Why is it necessary to have leading and lagging strands during DNA replication?

1 Answer
Dec 6, 2017
  • . DNA polymerase can add only add new DNA nucleotides to a pre-existing chain of nucleotides.

  • So RNA primers are used for initiation of new DNA chains.

  • RNA primer is short piece of RNA, synthesised at the #5^1#end of the new strand with the help of DNA specific RNA polymerase enzyme called primase and primers provide free #3^1#-OH for DNA polymerase to start polymersing.
  • leading template strand requires primer only ones. But lagging
    template strand requires many primers.

  • Once primer is ready DNA Pol(III) will come & start adding
    deoxyribonucleotide to the #3^1#-OH of primer by complementary basepairing.

  • Two nucleotides link each other by #3^1-5^1# phosphodiester bond.

  • The daughter DNA for #3^1-5^1# template is synthesised continuously, Therefore called leading strand.

  • DNA polymeraseIII has #3^1-5^1# Exonuclease activity#rarr#It can add nucleotides in the #5^1-3^1# direction.

  • Replication is discontinuous on the other template with polarity
    #5^1-3^1# because only a short segment of DNA strand can be built in #5^1-3^1# direction due to exposure of a small stretch of template at one time.

  • As one strand grows continuously while the other strand is formed discontinuously, DNA replication is semidiscontinuous.