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DNA REPLICATION
DNA replication, also known as semi-conservative replication, is the process by which
DNA is doubled
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In this article, we shall discuss the structure of DNA, the precise steps involved
in replicating DNA (initiation, elongation and termination) and the clinical
consequences that can occur when this process goes wrong
...
These are molecules composed of a deoxyribose
sugar, with a phosphate and a base (or nucleobase) attached to it
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The bond formed is between the third carbon atom on the deoxyribose sugar of
one nucleotide (known as the 3’) and the fifth carbon atom of another sugar on the next
nucleotide (known as the 5’)
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B: 3′ is pronounced ‘three prime’ and 5′ is pronounced ‘five prime’
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These strands are attached to each other throughout their lengths via the bases on each
nucleotide
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In normal DNA strands, cytosine binds to guanine, and adenine binds to
thymine
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By Difference _DNA_RNA- DE [CC BY-SA 3
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These origins are targeted by initiator proteins, which go
on to recruit more proteins that help aid the replication process, forming a replication

complex around the DNA origin
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Within the replication complex is the DNA helicase
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It
does this by hydrolysing the ATP used to form the bonds between the nucleobases, thereby
breaking the bond holding the two strands together
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It synthesises a
small RNA primer, which acts as a ‘kick-starter’ for DNA polymerase
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Elongation
Once DNA Polymerase has attached to the two unzipped strands of DNA (i
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the template strands), it is able to start synthesising new strands of DNA to match the
templates
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One of the template strands is read in a 3’ to 5’ direction, therefore the new strand will be
formed in a 5’ to 3’ direction
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Along the leading strand, DNA primase only needs to synthesise an RNA
primer once, at the beginning, to initiate DNA polymerase
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However, the other template strand (the lagging strand) is antiparallel, and is therefore read
in a 5’ to 3’ direction
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Instead, as the helix unwinds, RNA primers are added to the newly exposed bases on
the lagging strand and DNA synthesis occurs in fragments, but still in the 5′ to 3′ direction
as before
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Termination
The process of expanding the new DNA strands continues until there is either no more DNA
template strand left to replicate (i
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at the end of the chromosome), or two replication forks
meet and subsequently terminate
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Once DNA synthesis has finished, the newly synthesised strands are bound and
stabilised
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By LadyofHats Mariana Ruiz [Public domain], via Wikimedia Commons

Fig 2 – Diagrammatic representation of DNA replication



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