Their description of DNA as a twisted, ladder-like structure with rungs of complementary pairs of simple chemical bases offered a basic architecture for the genetic code. Although it had been known for hundreds of years that plants and animals had some sort of genetic material that carried information from generation to generation, what it was and how it worked remained unknown until 1953, when the American James Watson and the Englishman Francis Crick made their announcement from Cambridge, England. Following replication the new DNA automatically winds up into a double helix.This year marks the forty-first anniversary of one of the most important scientific discoveries ever made: the finding that the universal genetic material is made of deoxyribonucleic acid, commonly called DNA.This is why DNA replication is described as semi-conservative, half of the chain is part of the original DNA molecule, half is brand new. The result of DNA replication is two DNA molecules consisting of one new and one old chain of nucleotides.Finally, an enzyme called DNA ligase seals up the sequence of DNA into two continuous double strands.The new strand is proofread to make sure there are no mistakes in the new DNA sequence.The gaps where the primer(s) were are then filled by yet more complementary nucleotides. Once all of the bases are matched up (A with T, C with G), an enzyme called exonuclease strips away the primer(s).This type of replication is called discontinuous as the Okazaki fragments will need to be joined up later.Chunks of DNA, called Okazaki fragments, are then added to the lagging strand also in the 5’ to 3’ direction.Numerous RNA primers are made by the primase enzyme and bind at various points along the lagging strand.This sort of replication is called continuous.DNA polymerase binds to the leading strand and then ‘walks’ along it, adding new complementary nucleotide bases (A, C, G and T) to the strand of DNA in the 5’ to 3’ direction.The primer acts as the starting point for DNA synthesis. A short piece of RNA called a primer (produced by an enzyme called primase) comes along and binds to the end of the leading strand.Image credit: Genome Research Limited Leading Strand: As a result of their different orientations, the two strands are replicated differently:Īn illustration to show replication of the leading and lagging strands of DNA. The other strand is oriented in the 5’ to 3’ direction (away from the replication fork), this is the lagging strand. One of the strands is oriented in the 3’ to 5’ direction (towards the replication fork), this is the leading strand.The two separated strands will act as templates for making the new strands of DNA. The separation of the two single strands of DNA creates a ‘Y’ shape called a replication ‘fork’.This is carried out by an enzyme called helicase which breaks the hydrogen bonds holding the complementary bases of DNA together (A with T, C with G).The first step in DNA replication is to ‘unzip’ the double helix structure of the DNA molecule.
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