Nucleic acids
Two types of nucleic acids are present in all
mammalian cells. They are DNA - deoxy ribonucleic acid and RNA- ribonucleic
acid. DNA is present in the nucleus and mitochondria. RNA is present in the
nucleus, risosome and cytoplasm.
Nucleic acids are acidic sustances containing
nitrogenous bases, pentose sugar and phosphoric acid. Both DNA and RNA are
polynucleotides. They are polymers of mononucleotides.
In nucleic acids, nucleotides are joined
together by phosphodiester linkages.
A nucleoside is composed of purine or
pyrimidine base and a pentose sugar. Two types of pentose sugar are present in
nucleoside, they are ribose and deoxy ribose (Fig. 7.3). In the case of purine
nucleosides, the sugar is attached to N-9 of the purine ring, whereas in
pyrimidine nucleosides, the sugar is attached to N-1 of the pyrimidine ring.
The type of linkage is N-glycosidic linkage (Fig. 7.4).
Nucleotides are phosphorylated nucleosides
usually one or two of hydroxyl groups of ribose (or) deoxyribose are
phosphorylated. Thus a nucleotide has three structural components. They are
nitrogenous base, sugar and phosphate. Phosphate is attached to ribose (or)
deoxy ribose through an ester linkage (Fig.7.5).
Nucleotide sequence of a nucleic acid is known
as its primary structure which confers individuality to the polynucleotide
chain. Polynucleotide chain has direction. They are represented in 5’---> 3’
and 3’----> 5’ directions. Each polynucleotide chain has 2 ends. The 5’ end
carrying a phosphate group and 3’ end carrying an unreacted hydroxyl group (Fig
7.6).
In 1953, J.D. Watson and F.H.C. Crick proposed
a precise three dimensional model of DNA structure based on model building
studies, base composition and X-ray diffraction studies. This model is
popularly known as the DNA double helix (Fig.7.7).
The purine bases present in DNA are adenine and
guanine and the pyrimidine bases present are thymine and cytosine. The purine
and pyrimidine bases of DNA carry genetic information where as the sugar and
phosphate groups perform the structural role.
·
Two
polynuleotide chains are coiled around a central axis in the form of a right
handed double helix.
·
Each
polynucleotide chain is made up of 4 types of nucleotides. They are adenylate,
guanidylate, thymidylate and cytidinilate.
·
Each
polynucleotide chain has direction or polarity. Further each polynucleotide
chain has 5’ phosphorylated and 3’ hydroxyl ends.
·
The
backbone of each strand consists of alternating sugar and phosphate. The bases
project inwards and they are perpendicular to the central axis.
·
The 2
strands run in opposite direction (ie.) they are antiparallel.
·
The
strands are complementary to each other. Base composition of one strand is
complementary to the opposite strand. If adenine appears in one strand, thymine
is found in the opposite strand and vice versa. When guanine is found in one
strand, cytosine is present in the opposite strand and vice versa.
·
Bases of
opposite strands are involved in pairing. Pairing occurs through hydrogen
bonding and it is specific. Adenine pairs with thymine through two hydrogen bonds.
·
Guanine
pairs with cytosine with three hydrogen bonds.
·
Major
and minor grooves are present on the double helix. They arise because
glycosidic linkages of base pairs are not opposite to each other. Protein
interact with DNA through the minor and major grooves without disrupting the
DNA strands.
·
According
to Chargaff’s observation, the number of adenine base is equal to thymine base
and the number of quanine base is equal to number of cytocine base ie. A = T
and G = C. Also A + T = G + C and the ratio of A+T /G+C = nearly 1.0. The total
number of purine bases = the total number of pyrimidine bases.
1.
DNA is
the genetic material of living organisms. It is the greatest super chip ever
made by man.
2.
DNA
contain all the information required for the information of an individual
organism.
