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CHAPTER – VII
NUCLEIC ACID
INTRODUCTION
Nucleic acids have a variety of roles in cellular metabolism. Nucleic
acids are molecular repositories of genetic information. The structure of
every protein, and ultimately of every biomolecule and cellular component,
is a product of information programmed into the nucleotide sequence of a
cell’s nucleic acids. The ability to store and transmit genetic information
from one generation to the next is a fundamental condition for life. Thus the
nucleic acids are fundamental molecules of biological system. There are two
types of nucleic acids, DNA (Deoxyribonucleic acid) and RNA (Ribonucleic
acid). The amino acid sequence of every protein in a cell, and the nucleotide
sequence of every RNA, is specified by a nucleotide sequence in the cell’s
DNA. A segment of a DNA molecule that contains the information required
for the synthesis of a functional biological product, whether protein or RNA,
is referred to as a gene. A cell typically has many thousands of genes, and
DNA molecules, not surprisingly, tend to be very large. The storage and
transmission of biological information are the only known functions of
DNA. RNAs have a broader range of functions, and several classes are found
in cells. Ribosomal RNAs (rRNAs) are components of ribosomes, the
complexes that carry out the synthesis of proteins. Messenger RNAs
(mRNAs) are intermediaries, carrying genetic information from one or a few
genes to a ribosome, where the corresponding proteins can be synthesized.
Transfer RNAs (tRNAs) are adapter molecules that faithfully translate the
information in mRNA into a specific sequence of amino acids.
Structure of NUCLEIC ACID
Structure of Nucleic Acid includes following.
A Nucleic Acid is nothing but a long chain of polynucleotides.
Nucleotides have three characteristic components:
Nucleotide = Nitrogen base + Pentose sugar + Phosphate
(1) A Nitrogenous (nitrogen-containing) base – Adenine (A), Thymine (T),
Guanine (G), Cytosine(C),
Uracil (U, only in case of RNA, T is
replaced by U)
These nitrogen bases are divided into two groups
 Purines – Double ring compounds and includes A,G
 Pyrimidines – Single ring compounds and includes T, C,U
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(2)
Pentose
A DNA contains pentose sugar i.e. Deoxyribose and RNA contains Ribose
sugar. The difference between the two lies in fact that the oxygen is absent at
second position of pentose sugar as in case of DNA while in RNA it is
present at second position.
Structure of ribose and deoxyribose sugar
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(3) Phosphate
Phosphate group is attached to the either 3’ or 5’ of carbon of sugar in a
nucleotide.
In DNA the adenine pairs with thymine and cytosine pairs with guanine
by making 2 and 3 Hydrogen bonds respectively. The bond between N-base
and sugar is called as glycosidic bond.
In RNA, A pairs with U and G pairs with C by making 2 and 3 Hydrogen
bonds respectively.
Structure of Deoxyribonucleotide
Nucleoside
The molecule without the phosphate group is called a nucleoside.
Nucleoside = Sugar + nitrogen base
In case of RNA, the nucleotides are called as ribonucleotides.
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STRUCTURE OF DNA
The structure of DNA
was
discovered
by
Watson and Crick in
1953.
DNA is
structure.
double
helix

DNA
molecule
consists
of
two
polynucleotide chains.

he two polynucleotide
chains coil in an
inverted way around
axis.






The chains follow right
handed helices with ten base pairs in one turn of spiral.
The one chain runs inverted relative to the other, i.e. the sequence of
bases in two chains run in opposite directions. Thus, the two helix
show antiparallel arrangement.
The bases are present inside and phosphates on the outside of double
helix, with their planes set at right angles to the axis of helix and
spaced at intervals of 3.4 A°.
The two chains of DNA molecule remain held together by hydrogen
bonding between the bases, which join together in pairs, a single base
from one chain remains hydrogen bonded to single from other chain.
The purine base pairs with pyrimidine base as explained earlier.
The two strands of a DNA molecule are complementary to each other.
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 There are different types of DNA and they are A, B and Z DNA.
HYDROGEN BONDING BETWEEN NITROGEN BASES
STRUCTURE OF RNA
The RNA is generally single stranded and in some cases viz. Viruses
contains double stranded RNA. Synthesis of RNA depends on DNA. Like
DNA, RNA also consists of repeated units called as nucleotides and the
basic concepts RNA shows variation in some respects.
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DIFFERENCE BETWEEN DNA AND RNA
DNA
1. Pentose sugar
Deoxyribose sugar
RNA
Ribose sugar
2. Nitrogen bases
A, T, G, C
A, U, G, C
3. Occurence
Found in nucleus
generally
Found
Or
in
cytoplasm
associated
with
organelles.
4. It is self replicating.
depends on DNA.
The RNA synthesis
TYPES OF RNA
1. Messenger RNA (m RNA)
2. Ribosomal RNA (r RNA)
3. Transfer RNA (t RNA)
Messenger RNA
Cap UTR
start
UTR
PolyAtail

coding sequence
stops
3’


Constituting 5-10% of the total RNA. mRNA is synthesised from DNA. It
carries all the genetic information from DNA.
m RNA are short lived with a mol. wt of 5, 00000-2000000.
These are synthesised in nucleus by a process called as transcription from
one strand of DNA in presence of enzyme RNA polymerase and
transported to cytoplasm.

5' cap
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
The 5' cap is a modified guanine nucleotide added to the "front" (5' end)
of the pre-mRNA using a 5'-5'-triphosphate linkage. This modification is
critical for recognition and proper attachment of mRNA to the ribosome,
as well as protection from 5' exonucleases. It may also be important for
other essential processes, such as splicing and transport.
Ribosomal RNA
 Found in ribosomes of cell, rRNA constitutes upto 85-90
percent of total RNA present in an organism.
 RNA molecules are found in ribosome.
 It is the most stable type of RNA.
 These participate in protein synthesis.
 r RNA exposed at the surface of ribosome is double stranded
while portion associated with internal proteins remains single
stranded.
Transfer RNA
 These are smallest type of RNA.
 It is composed of 75-80 nucleotides and are freely present in
cytoplasm.
 It plays important role in protein synthesis.
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 All tRNA have tertiary structure and for this kind of stabilisation,
presence of Mg ion is present.
 All tRNAs have G (guanine) base at the 5’ terminal end and
unpaired or single stranded portion containing CCA at 3’ end
.Amino acid is attached at 3’ end.
 In 3D structure, RNA looks like L shaped molecule.
 The nitrogen bases present on tRNA are called as anticodon.
According to the anticodon sequence protein is synthesised and
translocated to cytoplasm.
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