HOW IS DNA REPLICATED ?
Replication occurs in four major steps : 1 . Replication fork formation 2 . Primer binding 3 . Elongation 4 . Termination
STEP 1 In order to fit within the nucleus , DNA is packed into tightly coiled structures called chromatin . Prior to DNA replication , the chromatin loosens giving the enzymes and proteins necessary for cell replication access to the DNA strands . Before DNA can be replicated , the double-stranded molecule must be ' unzipped ' into two single strands .
DNA has four bases ( adenine , thymine , guanine , and cystine ) that form complementary pairings between the two strands . In order to unwind DNA , the interactions between the base pairs must be broken . First , an initiator protein unwinds a short stretch of the DNA double helix . Topoisomerase enzymes further unwind DNA by cutting the phosphate backbone of the strands .
Then , a protein known as helicase attaches to and disrupts the hydrogen bonds between the base pairs , thereby pulling apart the two strands . Single stranded binding ( SSB ) proteins bind to the DNA strands after they have been separated and prevent the strands from reattaching . As the helicase moves along the DNA molecule , it continues breaking apart the hydrogen bonds and separating the two strands into a Y shape known as the replication fork .
DNA strands are bi-directional ( one strand runs 5 ’ to 3 ’ while its complementary strand runs 3 ’ to 5 ’). The replication fork is therefore also bi-directional ; the leading strand runs in the 3 ’ to 5 ’ direction while the lagging strand runs 5 ’ to 3 ’. This directionality is important as replication only occurs in the 5 ’ to 3 ’ direction .
STEP 2 As the helicase separates the strands , another enzyme called primase attaches to each strand and assembles a short RNA foundation ( primer ) at which replication can begin .