DNA replication in prokaryotes
- Initiation.
- Elongation (formation of new strands).
- Termination.
Initiation
The replication begins a sequence of nucleotides in the DNA called origin of replication, oriC or initiation point, which acts as an initiation signal. This sequence is different depending on the species, but it has abundant thymine (T) and adenine (A). The T and A are linked by two hydrogen bonds, instead of three, like the C and G, so these bonds will be weaker and easier to break.
The following proteins are involved in the initiation of replication:
- Helicasas. They are enzymes that recognize the nucleotide sequence origin of replication and break hydrogen bonds between complementary nitrogenous bases. They are responsible for opening the double helix so that the chains can serve as a mold for the new chains.
- Topoisomerases. The unwinding of the double helix creates stresses between the two strands, and the topoisomerases are responsible for cutting the strands to release the supercoiling stresses. Cutting a (the topoisomerases) or both chains (the topoisomerase II or gyrase) of DNA, and when there are no such tensions, ligases the spliced again.
- SSB proteins (Single Strand Binding-DNA). They are the stabilizing proteins that bind to each strand of DNA separated by the helicase so that they do not rejoin. Thus, they allow the correct passage of DNA polymerase , preventing complementary strands from joining before the nucleotides of the new strand that is being formed are added.
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A helicase works in each direction, so this process is bidirectional. The two replication forks that have been created form the replication bubbles or eyes.
As DNA polymerase needs to have a primer to which nucleotides can be added, an RNA polymerase must first intervene that synthesizes a small fragment of about ten nucleotides of RNA that serves as a primer. To this RNA polymerase it is called primase, and a fragment of RNA that serves as a primer, first.