DNA REPLICATION The dividing cell is an essential operation. This is important.

The structure of DNA, the exact steps involved in replicating DNA (initiation, elongation and termination) and the clinical implications that may arise when that goes wrong are briefly discussed in this Article.

The structure of DNA, the exact steps involved in replicating DNA (initiation, elongation and termination) and the clinical implications that may arise when that goes wrong are briefly discussed in this Article.

Structure of DNA

Millions of nucleotides are composite of DNA. These are molecules consisting of deoxyribose sugar, which is bound to a phosphate, a basis (or nucleobase). These nucleotides are joined in strands in order to form a 'sugar phosphate backbone' by phosphodiesters. The bond is formed between the third carbon atom in one nucleotide's desoxyribose sugar (the 3') and the fifth carbon atom of the next sugar (the 5').

In opposite or antiparallel directions there are two strands of one another. The core of each nucleotide is bound to each other along the length of the strand. DNA is associated with four different bases: cytosine, guanine, adensine and thymine. Cytosine binds to Guanine and Adenine binds to Thymine in regular DNA strands. The two beams are a double helix together.

Replication phases of DNA

In the three stages, the replication of DNA can be thought of;


DNA synthesis is initiated at certain points in the DNA chain, which are unique coding regions and are known as 'origins' The sources are attacked by initiator proteins, which recruit more proteins that contribute to the replication process and form a replication complex around the DNA source. There are several sites of origin and these sites are referred to as replication forks when replication of DNA commences.

The replication facility comprises the DNA Helicase enzyme, which releases the double helix and reveals each of the two strands to be used as a replication template. It does this by hydrolysing the ATP, which forms the connections between the nucleo-bases, breaking the connection between these two strands.

Another essential enzyme in the replication of DNA Primase is DNA Primase. It synthesises a small RNA primer, which serves for DNA polymerase as a 'shoot starter.' DNA Polymerase is the enzyme which ultimately creates and expands the new DNA strands.


The DNA Polymerase would begin the synthesisation of a new DNA to fit the templates until the original unzipped two DNA strands (i.e. the template strands) is attached. DNA polymerase must be noted that it is not possible to expand the first substance until 3' is added free nucleotides.

In 3' to 5' direction one of the templates is read, meaning that the new strand is shaped in a 5' to 3' direction. The newly formed beach is known as the leading beach. DNA primase must synthesise an initial RNA primary only once in this strand in order to initiate DNA polymerase. DNA polymerase is therefore willing, by reading the template 3′′ to 5′′, to extend the new DNA strand, synthesising in the 5′′ to 3′′ direction as described above.

The other template strand (the lagging strand) however is antiparallel and can be read in a direction of 5 to 3.' As in the leading strand, continuous DNA synthesis must be 3′′ to 5′′, which is unlikely because we can't add bases at the 5′′ end. As the helix discharges, instead, RNA primers are applied to the lagging strand's newly-exposed bases, and the synthesis of DNA takes place in fragments but still in the 5′′ to 3′′ direction as before. These fragments are known as fragments of Okazaki.


The expansion process of the new DNA strands will go on until the DNA templates (i.e. at the end of the chromosomes) are either not replicated again, or until two replication forks meet and then terminate. Two replication forks are not adjusted and occur uniformly around the chromosome.

It is essential that the newly synthesized strands are bound and stabilized, once DNA synthesis has been completed. Two enzymes are required for this purpose in respect of the lagging strand; RNAase H removes the RNA first at the beginning of each fragment of Okazaki, and DNA Ligasus binds fragments together to form the full strand.


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