Spectrum biology

Molecular Basis of Inheritance

Genetic Material : DNA Vs RNA

Studies have revealed that RNA is also the genetic material in some viruses. It has a reactive 2 ′–OH group and is unstable due
to the presence of uracil instead of thymine as in DNA.

It thus, mutates at a faster rate and evolves faster. Such rapid changes are harmful for other life forms. DNA, on the other edge,
is less reactive and stable enough to cause major change, hence is considered better for transmission of genetic information.

DNA : Structure

In 1953, Watson and Crick proposed the double helical structure of DNA, on the basis of observations deduced from X-ray
diffraction crystallographic studies of DNA by Rosalind Franklin.

Together, they were awarded with Nobel Prize in Physiology and Medicine for their remarkable discovery in 1962.

In 1950, Erwin Chargaff formulated important postulates regarding DNA structure, i.e. its base composition, which are
summarised as Chargaff’s rules.

A base pair must consist of a purine and pyrimidine because
(i)The space available between two chains of DNA, i.e. 2 nm, can ac com mo date only such pair.
(ii)There is a perfect match between hydrogen donor and acceptor sites on two bases such that A pairs with T and G pairs
with C.

Packaging of DNA

In prokaryotes, even though there is absence of well organised nucleus, DNA is held with some proteins in nucleoid. In
eukaryotes, the major protein involved in packaging of DNA is histone. A set of pos i tively charged pro teins (rich in lysine and
arginine) or gan ises to form histone octamer (eight protein molecules).

The best ac cepted model, pro posed for ex plain ing the pack ag ing of DNA is nucleosome model.

DNA AS GENETIC MATERIAL

5 ′ 3 ′

2 nm

3.4 nm

0.34 nm

Minor

groove

Major

groove 5th C atom of second

sugar molecule

3rd C atom of first

sugar molecule

PO 4

C H 2

OH

3 ′

5 ′

CH 2

Base

C

O

O P = O

O

O

Central axis

The axis at which

whole DNA strands

revolve around.

Hydrogen bonds

The nitrogenous bases are held together by hydrogen

bonds. The bonds ultimately held to strands of DNA.

The base GMMMMMC has 3 H-bonds, while base

A T have two hydrogen bonds.:::::

Phosphodiester bond

It is a linkage between two

sugar and a phosphoric acid

is involved in bonding.

(–C–O–P–O–C–)

Sugar phosphate backbone

Phosphoric acid, i.e. H PO having (^34)
3 reactive —OH groups out of which
2 are involved in forming backbone.
The both strands are antiparallel
to each other.
Deoxyribose sugar
The five carbon sugar,
|
which has — C — H linkage
|
at carbon no. 2.
Nitrogenous base
There are two types of base, i.e. Purine
and Pyrimidine. Purine include
adenine and guanine, while pyrimidine
includes thymine and cytosine.
Phosphoric acid
As a component of
nucleotide, it is also
involved in
phosphodiester
linkage.
Major
axis
O