nucleic acid

2.  Learn about
Nucleic acid
Nucleotide
Nucleoside
Basis
Importance

3.  Nucleic acids are among the most important biological
macromolecules (others being amino acids-proteins, sugars-
carbohydrates, and lipids-fats).
 They are found in abundance in all living things Nucleic
acids, which include DNA (deoxyribonucleic acid)
and RNA (ribonucleic acid), are made from monomers known
as nucleotides.

4.  Each nucleotide has three components: a 5-
carbon sugar, a phosphate group, and
a nitrogenous base.
 If the sugar is deoxyribose, the polymer is
DNA. If the sugar is ribose, the polymer is
RNA. When all three components are
combined, they form a nucleic acid.
Nucleotides are also known as phosphate
nucleotides.

5.  SUGARS
D-ribose and D-2-deoxyribose are the only sugars
so far found in the nucleic acids from which the
sugars have been isolated and identified, and they
are assumed to be the sugars universally present in
nucleic acids.

6. .

7.  Both sugars are present in nucleic acids
as the β-Furanoside ring structures as
depicted below.

8.  It has 2 types
 Pyrimidine and purine
 Pyrimidine bases found in nucleic acids are
mainly three:
• Cytosine is found both in DNA and RNA
• Thymine is found in DNA only
• Uracil is found in RNA only.

9.  Pyrimidine nucleus is represented below.

10. .

11.  The Purine ring is more complex than the
Pyrimidine ring.
 It can be considered the product of
fusion of a pyrimidine ring with an
imidazole ring.

12. .

13.  Adenine and guanine are the two principal
purines found in both DNA and RNA.
1. Adenine
 Chemically it is 6-aminopurine.

14.  2. Guanine
Chemically it is 2-amino-6-oxypurine.

15. The nucleosides are composed of purine or
pyrimidine base linked to either D-ribose (in
RNA) or D-2-deoxyribose (in DNA).

16. .

17.  They are joined by β-N-glycosidic linkage.
 This linkage in purine nucleosides is at
position –9 of the purine base and carbon 1’
of sugar or deoxy sugar.
 Example: Adenosine (adenine-9-riboside)

18.  In pyrimidine nucleosides, β-N-glycosidic
linkage is formed at position –1 of the
pyrimidine base linked to carbon –1’ of
ribose or deoxyribose sugar.
 Example: Uridine (uracil-1-riboside)

19. .

20.  A nucleotide is a nucleoside to which a
phosphoric acid group has been attached to
the sugar molecule by ‘esterification’ at a
definite – OH group and thus has the general
composition base – sugar – PO4.

21.  In ribose nucleosides, there are three possible
positions for phosphate esterification namely 2’,
3’ and 5’.
 In deoxynucleosides, there are free –OH groups
only at the 3’ and 5’ positions in the deoxyribose
nucleosides. PO4 can be attached only at these
positions. The name of each nucleotide may be
derived from that of its constituent nitrogenous
base.

22. .

23.  Examples
 ATP, ADP, AMP and Cyclic AMP, GTP etc

24. Mediator of hormone action—acts as “Second
messenger” in the cell.
 • Regulates glycogen metabolism—increased cyclic
AMP produces breakdown of glycogen
(glycogenolysis).
 • Regulates TG metabolism—increased cyclic AMP
produces lipolysis (breakdown of TG).
 • Cholesterol biosynthesis is inhibited by cyclic
AMP.
 • Cyclic AMP stimulates protein kinases so that
inactive protein kinase is converted to active protein
kinase.
 • Cyclic AMP modulates both transcription and
translation in protein biosynthesis.

25.  • Cyclic AMP activates different steps of steroid biosynthesis.
 • Cyclic AMP also regulates permeability of cell membranes
to water, sodium, potassium and calcium.
 • Plays an important role in regulation of insulin secretion
catecholamine biosynthesis, and melatonin synthesis.
 • Histamine increases cyclic AMP production in parietal cells
which in turn increases gastric secretion.
 • Decrease in cyclic AMP level is involved in the excitation of
bitter taste receptors in tongue.
 • Cyclic AMP plays an important role in cell differentiation.
Addition of cyclic AMP to malignant cell lines in vitro
reduces growth rates and restores their morphology to normal

26.  DNA is a polymer of
deoxyribonucleotides and is found in
chromosomes, mitochondria an chloroplasts.
 The nuclear DNA is found bound to basic
proteins called histones. DNA is present in
every nucleated cell and carries the genetic
information. It is conveniently isolated from
viruses, thymus gland, spleen, leucocytes,
etc.

