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Replication
Each time a cell divides into two daughter cells, all the DNA molecule must be duplicated. Duplication of an old DNA molecule into two new DNA molecules is called Replication.
During replication, the DNA helix is unraveled and its two strands are separated. An area known as the replication bubble forms and progresses along the molecule in both direction. Then each DNA strand serves as a template for the synthesis of a new complementary strand.
Each daughter DNA molecule is an exact copy of its parent molecule, consisting of one old and one new DNA strand. Thus the replication is semi-conservative
Each time a cell divides into two daughter cells, all the DNA molecule must be duplicated. Duplication of an old DNA molecule into two new DNA molecules is called Replication.
During replication, the DNA helix is unraveled and its two strands are separated. An area known as the replication bubble forms and progresses along the molecule in both direction. Then each DNA strand serves as a template for the synthesis of a new complementary strand.
Each daughter DNA molecule is an exact copy of its parent molecule, consisting of one old and one new DNA strand. Thus the replication is semi-conservative
In humans, the replication occurs at a rate of about 50 nucleotides per second, in prokaryotes this rate may reach 500 nucleotides/second. However eukaryotes have multiple replication bubbles on the same parent DNA, while prokaryotes open a single bubble per DNA molecule.
Since the DNA strands are antiparallel, and replication proceed only in the 5' to 3' direction, one strand named the leading strand forms a continuous copy, while the other named lagging strand forms discontinuous short DNA strings called Okazaki fragments.
Due to the high level of efficiency and fidelity required to ensure the integrity of the genetic flow, the replication is a very complex process wich involves multiple enzymatic activities.
The major activity is taken on by a group of enzymes termed DNA polymerases. The DNA polymerases recruit free nucleotides and match them by base pairing with the complementary nucleotides of the parent strand.
In prokaryotes there are two main DNA polymerases : pol I and pol III.
Though, the polymerase activity of eukariotic cells is a little more complex than those of prokaryotes, there is an equivalent of prokaryote pol III named pol-a and one of pol I named pol-b.
In prokaryotes there are two main DNA polymerases : pol I and pol III.
Though, the polymerase activity of eukariotic cells is a little more complex than those of prokaryotes, there is an equivalent of prokaryote pol III named pol-a and one of pol I named pol-b.
The DNA pol III (or pola) exhibits outstanding proofreading capabilities, wich prevent it to initiate a polynucleotide strand synthesis. Therefore it requires a primer, a short piece of RNA (RNA primer) that it can recognise and elongate.
In order for DNA polymerases to access free unraveled DNA parent strands and start replication, some accessory proteins are involved in the many steps associated with the synthesis and the unwinding -rewinding of the DNA helix. The main accessory proteins are the topoisomerase, the helicase, the SSB protein and the primase.
- The topoisomerases introduce negative supercoils and relieve strains in the double helix at either end of the bubble.
- The helicase unwinds and unzips the DNA helix by breaking the Hydrogen bonds between the base pairs, thus allowing the two strands to separate.
- The SSB proteins (Single Strands Binding) stabilize the single strands thus preventing them to zip back together and to form hairpin loops.
- The primase (RNA polymerase) added to other proteins (forming a Primeosome) makes short pieces of RNA (RNA primers) that are recognised by DNA polymerase III to initiate replication.
- Following the DNA pol III, the DNA pol I removes the RNA primers, and fills in with DNA. Finally, the DNA ligases repair the gaps between DNA Okazaki fragments.
The combination of DNA polymerases with some of the accessory proteins yields an activity identified as DNA polymerase holoenzyme.
The entire activity located in the replication fork is called a Replisome.
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