Question

Why does synthesis of DNA proceed only in the $5^{\prime }$ to $3^{\prime }$ direction?

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

Answer 1

The correct option is A DNA polymerases can only add nucleotides to the $3^{\prime }$ 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'.