27.  Histones are a small family of closely related basic proteins in
chromatin.
 four types of histones.
They are—
 H2A, H2B, H3 and H4, called core histones.
 The structure of all four types of histones are highly conserved
between species. This extreme conservation implies that the function
of histones is identical in all eukaryotes.
 Structure: The –COOH terminal 2/3rd of the molecules have a typical
random amino acid composition, but 1/3rd of the –NH2 terminal are
rich in basic amino acids.
 Interaction of histones: The four types of histones interact with each
other in very specific ways as follows:
 H3 and H4 form a tetramer containing 2 molecules of each (H3H4)2
 H2A and H2B form dimers (H2A–H2B).

29.  In the nucleosome, the DNA is supercolied in
a left hand helix over the surface of the disc-
shaped histone octamer.
 Function: This association of histone
octamer with DNA protects the DNA from
digestion by a nuclease.

30.  Chromosomal DNA consists of very long DNA
molecules.
 Each DNA is a polymer of about 1010
deoxyribonucleotides.
 Normally there are only four different types of
deoxyribonucleotides that are found in DNA
molecule, namely,
 1. adenine deoxyribonucleotide(dA),
 2. thymine deoxyribonucleotide (dT),
 3. guanine deoxyribonucleotide (dG), and
 4. cytosine deoxyribonucleotide (dC).

31. ..

32.  Nucleotides of each of the two helical strands are
bound to each other by covalent 3’-5’
phosphodiester linkage.
 Each such bond is formed by the ester linkages of
a single phosphate residue with the 3’ –OH (i.e.
C – 3’ –OH group of the ribose sugar) of one
nucleotide with the C – 5’ –OH group of ribose
of the next nucleotide.
 This kind of bonding gives rise to a linear
polydeoxyribonucleotide strand with 2 free ends
on both sides.

34.  The primary structure is the number and
sequence of different deoxyribonucleotides in its
strands joined together by phosphodiester
linkages.
 The backbone of the primary structure is the
linear strand of interconnected sugar phosphate
residues while the purine or pyrimidine
connected with the sugar residue projects
laterally from the backbone.

36.  This consists of a double stranded helix
formed by the two polydeoxyribonucleotide
strands around a central axis. This type of
model was first proposed by Watson and
Crick

37.  DNA is a double helix. Each of its two
strands is coiled about a central axis,
usually a right handed helix.
 The two sugar phosphate backbones wind
around the outside of the bases like the
banisters of a spiral staircase and are
exposed to the aqueous solution.
 The phosphodiester bonds in the two
interwoven strands run in opposite
directions. Therefore the strands are
called antiparallel.

38.  Thus the polarity of the two strands will be 3’
– 5’ and 5’ – 3’. The 3’ – 5’ strand is called
coding or “template strand” and 5' – 3'
strand is called noncoding‘strand

40.  The aromatic rings of bases are hydrophobic and
they are stacked in the interior, nearly
perpendicular to the long axis of the helix.
 Adenine base of one strand of DNA is hydrogen
bonded to a thymine in the opposite strand;
while the guanine is hydrogen bonded to a
cytosine.

41.  Two strands of DNA double helix can separate or
unwind during processes such as DNA
replication, RNA transcription and genetic
recombination.
 Complete unwinding of DNA can take place in
vitro and is called denaturation of DNA or it is
also known as a helix to coil transition.

42.  The melting temperature of DNA is determined by
its base composition. Since there are two hydrogen
bonds between A and T while three between G and
C, increasing G-C base pairs raises Tm.
 Tm is strongly influenced by the base composition
of the DNA.
 DNA rich in G-C pairs has a higher Tm than DNA
with high proportion of A-T pairs.
 Mammalian DNA, which has about 40 per cent G-C
pairs, has a Tm of about 87°C.

43.  Once the strands are separated, they can be
renatured. If a melted sample of DNA is slowly
cooled, the absorbance of the solution decreases.
This is indicative of complementary strands
being paired again.
 This process is called as annealing. Annealing
can occur only at a temperature below Tm of
DNA which is about 70oC.
 It is fastest at 20oC below Tm or 50oC.

44.  Ribonucleic acid is a polymer of ribonucleotides
of Adenine, Uracil, Guanine and Cytosine, joined
together by 3’ – 5’ phosphodiester bonds.
 Thymine is absent in RNA. RNA is found in the
nucleolus, ribosomes, mitochondria and
cytoplasm.

