இந்தக் கட்டுரையில், TNPSC குரூப் 1, குரூப் 2, குரூப் 2A, குரூப் 4 மாநிலப் போட்டித் தேர்வுகளான TNUSRB, TRB, TET, TNEB போன்றவற்றுக்கான முறைகள் இலவசக் குறிப்புகளைப் பெறுவீர்கள்.தேர்வுக்கு தயாராவோர் இங்குள்ள பாடக்குறிப்புகளை படித்து பயன்பெற வாழ்த்துகிறோம்.
DNA
DNA is the hereditary material as it contains the genetic information.
It is the most important constituent of a chromosome.
The most widely accepted model of DNA is the double helical structure of
James Watson and Francis Crick.
They proposed the three-dimensional model of DNA on the basis of X-ray
diffraction studies of DNA obtained by Rosalind Franklin and Maurice
Wilkins.
In appreciation of their discoveries on the molecular structure of nucleic
acids Watson, Crick and Wilkins were awarded Nobel prize for Medicine
in 1962.
Chemical Composition of DNA molecule:
DNA is a large molecule consisting of millions of nucleotides. Hence, it is
also called a polynucleotide.
Each nucleotide consists of three components.
1. A sugar molecule – Deoxyribose sugar.
2. A nitrogenous base.
There are two types of nitrogenous bases in DNA.
(a) Purines (Adenine and Guanine)
(b) Pyrimidines (Cytosine and Thymine)
3. A phosphate group
Nucleoside and Nucleotide
(a) Nucleoside = Nitrogen base + Sugar
(b) Nucleotide = Nucleoside + Phosphate
The nucleotides are formed according to the purines and pyrimidines
present in them.
Watson and Crick model of DNA:
DNA molecule consists of two polynucleotide chains.
These chains form a double helix structure with two strands which run
anti-parallel to one another.
Nitrogenous bases in the centre are linked to sugar-phosphate units which
form the backbone of the DNA.
Pairing between the nitrogenous bases is very specific and is always
between purine and pyrimidine linked by hydrogen bonds.
Adenine (A) links Thymine (T) with two hydrogen bonds (A = T)
Cytosine (C) links Guanine (G) with three hydrogen bonds( C ≡ G)
This is called complementary base pairing.
Hydrogen bonds between the nitrogenous bases make the DNA molecule
stable.
Each turn of the double helix is 34 A° (3.4 nm). There are ten base pairs in a
complete turn.
The nucleotides in a helix are joined together by phosphodiester bonds.
DNA Replication:
DNA replication is one of the basic process that occurs within a cell.
DNA molecule produces exact copies of its own structure during replication
process.
The two strands of a DNA molecule have complementary base pairs the
nucleotides of each strand provide the information needed to produce its
new strand.
The two resulting daughter cells contain exactly the same genetic
information as the parent cell.
DNA replication involves the following steps
Origin of replication
The specific point on the DNA, where the replication begins, is the site of
origin of replication.
The two strands open and separate at this point forming the replication
fork.
Unwinding of DNA molecule:
The enzyme called helicase, bind to the origin of replication site.
Helicase separates the two strands of the DNA.
The enzyme called topoisomerase separates the double helix above the
replication fork and removes the twists formed during the unwinding
process.
Each of the separated DNA strands function as a template.
Formation of RNA primer
An RNA primer is a short segment of RNA nucleotides.
The primer is synthesized by the DNA template close to the origin of
replication site.
Synthesis of new complementary strand from the parent strand
After the formation of RNA primer, nucleotides are added with the help
of an enzyme DNA polymerase and a new complementary strand of
DNA is formed from each of the parent strand.
The synthesis is unidirectional.
In one strand, the daughter strand is synthesized as a continuous strand
which is called leading strand.
In the other strand, short segments of DNA are synthesized. This strand is
called lagging strand.
The short segments of DNA are called Okazaki fragments. The fragments
are joined together by the enzyme, DNA ligase.
The replication stops when the replication fork of the two sides meet at a
site called terminus, which is situated opposite to origin of replication
site.
Significance of DNA
It is responsible for the transmission of hereditary information from one
generation to next generation.
It contains information required for the formation of proteins.
It controls the developmental process and life activities of an organism.
Ribonucleic Acid (RNA)
Ribonucleic acid (RNA) is a polymeric molecule essential in various
biological roles in coding, decoding, regulation and expression of genes.
RNA is single stranded and is unstable when compared to DNA .
Fraenkel-Conrat and Singer (1957) first demonstrated that RNA is the
genetic material in RNA containing viruses like TMV (Tobacco Mosaic
Virus) and they separated RNA from the protein of TMV viruses.
Three molecular biologists in the early 1980’s (Leslie Orgel, Francis Brick
and Carl Woese) independently proposed the ‘RNA world’ as the first
stage in the evolution of life, a stage when RNA catalyzed all molecules
necessary for survival and replication.
The term ‘RNA world’ first used by Walter Gilbert in 1986, hypothesizes
RNA as the first genetic material on earth.
There is now enough evidence to suggest that essential life processes
(such as metabolism, translation, splicing etc.,) evolved around RNA.
RNA has the ability to act as both genetic material and catalyst. There are
several biochemical reactions in living systems that are catalyzed by RNA.
This catalytic RNA is known as ribozyme. But, RNA being a catalyst was
reactive and hence unstable.
This led to evolution of a more stable form of DNA, with certain chemical
modifications.
Since DNA is a double stranded molecule having complementary strand,
it has resisted changes by evolving a process of repair.
Some RNA molecules function as gene regulators by binding to DNA and
affect gene expression.
Some viruses use RNA as the genetic material.
Andrew Fire and Craig Mellow (recipients of Nobel Prize in 2006) were of
the opinion that RNA is an active ingredient in the chemistry of life.
Types of RNA
mRNA (messenger RNA):
Single stranded, carries a copy of instructions for assembling amino acids
into proteins.
It is very unstable and comprises 5% of total RNA polymer.
Prokaryotic mRNA (Polycistronic) carry coding sequences for many
polypeptides.
Eukaryotic mRNA (Monocistronic) contains information for only one
polypeptide.
tRNA (transfer RNA):
Translates the code from mRNA and transfers amino acids to the ribosome
to build proteins.
It is highly folded into an elaborate 3D structure and comprises about 15%
of total RNA.
It is also called as soluble RNA.
rRNA (ribosomal RNA):
Single stranded, metabolically stable, make up the two subunits of
ribosomes. It constitutes 80% of the total RNA.
It is a polymer with varied length from 120–3000 nucleotides and gives
ribosomes their shape.
Genes for rRNA are highly conserved and employed for phylogenetic
studies.
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