Nucleotides are the monomer units that make up nucleic acids like DNA and RNA. They consist of a nitrogenous base, a pentose sugar, and a phosphate group. Nucleotides join together via phosphodiester bridges between the 3' carbon of one sugar and the 5' carbon of the next, forming polynucleotide chains. There are two main types of nucleic acids: DNA contains the sugar deoxyribose and is usually double-stranded, while RNA contains the sugar ribose and is generally single-stranded. The structures and functions of DNA, RNA, and their components are essential for genetic inheritance and protein synthesis.

1. Nucleotides & Nucleic Acids
Nucleotides—the monomer units or building blocks
of nucleic acids—serve multiple additional functions.
They form a part of many coenzymes and serve as
donors of phosphoryl groups (eg, ATP or GTP),
of sugars (eg,UDP- or GDP-sugars),
or of lipid (eg, CDP-acylglycerol).
Regulatory nucleotides include
the second messengers cAMP and cGMP,
allosteric regulation of enzyme activity by ATP, AMP,
and CTP.

2. • Synthetic purine and pyrimidine analogs that
contain halogens, thiols, or additional nitrogen
are employed for chemotherapy of cancer and
AIDS and
• as suppressors of the immune response during
organ transplantation.
• There are 2 types of Nucleic acids :
• Deoxyribonucleic Acid ( DNA ) &
• Ribonucleic acid ( RNA)
• Components of Nucleic acids :
• Nucleic acids are polymers of nucleotides held
by 3’ & 5’ phosphate bridges.

3. • Nucleotides are composed of
• nitrogenous base + pentose sugar + phosphate
• Nucleoside refers to base + sugar
• So, Nucleotide = Nucleoside + phosphate
• Nitrogenous bases found in nucleotides are
aromatic heterocyclic compounds.
• Bases are of 2 types :
• Purines & Pyrimidines

5. • Thymine & Uracil differ in structure by the presence
(in T) or absence of (in U) a methyl (--CH3 ) group.
• Sugars of Nucleic acids :
• RNA contains D- ribose &
• DNA contains D- deoxyribose

6. • Nucleic Acids Are Polynucleotides
• Nucleic acids are linear polymers of nucleotides linked 3’ to
5’ by phosphodiester bridges.
They are formed as 5’-nucleoside monophosphates
are successively added to the 3’-OH group of the preceding
nucleotide, a process that gives the polymer a directional
sense.
By definition, the 5’ end lacks a nucleotide at the 5’ position
& the 3’ end lacks a nucleotide at the 3’ position.
Other groups (most often one or more phosphates) may be
present on one or both ends.
• Polymers of ribonucleotides are named ribonucleic acid,
or RNA.
• Deoxyribonucleotide polymers are called
• deoxyribonucleic acid, or DNA.

8. 3’-5’ phosphodiester bridges link nucleotides together to
form polynucleotide chains

11. • Chargaff’s rule of DNA composition :
• He observed (late 1940s) that in all the species,
• DNA had equal numbers of adenine & thymine
residues(A=T) &
• equal numbers of guanine & cytosine(G=C)
residues.
• This is known as Chargaff’s rule of molar
equivalence between the purines & pyrimidines in
DNA structure.

12. DNA DOUBLE HELIX
• Double helical structure of DNA was proposed by
James Watson & Francis Crick in 1953( Nobel Prize,
1962)
• The main features of Watson – Crick model of DNA
(Now known as B- DNA ) are :
• 1. DNA is right handed double helix .It consists of 2
polydeoxynucleotide chains(strands) twisted
around each other on a common axis.
• 2. Two strands are antiparallel
• i.e. 5’---------------3’ & 3’------------------5’

16. • 3.Width or diameter of a double helix is 20 A0 (2nm)
• 4. Each turn (pitch) of the helix is 34 A0 (3.4nm)
with 10 pairs of nucleotides, each pair placed at a
distance of about 3.4A0
• 5. Each strand of DNA has a hydrophilic deoxyribose
phosphate backbone(3’—5’ phosphodiester bonds)
on the outside(periphery) of the molecule while the
hydrophobic bases are stacked inside(core).
• 6. Two polynucleotide chains are not identical but
complementary to each other due to base pairing.

17. • 7. Two strands are held together by H- bonds
formed by complementary base pairs.
• A-T pair has 2 Hydrogen bonds & G-C pair has 3
hydrogen bonds.
• G = C is stronger by about 50% than A = T.
• 8. H Bonds are formed between a purine & a
pyrimidine only. A-T, T-A, G-C & C-G.
• 9. Complementary base pairing in DNA helix proves
Chargaff’s rule.
• 10. Genetic information resides on one of the 2
strands, known as template strand or sense strand.
• Opposite strand is antisense strand.

18. Conformations of DNA Double helix
Double helical structure of DNA exists in at least
6 different forms A- E & Z.
Among these, B, A & Z forms are important.
B- form of DNA double helix described by
Watson & Crick , is the most predominant form
under physiological conditions.

19. Other Types of DNA structure
It is now recognized that besides double helical
structure , DNA also exists in certain unusual
structures.
• Bent DNA
• Triple stranded DNA
• Four stranded DNA
Polynucleotides with very high contents of
guanine can form G- quartets(parallel)
G- tetraplexes(Antiparallel)

23. Structure of RNA
• RNA is a polymer of ribonucleotides held together
by 3’,5’-phosphodiester bridges.
• Although RNA has certain similarities with DNA
structure, they have specific differences.
• 1. Pentoses : Suger in RNA is ribose in contrast to
deoxyribose in DNA.
• 2. Pyrimidine : RNA contains the pyrimidine uracil
in place of thymine (in DNA).
• 3. Single strand : RNA is usually a single stranded
polynucleotide. However this strand may fold at
certain places to give a double stranded structure ,
if complementary base pairs are in close proximity.