3. The genetic information in DNA is converted to characteristic features of living organisms like colour of the skin and eye, height, intelligence, ability to metabolise particular substance, ability to withstand stress, susceptibility to desease and ability to produce or synthesise certain substances.
4.
DNA is
the source of information for the synthesis of all cellular proteins. The
segment of DNA that contain information for a protein is known as gene.
5.
DNA is
transmitted from parents to offsprings and hence transmit genetic information
from one generation to another
6.
The
amount of DNA in any given species or cell is constant and is not affected by
nutritional and metabolic states.
7. Avery Macleod and Mc Carty in 1944 first demonstrated that DNA contained the genetic information and they referred DNA as ‘transforming factor’
8. The
nucleotides present in
DNA are deoxy adenylic acid, deoxy quanidylic acid,
deoxy cytidylic acid and deoxy thymidylic acid.
RNAs are present in the nucleus, ribosomes and
cytoplasm of eukaryolic cells. They are involved in the transfer and expression
of genetic information. They act as primer for DNA formation. Some act as
enzymes and as coenzymes. RNA also function as genetic material for viruses.
RNAs are also polynucleotides. In RNA polymer,
purine and pyrimidine nucleotides are linked together through phosphodiester
linkages. The sugar present is ribose. The nitrogenous bases present in RNA are
adenine and guanine (purine bases), uracil and cytosine (pyrimidine bases). The
nucleotides present in RNA are adenylic acid, quanidylic acid, cytidylic acid
and uridylic acid.
There are mainly three types of RNAs in all
prokaryotic and eukaryotic cells. They are (1) Messenger RNA (mRNA) 2) Transfer
RNA (tRNA) 3) Ribosomal RNA (rRNA). They differ from each other by size,
formation and stability.
It accounts for 1-5% of cellular RNA. They have
a primary structure. They are single standed linear molecules. They consist of
1000-10,000 nucleotides. They have a free or phosphorylated 3’ and 5’ end. They
have different life span ranging from few minutes to days.
mRNA molecules are capped at 5’ end by
methylated guanine triphosphate. Capping protects mRNA from nuclease attack. At
3’ end a polymer of adenylate (poly A) is found as the tail. Poly A tail
protects mRNA from nuclease attack.
Intrastrand base pairing among complementary
bases allows folding of the linear molecule. As a result, haripin or loop like
secondary structure is formed.
·
mRNA is
a direct carrier of genetic information from the nucleus tothe cytoplasm.
·
It
contain information required for the synthesis of protein molecules.
It accounts for 10-15% of total cell RNA. They
are the smallest of all the RNAs. Usually they consist of 50-100 nucleotides.
They are single standard molecules. They contain unusual bases such as
methylated adenine, guanine, cytosine and thymine,dihydrouracil and
pseudouridine. These unusual bases are important for binding 6-RNA to intra
chain base pairing. Further some bases are not involved in base pairing,
resulting in loops and arms formation in tRNA. These folding in the primary
structure generates a secondary structure (Fig.7.8).
Secondary structure of t-RNA is in the form of
a clover leaf. The important feature of the clover-leaf structure are,
1.
An
acceptor arm with base sequence “CCA” 3’-OH of adenosine moiety of t-RNA.
2.
An
anticodon arm which recognises codon on mRNA.
3.
TφC arm
which contain unusual base cytosine.
4.
D- arm
which contain many dihydrouracil residues.
It is the carrier of amino acids to the site of
protein synthesis.
There is at least one t-RNA molecule to each of
20 amino acids required for protein synthesis.
This accounts for 80% of the total cellular
RNA. It is present in ribosomes. In ribosomes, r-RNA is found in combination
with protein. It is known as ribonucleoprotein. The length of rRNA ranges from
100-600 nucleotides. rRNA molecules have a secondary structure. Intra strand
base pairing between complementary bases generate double helical segments or
loops.
1.
They are
required for the formation of risosomes
2.
They are
involved in the initiation of protein synthesis.
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