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