During replication one strand of DNA replicates into 2 strands of DNA. Replication starts at the “origin of replication”. It is “bidirectional” meaning both strands of DNA are being replicated at the same time but in the opposite direction. One of the original strands is called Leading strand which gets replicated in a continuous fashion while the other strand called Lagging strand is replicated in a discontinuous fashion.
To summarize, initially the parent DNA double stranded helix uncoils, followed by strand separation by breaking of hydrogen bonds between the complementary strands, then synthesis of 2 new strands by complementary base pairing.
DNA Helicases are the enzymes that unwind the DNA helix forming a Y shaped replication fork. Single strand binding proteins bind to each of the two strands to prevent them from rejoining. During this process, the DNA molecule ahead of the replication fork gets positive supercoils. Topoisomerases counteract this by causing DNA breaks and rejoining to form negative supercoils.
New DNA nucleotides connect by hydrogen bonds to the parent strand. These new nucleotides join each other by phosphodiester bonds which are formed by DNA Polymerase. DNA polymerase is able to join the phosphate group at the 5’ carbon of a new nucleotide to the OH group of the 3’ carbon of a nucleotide already in the chain. Hence DNA can only be synthesized in a 5’ to 3’ direction while copying a parent strand in a 3’ to 5’ direction.
The parent strand running in the 3’ to 5’ direction can be copied continuously hence is called the Leading strand. While the parent strand running in 5’ to 3’ direction is copied discontinuously in short fragments called Okazaki fragments. DNA ligase then joins the Okazaki fragments together.
DNA Polymerase can only join new nucleotides to a growing chain, it cannot synthesize DNA from scratch. RNA polymerase does that function. An RNA Polymerase complex called a Primase initially adds several complementary RNA nucleotides opposite the DNA nucleotides of the parent chain to form an RNA Primer. DNA Polymerase III then replaces the primase and is able to add DNA nucleotides to the RNA primer. This is followed by DNA polymerase II which digests the RNA primer and replaces the RNA nucleotides with DNA nucleotides. Finally, the DNA fragments are joined by DNA Ligase.
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