45.  Structures of RNA
 (a) Primary Structure of RNA
The primary structure of RNA is defined as the
number and sequence of ribonucleotides in the
chain.
 •Each linear strand is held together by the
ribonucleotides bound to each other by 3’,–5’
phosphodiester bonds joining 3’–OH of one
nucleotide with the 5’–OH of the next

46.  Secondary Structure of RNA
 The secondary structure of RNA involves various
coil formation of the polyribonucleotide chain.
 These coil structures are stabilised by
hydrophobic interactions between the purine and
pyrimidine bases.

47.  Tertiary Structure of RNA
 The tertiary structure of RNA involves the
folding of the molecule into three-
dimensional structure.
 The crosslinking also occurs at various sites
stabilised by hydrophobic and hydrogen
bonds producing a compactly coiled globular
structure.

48.  Types of RNA
 There are mainly three types of RNA found
in human beings. These are:
 1. Messenger RNA or m-RNA,
 2. Transfer or soluble RNA or t-RNA, and
 3. Ribosomal RNA or r-RNA.
 The main function of each of these RNA
is protein synthesis.

49.  In human cells there are small nuclear RNA or
Sm-RNA which are not involved in protein
biosynthesis directly. They may have some role
in processing of RNA and cellular architecture.
They are found in nucleoplasm, nucleolus,
perichromatic granules, and cytoplasm and vary
in size from 90 nucleotides to 300 nucleotides.
large precursor of m-RNA called as
heterogeneous nuclear RNA or hn-RNA is also
found in the nucleus

50.  This is the most heterogeneous class of RNA
with respect to its size and stability. The
molecular weight varies from 3 × 104 to 2 × 106.
They consist of 103 to 104 ribonucleotides.
 It carries mainly adenine guanine, cytosine and
uracil as the major bases and methylpurines and
methylpyrimidines as minor bases.

51.  These are also called as soluble or s-RNA. They
remain largely in cytoplasm. The t-RNAs are
relatively small, single-stranded, globular molecules
with molecular weight of 2 to 3 × 104. There are at
least 20 different t-RNA molecules.
 (a) Primary structure of t-RNA: t-RNA molecules
consist of approximately 75 nucleotides. Their bases
include adenine, guanine, cytosine, uracil,
pseudouridine (ψ) or uracil 5-ribofuranoside and
thymine are present in one loop.

52.  Secondary structure of t-RNA: Each single
stranded t-RNA molecule remains folded to form
a clover-leaf secondary structure. These folds of
the secondary structure are stabilized by H-bonds
between complementary bases in different
portions of the same strand. These double
stranded helical structures are called as stems.
 (c) All t-RNA molecules contain 4 main arms or
loops:

53. .

54.  Ribosomal or r-RNA
 A ribosome is present in the cytoplasm and is a
nucleoprotein. It is on the ribosome that the m-
RNA and r-RNA interact during the process of
protein biosynthesis. The r-RNA forms 80 per
cent of the total cellular RNA.

55.  The function of r-RNA in ribosome particle is
still not clearly understood. However, they are
necessary for ribosomal assembly and seem to
play key roles in the binding of m-RNA to
ribosomes and its translation.

56.  1. Both have adenine, guanine, cytosine.
 2. The nucleotides are linked together by
phosphodiester bonds.
 3. The bonding is in 3’– 5’ direction.
 4. Main function involves protein biosynthesis

57. Differences
1. In addition to A, G, C, the fourth base is
T—Uracil absent
2. Pentose sugar is deoxyribose
3. Present in nucleus, mitochondria but
never in cytoplasm
4. They consist of 2 helical strands
5. There are A, B, C, D and E forms of
DNA
6. Large molecules
7. One strand 3’ – 5’ carries genetic
information
8. DNA can form RNA by the process of
“transcription”
9. Purine and pyrimidine contents are
almost equal.
1. In addition to A, G, C, the fourth base is
U—Thymine absent
2. Pentose sugar is ribose
3. In addition to nucleus, RNA is found in
cytoplasm
4. Single stranded
5. There are t-RNA, m-RNA, r-RNA, hn-
RNA and SnRNA
6. Only hn-, m- and r-RNA are large
molecules
7. m-RNA transcribed from DNA carries
genetic information.
8. RNA cannot give rise to DNA under
normal conditions, but it can under special
experimental conditions using reverse
transcriptase
9. Not equal