24. 4. Chargaff’s rule ---- not obeyed : Due to the single
stranded nature , there is no specific relation between
purine & pyrimidine contents.
Thus Guanine content is not equal to cytosine(as in
the case in DNA)
5. Susceptibility to alkali hydrolysis : Alkali can
hydrolyse RNA to 2’,3’-cyclic diesters. This is possible
due to the presence of a hydroxyl group at 2’ position.
DNA can not be subjected to alkali hydrolysis due to
lack of this group.
6. Orcinol color reaction : RNAs can be histologically
identified by orcinol color reaction due to the
presence of ribose.

25. • Types of RNA :
• 3 major types of RNAs with their respective cellular
composition ;--
• 1. Messenger RNA( m RNA) : 5 – 10 %
• 2. Transfer RNA ( t RNA ) : 10 – 20 %
• 3. Ribosomal RNA (r RNA) : 50 – 80 %
• Besides the above 3 , other RNAs are also present in
the cells.
• RNAs are synthesized from DNA, and are primarily
involved in the process of protein biosynthesis.

26. Cellular RNAs & their functions
• Messenger RNA ( mRNA) :
• Transfers genetic information from genes to
ribosomes to synthesize proteins.
• Heterogeneous nuclear RNA ( hnRNA) :
• Serves as precursor for mRNA & other RNAs.
• Transfer RNA( tRNA) :
• Transfers amino aid to mRNA for protein
biosynthesis.
• Ribosomal RNA(rRNA) :
• Provides structural framework for ribosomes.

27. • Small nuclear RNA( snRNA) :
• Involved in mRNA processing.
• Small nucleolar RNA (snoRNA) :
• Playes a key role in the processing of rRNA
molecules.
• Small cytoplasmic RNA(scRNA) :
• Involved in the selection of proteins for export.
• Transfer- messenger RNA(tmRNA) :
• Mostly present in bacteria. Adds short peptide tags
to proteins to facilitate the degradation of
incorrectly synthesized proteins.

28. Messenger RNA(mRNA)

30. • m RNA is synthesized in the nucleus (in eukaryotes)
as heterogeneous nuclear RNA( hnRNA).
• hn RNA, on processing ,liberates the functional
m RNA which enters the cytoplasm to participate in
protein synthesis .
• m RNA has high mol wt. with a short half –life.
• Eukaryotic mRNA is capped at the 5’ terminal end
by 7-methylguanosine triphosphate.
• This cap helps to prevent the hydrolysis of mRNA by
5’-exonucleases.
• Cap may also be involved in the recognition of
mRNA for protein biosynthesis.

31. • 3’-terminal end of m-RNA contains a polymer of
adenylate residues( 20 – 250 nucleotides) which is
known as poly(A) tail.
• This tail may provide stability to mRNA , besides
preventing it from the attack of 3’-exonucleases.
• mRNA molecules often contain certain modified
bases such as 6-methyladenylates in the internal
structure.

33. Above picture, The cap structure attached to
the 5’ terminal of most eukaryotic messenger
RNA molecules.
A 7-methylguanosine triphosphate (black) is
attached at the 5’ terminal of the mRNA (shown
in blue), which usually contains a
2’-O-methylpurine nucleotide.
These modifications (the cap and methyl group)
are added after the mRNA is transcribed from
DNA.

34. Transfer RNA (tRNA)
• tRNA molecules vary in length from 74 to 95
nucleotides.
• They also are generated by nuclear processing
of a precursor molecule .
• The tRNA molecules serve as adapters for the
translation of the information in the sequence of
nucleotides of the mRNA into specific amino
acids.

35. • There are at least 20 species of tRNA
molecules in every cell, at least one (and often
several) corresponding to each of the 20 amino
acids required for protein synthesis.
• The structure of tRNA, resembles that of a
clover leaf.
• tRNA contains 4 arms,each with a base paired
stem.

36. Structure of tRNA

39. • 1. The acceptor arm : This arm is capped with a
sequence CCA(5’ to 3’). The amino acid is attached
to the acceptor arm.
• 2. The anticodon arm : This arm with the 3 specific
nucleotide bases (anticodon), is responsible for the
recognition of triplet codon of mRNA.
• Codon and anticodon are complementary to each
other.
• 3. The D arm: It is so named due to the presence of
dihydrouridine.
• 4. The TψC arm : This arm contains a sequence of T,
pseudouridine(represented by ψ) and C.

40. • 5. The variable arm : This arm is the most variable
in tRNA.
• Based on this variability,
tRNAs are classified into 2 categories:
a) Class I tRNAs :
The most predominant(about 75%) form with 3-5
base pairs length.
b) Class II tRNAs :
They contain 13- 20 base pair long arm.

41. • Base pairs in tRNA :
• Structure of tRNA is maintained due to the
complementary base pairing in the arms.
• The 4 arms with their repective base pairs are
given below :
• The acceptor arm - 7 bp
• The TψC arm - 5 bp
• The anticodon arm - 5 bp
• The D arm - 4 bp

42. Ribosomal RNA ( rRNA)
• Ribosomes are the factories of protein synthesis.
• Eukaryotic ribosomes are composed of 2 major
nucleoprotein complexes----
60S subunit & 40s subunit.
• 60S subunit contains 28S rRNA, 5S rRNA &
5.8S rRNA.
• 40S subunit contains 18S rRNA.
• Most probably rRNAs play a significant role in the
binding of mRNA to ribosomes & protein synthesis.