Question

Both the leading strand and lagging strand in replication synthesize a new strand of DNA but explain how each strand is made. Make sure to include important enzymes like ligase, helicase, topoisomerase, DNA polymerase III/I, and primase.

Answer 1

During DNA replication, the helicase enzyme unwinds the double helix structure of DNA by breaking the hydrogen bonds and the single stranded binding proteins prevents the strand from winding back . Topoisomerase or DNA gyrase relieves the tension due uncoiling of DNA strand. Now there are two strands, one in the 5' to 3' direction and the other in 3' to 5' direction. RNA primase syntheses a short RNA primer to which the enzyme DNA polymerase III attaches and proceeds with adding nucleotides. Since the enzyme can only work in 5' to 3' direction, the strand in which 5' to 3' is the template, the daughter strands are produced as a single leading strand. But the other parental strand is in the direction 3' to 5' - here the DNA polymerase-III enzyme adds nucleotides making gaps in between forming a lagging discontinuous strand. Later the RNA primer is released and DNA polymerase-I fills the gaps. Finally DNA Ligase joins all the gaps and makes the Okasaki fragment into a single strand.