DNA REPLICATION (LEADING & LAGGING)

=DNA REPLICATION= DNA replication is carried out by a series of enzymes. '''''DNA Helicase''''' is an enzyme that breaks the hydrogen bonds holding the two strands together. It essentially acts like the slider on zipper. In the diagram below DNA helicase is unzipping the DNA strands as it moves from right to left. The point where the two strands are split is called the '''''replication fork'''''. The exposed strands can then be copied by an enzyme called '''''DNA polymerase''''' which matches the exposed bases with nucleotides carrying a complementary base. However, because the two strands are anti-parallel (they run in opposite directions) the two new strands are synthesised in opposite directions. [image:http://i.imgur.com/4Fjcqm0.png?1] ==Which end is which?== [image:http://i.imgur.com/QJSnDpv.png?1] Because the two strands are anti-parallel biologists found it necessary to some how label the two strands accordingly. Eventually it became common practise to refer to one end of each strand as the '''''3' (three prime)''''' end and the other end of each strand as the '''''5' (five prime)''''' end. This is actually based on the sugar molecules within each strand. The carbon atoms in the sugar molecule of each strand can be numbered starting with the carbon that is bound to the base and going clockwise from there. You will find that at one end of each strand the 5th carbon is bound to the phosphate at the end of the strand. At the other end the 3rd carbon has an OH group at the end of the strand. DNA nucleotides can only be added to this end and the phosphate of a free nucleotide will form a bond with this OH group. ==The Leading & Lagging Strand== DNA polymerase can only synthesise DNA in a '''''5' → 3' ''''' direction (because nucleotides can only be added to the 3' OH). One strand can be synthesised continuously as DNA polymerase can simply follow-on behind DNA helicase as it unzips the two strands. This is known as the '''''leading strand'''''. The other stand, known as the '''''lagging strand''''' must be synthesised in short bursts. On this strand DNA polymerase moves away from the replication fork. As soon as DNA polymerase has finished copying one section another section has already been unzipped by DNA helicase and is waiting to be copied. Thus the lagging stand is synthesised in sections called '''''Okazaki fragments''''' (after the scientist who discovered them). Another enzyme joins these fragments together ensuring the lagging strand becomes one continuous strand. [image:http://i.imgur.com/Ay78nUJ.png?2]