The 5' carbon of DNA has a phosphate group and the 3' carbon has
hydroxyl (-OH) group.
As DNA polymerase (enzyme synthesize DNA) requires a free 3' OH
group for synthesis, it can synthesize by extending the 3' end of
the preexisting nucleotide chain. So when DNA polymerase moves
along the template strand in a 3' to 5' direction then the daughter
strand is formed from 5' to 3' direction.
The drug dideoxycytidine stops the growth of a DNA chain when added
to DNA. As drug gets incorporated into a growing DNA strand, but it
has no 3' OH group which is essential for attachment of the next
base I.e. for DNA synthesis . So, growth of the DNA strand
stops....
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