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
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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