BIOLOGY-DNA IN DETAIL, FUNCTION ,FORMS OF DNA

DNA INFORMATION IN DETAIL

Watson and crick in 1950 determined the structure of DNA using X Ray crystallography in 1953 describe the structure of DNA
The important feature of watson crick model or double helix model of DNA are as follows
1· The DNA molecule consists of two polynucleotide chains or stands spirally twisted around each other and cold around a common Axis to form a right handed double helix.
2· The two strands are antiparallel they run in opposite directions so that the third end of one chain facing v end of the other.
3·The sugar phosphate backbone remain on the outside while the core of helix contain the purine and pyrimidine bases.
4·The two strands are held Together by hydrogen bond between the purine and pyrimidine base of the opposite stands.
5·The diameter of DNA is 2.0nm or 20 Å.
6·Adjacent bases are seperated 0.34nm or by 3.4Å along axis.
7·The length of complete turn of helix is 34Å .
8· There are 10 BP per turns
9· Adenine(A) always pair with thymine(T) by  to hydrogen bond & guanine(G) always pair with cytosins(c) by three hydrogen bond. This complimentary is know as the base pairing rule.
10· They are two types of groove in DNA- major groove and minor groove.

Function of DNA

1:-DNA is a genetic material which carries all the the hereditary information the genetic information is coded in the sequence of nitrogenous base.
2:-DNA has a unique property of replication production of carbon copies this is essential for transfer of genetic information from one cell to its daughter and from one generation to Next generation
3:-DNA occur inside chromosomes during meiosis crossing over gives rise to the new combination of gene called recombination
4:- change in the sequence of nitrogenous base due to the addition,deletion /wrong. Replication is called Mertation, which the fountain head of variations and evolution.
5:-DNA form RNA true process of replication.
6:- It controls metabolic reaction of the cell through synthesis of enzyme.
7:- Due to the  differential function of specific region of DNA cell are differentiated to perform specific function.
Forms of DNA.

There are various forms of DNA

A-form of DNA 
The a form of DNA is found at 75% relative humidity in a presence of NA+,K+ & Cs+ ions. it contain 11 base pair as compared to the 10 base pair of B DNA which tilt form the  axis of helix by 20 degree.
B form of DNA
 The structure of B form of DNA has been produced bi patsan and Crick it is present in every cell at a very high relative humidity 92% and low concentration of ions it is antiparallel double helix rotating clockwise (right hand) & made up of sugar phosphate backbone combined with base pair of purine pyrimidine.
C form of DNA
 C form of DNA is found at 66% relative humidity in the presence of Lithium ion as compared to A and B DNA in C DNA the numbers of base pair per turn is less.
D form of DNA 
The D-form of DNA is found rarely as extreme variants total number of base pairs per turn of helix is 8 therefore it shows 8 fold symmetry this form is also called Poly (DA-DT) & poly (DA-DC) form.
 Z form of DNA
 In 1979 rich and co-workers at MIT in USA obtained Z DNA by artificially synthesizing d(C-G) 3 molecules in the form of crystals they proposed a left handed double helix model with zigzag sugar phosphate backbone running in antiparallel direction