Wednesday, August 17, 2011

WATSON AND CRICK'S MODEL OF DNA

In 1953  ,James Watson and FrancisCrick proposed that the DNA molecule consists two poly nucleotide strands , each coiled  in a righthanded helix .The two strands of the double helix  are held together  by hydrogewn bonding between the nitrogenous bases  of adjacent nucleotides.For each complete twist of the double helix  there are 10 pairs of nucleotides , each twist measures 3.4 nm.      

The two polynucleotide  strands of a DNA molecule are not identical .They are complementary . Where adenine strands , thymine  is found in other .Where cytosine occurs in one strands , the guanine is present in the complementary strand .Altogether  there can be several thosands of nucleotides in aDNA molecule .The exact number depends upon the  origin of the DNA .The sequence in which the pairs of bases occurs  also differs in DNA from different species .In theory  the numbers of permutation s of the bases  is indefinite .    REPLICATION OF DNA ; The DNA produces its exact copy .The formation of its replica by DNA  molecule is called replication .They include despersive conservative and  semi conservative methods .Synthesis of DNA  requires  an enzyme  called  DNA  ligase. In despersive method , the old DNA mlecule should desintegrate and two new DNA  molecules  would be synthe sized again from the despersed constituents .Inconservative  method  of replication , out of two DNA  molecules formed ,one molecule would retain or conseve two old strands  and other will carry two newly synthesized  strands ,in this , no part  of the parent molecule would present  be half conseved  in each DNA molecule .Ther is sufficient  evidence  to prove  that the  replication  of the DNA takes  place by the semi conservative methd .The semi  conservative replication  takes place as follows; The hydrogen bonds between the base  pairs break at one end .Consequently,two strands unwind .Each strands  act as mould or template on which a new polynucleotide  strand is made.This gives a rise to two molecules of DNA ,identical to each other and which are replica of the original molecule ,carrying an old an new strand , Experimental evidence that DNA replication is semi conservative was  obtain by messelon and stahl [1957]  in escherichia coli . DNA as the herediatory  substance  Chromosomes the carriers of herediatory  characters , are mainly composed with nuclearproteins along with some other chemicals .The nucleoproteins  have two components,the nucleic acids and prteins and therefore .one of those should constitute  or act as herediatary  genetic material .Findings of certain experment  have  established  that the nucliec acid ,.not the proteins contains genetic information .                                       Transformation experiment ,first conducted by griffith and then repeated by others , have proved that the DNA is a genetic material. Griffith [1928] noted two distinct strains of bacterium pnemococcus , the s-strain and r-strain . The s-strain bacterium is enclosed in a capsule  ,and when when inoculate on to an agar-based  medium ,it  grows into a smooth shining colony .The r -strain bactrium lacks capsule  and its colony is rough and dull in  appearence . The s-strain bacterium is virulent ,whereas  the bacterium of r-strain is non virulent                       
 

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