted from surface water to dark
caves of deep water). So now the definition of evolution is given by gene alleles. (One
specific gene can remain in more than one form. Then each and every form of that gene is
called its allele.)

 According to Curtis-Burns (1939) the modem definition, of evolution is
the change of the gene allele frequency from generation to generation
within the species more or less similar in nature.
For example, a list was made after determining the genes of all the tigers of the
Sundarbans. At the same time, the number of alleles of the genes was also
counted. After some years, another list will be made by determining genes of
the next generation of tigers along with determining the alleles. When
comparing the two lists, if it is found that there is a noteworthy change in the
alleles then one can say that evolution occurs in the tiger population.

Origin of life:
There are many opinions about how life originated on earth. But there is no controversy
about life originating from sea water. In this regard scientists put forward logic in
different ways.
First, the presence of different mineral salts in most living cells, blood and other liquid of
body are similar to that of sea water minerals.
Second, there are still simple and unicellular organisms found in sea water.

The scientific hypothesis regarding how life originated on earth is like this:
 260 years ago the air of the earth was filled with large amounts of methane, ammonia, hydrogen
sulphide, water vapour, nitrogen and carbon dioxide. There was no trace of oxygen.
 Due to frequent eruptions of volcanos, and the effect of ultra violet rays and lightening the
atmospheric temperature increased and the organic materials combined to produce amino acid and
nucleic acid. The above process was proved by doing an experiment in the laboratory.
 Nucleoprotein is produced from the combination of amino acid and nucleic acid. Gradually the
nucleoprotein attains the power of replication and thus a life starts.
 The origin of earth and its consequent evolution of life is called chemical evolution. It is
assumed that the nucleoprotein is produced by the chemical combination of protein and nucleic
acid. A protovirus is evolved from the nucleoprotein and from it, a virus evolved. A virus is a
state between living and non-living. After that, bacteria were produced and later on protozoa was
originated. The nucleus of bacteria is primitive in nature, it is called a prokaryotic cell. Later
on, true nucleus appears in Protozoa. In some unicellular organisms, chlorophyll is formed and as
a result synthesis of food became possible and oxygen is produced as a by product. Then the aerobic
organisms increase in number. Initiation of multicellular organisms appear from the unicellular
organisms.

In this way evolution occurs in two stream, plant and animal. There are many
explanations about chemical evolution and the origin of living organisms.
Evolution never happens in a linear way. It has always been happening in a
complicated way in different branches.
 After many hypothetical and research based experiments, modern people
have attained the concept that evolution is the basis of the origin of life.

Theory of Darwin or Darwinism:
 Charles Darwin, the British naturalist, brought a revolutionary idea to the history of Biology
and to science.
 Scientist Charles Robert Darwin (1809-1822) was born in Shrewsbury, England.
 During his voyage to the Galapagos Island in the Pacific Ocean he was attracted to the
amazing diversity of plants and animals of that region and he came back to England in 1837
with a vast collection of data and samples.
 Twenty years after his return i.e., in 1859, he expressed his opinions in a book named the
‘Origin of Species by means of Natural Selection.’
 It is mentionable that Darwin is not the founder of evolution but his theory is known as
the theory of evolution. The success of Darwin was that he developed a mechanism based on
scientific data and proof which could explain all matters related to evolution.
 Alfred Russell Wallace, (1823-1913), a contemporary British naturalist and scientist who
independently developed a theory on natural selection, also mentioned natural selection as a
cause of organic evolution. Due to some historical reasons, Darwin's theory was more popular
than his.

 According to Darwin, general facts about natural events:
Increase of generation in excessive rate: A general characteristic of living
beings is to reproduce its species in an excessive rate. Due to this the number
of species increases both geometrically and mathematically. For example,
one mustard plant produces 7,30,000 seeds in a year. It is possible to get
730,000 plant from those seeds. A female salmon fish lays thirty million
eggs in one breeding season. According to Darwin, if all the elephants born
from a pair of elephants would survive, the number of elephants would be
1,90,000 (one hundred ninety thousand) in 750 years.
Limited food and shelter: As the area is limited on Earth, shelter and food for
living organisms is limited.

Struggle for survival: As the number of living organisms increases both geometrically and mathematically, having
limited food and shelter, living organisms have to face hard competition for survival. Darwin called this the struggle
for existence.
Darwin noticed that living organisms had to struggle in three stages. These are:
Interspecific Struggle: For example, a frog cats an insect and a snake eats a frog. A Peacock eats both snake and
frog. Thus only due to physical needs (hunger), a cruel relationship develops among different species, where each
species plays the role of food or consumer.
Intraspecific struggle: As the same species has the same habitat and the same food habits, when their number
increases, they compete among themselves for survival.
For example, when in an island the number of herbivorous animal increases, they fight for food and shelter among
themselves. The stronger animal (herbivorous) beat the weaker ones and take the command of that area. As a result,
the weaker animals die after a certain period from starvation.
Struggle with Environment: Normal life of living organisms is interrupted by unfavourable conditions like flood,
drought, cyclone and tornado, earth quake, volcano eruption etc. So living beings always have to fight with these
unfavourable environments to survive. Those who won the struggle can survive, while others become extinct. For
example, the koel of Mid and North America became extinct due to severe cold and snowfall.

Variation: According to Darwin, two animals (or group of animals) are never exactly the
same. There are always some differences, even if in a very small scale. The difference
between the two specimens of the same species is called variation or mutation. Variation
helps living organisms to survive.
Natural selection: The process through which organisms develop through adaptation,
gain more success in struggling in comparison to others, and enjoys more advantages in
competition, is called natural selection. The organisms which win the competition in
struggling through adaptation are selected by nature and reproduce at a greater rate.
On the other hand, organisms which cannot adjust to the new changes cannot survive and
gradually become extinct. According to Darwin, the organism which can survive in
unfavourable conditions by adaptation should be addressed as the fittest. Thus it can be
easily said that the fittest organism survive by winning the competition through adaptation
(Survival of the fittest).

Origin of new species: Nature selects plants and animals and
nourishes them. Plants and animals that survive can cope with the
changes of the environment and are able to reproduce more than those
who are unfit for survival. The characteristics of variation are
transferred generation after generation. The generation which has the
quality of environmental friendly variation are again selected by
nature. Over the years through natural selection, plants and animals are
selected and thus new varieties of species are produced.

At present geneticists, cytologists and taxonomists following the theory on genetics and
evolution, believe that new varieties could be produced in three different ways, such as:
a. Being isolated from basic species
b. Hybridization
c. In hybridized species the accidental increase of chromosomes (due to polygamy)
occurs during cell division and as a result, a new variety produced can adapt with the
environment and is selected by nature for survival.

Significance of Evolution in surviving of species:
During emergence of a new species through evolution, many species are lost in the passage
of time. For example, Dinosaurs The species which possess a greater ability to adapt through
evolution can go far.
So the species which can attain more ability to adapt with the environment, flow of life and
demography will survive for a longer period of time. This process is called adaptation.
It is not that evolution occurs only in nature. Evolution can also be done by experiment in the
laboratory. This is also evidence of realistic evolution. There is no scientific evidence found
against evolution till today. The more knowledge we gain about the biological kingdom, the
more difficult it becomes to deny evolution.

Heredity and Evolution

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