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Question

Why does synthesis of DNA proceed only in the 5 to 3 direction?

A
DNA polymerases can only add nucleotides to the 3 end of a polynucleotide strand.
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B
The 3 end of the polynucleotide molecule is more electronegative than the 5 end.
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C
This is the direction in which the two strands of DNA unzip.
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D
The chromosomes are always aligned in the 5 to 3 direction in the nucleus.
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Solution

The correct option is A DNA polymerases can only add nucleotides to the 3 end of a polynucleotide strand.
  • DNA polymerase enzyme helps in the synthesis of DNA molecules from deoxyribonucleotides, the building blocks of DNA. They are essential for DNA replication and usually works in a pair to create identical DNA strands.
  • DNA polymerases require 3' OH group for the initiation of synthesis of DNA strand. Therefore, it can synthesize in only one direction by extending the 3' end of the pre-existing nucleotide chain. Hence, DNA polymerase moves along the template strand in 3' - 5' direction and the daughter strand is formed in a 5' - 3' direction.
So, the correct option is 'DNA polymerases can only add nucleotides to the 3′ end of a polynucleotide strand'.

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