Dna

INTRODUCTION TO

DNA

By,R.Harish kumar

Introduction

The genetic information of a cell is stored within its DNA (Deoxyribonucleic acid) macromolecules.

The segment of DNA carry this genetic information called “Genes”.

History of DNA

Frederick Griffith (1928) demonstrated the transformation from heat killed S-type to live R-type streptococcus.

In 1869Johann Friedrich miescher had isolated from the pus cells obtained from the discarded bandages.

Rosalind Franklin (1952) X ray diffraction image of DNA.

Watson and Crick (1953) concluded DNA is the helical form.

DNA is a polymer

DNA is a long polymeric molecule . A polymer is a long chain like molecule comprising

numerous individual units called “monomers”

Nucleotides- the monomers in DNA

DNA which contains four kinds of smaller building blocks (monomers)called deoxyribotides or deoxyribonucleotides .

The nucleotide itself a complex molecule being made up of three components.

Nucleotide

Phosphoric acid

sugar

Nitrogenous base

Composition of nucleotide

The sugar componentThe sugar component of the nucleotide is a pentose

(containing five carbon atom) called 2’-deoxyribose.Pentose sugar can exist in two forms ,

1.straight chain (or) Fischer structure 2.ring (or) Haworth structure The 2’-deoxyribose that occurs in nucleotide.

NITROGENOUS BASE

PURINES1. Adenine (A)

2. Guanine (G)

PYRIMIDINES3. Thymine (T)

4. Cytosine (C)

T or C

A or G

Base pairing

A pairs with T by two hydrogen bonds

C pairs with G by three hydrogen bonds

A-DNA

It is a right-handed double helix fairly similar to the more common and well-known B-DNA form, but with a shorter more compact helical structure.

It contains 11 base pairs per turn. Its is right handed but less than the B-form DNA.

It appears likely that it occurs only in dehydrated samples of DNA, such as those used in crystallographic experiments, and possibly is also assumed by DNA-RNA hybrid helices and by regions of double-stranded RNA.

B-DNA

Each turn of the B-form has 10 base pair spanning a distance of 3.4nm.

B-DNA is the form commonly observed in chromosomes. B-DNA is a right-handed helix with 10 base pairs per turn.

This structure was described by Watson and crick is the most predominant under physiological conditions.

Z-DNA

Z-DNA is a left handed helix generated because of different conformation between one of its bases and the deoxyribose sugar.

This bond is called a glycosidic bond.

The syn conformation is found in Z-DNA .

The alteration of a purine (G) and a pyrimidine (C) along a Z-DNA double strand produces the anti-syn alteration that gives the backbone of Z-DNA a zigzag appearance.

Feature B-DNA A-DNA Z-DNA

Helix type Right handed Right handed Left handed

Helical diameter (nm)

2.37 2.55 1.84

Distance per each turn (nm) 3.4 3.2 4.5

Number of base pair per turn 10 11 12

Helix axis rotation Major groove Through base pairs (variable)

Minor groove

ORGANISATION IN EUKARYOTES

Biological function Gene expression

DNA

mRNA

protein

transcription

translation

Over view of gene expression

Replication

During the cell division, the daughter cells receive an identical copy of genetic information from the parent cell.

Replication is a process in which DNA copies itself to produce identical daughter molecules of DNA .

Replication