Spectrum biology

It can be understored by following diagrammatic representation.

Comparative Structure of Various Types of DNA

Char ac ters A B C D Z

Handedness Right Right Right Right Left

Base pairs / Turn 11.0 10.0 9.3 8.0 12.0

Helix diameter (Å) 23 19 19 16.7 18

Helix rise per bp 2.92 3.36 3.32 3.03 3.52-4.13

Occurrence in biological world Rare Common Less common No In some cells

DNA Replication

l DNA replicates semiconservatively, the process is guided by the complementary H-bonding. This biological process occurs

in all living organisms and is the basis for biological inheritance.
l Semiconservative replication of chromosome was experimentally proved by Taylor (1957) in Vicia faba using triradiated

thymidine. It is also experimentally proved by Meselson and Stahl (1958) by conducting following experiment:
(i)Culturing E. coli for many generation in N^15 containing culture medium.
(ii)N^15 was incorporated as nitrogenous base, in newly synthesised DNA.
(iii)Heavy DNA separated from normal by centrifugation in CsCl (Cesium chloride) density gradient.
(iv)E. coli cells having N^15 was transferred to N^14 medium.
(v)Extraction of DNA from E. coli cells in earlier step and density of ds DNA in CsCl centrifugation was measured.
l After one generation, resultant DNA was hybrid with one strand having N^15 isotope and other having N^14.
l Three modes of DNA replication were assumed.
l In DNA it was found that rep li ca tion is of semicon ser va tive type, al though it can be thought of to op er ate in con ser va tive or

dispersive mode too.

A chromatosome

consists of a nucleosome

plus the H1 histone.

At the simplest level, chromatin

is a double-stranded helical

structure. DNA double helix

DNA is complexed

with histones to

form nucleosomes.

Each nucleosome consists

of eight histone proteins around

which the DNA wraps 1.65 times.

Histone H1

11 nm

The nucleosomes

fold up to produce

a 30 nm fibre.

30 nm Tight coiling of the 250 nmfibre produces the chromatid

of a chromosome.

Chromatosome

Nucleosome core

of eight histone

These 30 nm fibres molecules

form loops averaging

300 nm in length.

300 nm

250 nm wide fibre

700 nm

The 300 nm fibres are

compressed and folded to

produce a 250 nm wide fibre.

1400 nm

2 nm

Packaging of DNA at different levels