DNA replication is a process in which DNA copies itself to produce identical daughter molecules of DNA. DNA replication is a complex process which involve a series of steps and it is carried out with high fidelity that is essential for the survival of the species. Synthesis of a new DNA molecule is a complex process involving a series of steps.
DNA replication in Prokaryotes
DNA replication is somewhat simpler than eukaryotes. Prokaryotic DNA molecule is also smaller in size as compared the eukaryotes. Let us see in details about DNA replication in prokaryotes-
Replication is semiconservative
In this type of DNA replication, one of parental DNA strand replicate and goes to the daughter molecules. Each of one newly synthesized DNA contained one half of the parental DNA and one half of newly DNA. This type of replication is known as semiconservative since half of the original DNA is conserved in the daughter DNA. In 1958, Meselson and Stahl first forwarded experimental evidences for this type of DNA replication.
Initiation of replication
The initiation of DNA replication start at a site called origin of replication. In prokaryotes, there is a single site where DNA replication occurs, whereas there are multiple sites of original of replication in eukaryotes. This origin of replication sites mostly consist of a short sequence of A=T base pairs. Here, a specific protein called as dna A with 20-50 monomers binds with the sites of origin of replication. This structures facilitates the double stranded DNA molecule to separate from each other.
Replication bubbles
The two complementary strands of DNA molecule separate at the site of replication to form a bubble (swelling form). In case of prokaryotes there is only one bubble forms as they have single origin of replication but in eukaryotes, bubbles may be multiples as they have multiple sites of origin of replication.
RNA primer
A short form of RNA contains about 5-50 nucleotides, which is called RNA primer required for the synthesis of new DNA molecule. A specific RNA polymease enzyme known as primase in association with single stranded binding proteins forms a complex called primosome, and produces RNA primer. A constant synthesis and supply of RNA primers is required for the synthesis of lagging or discontinuous strand of DNA. In leading or continuous strand a single primer is enough to carry out the same.
DNA replication in Eukaryotes
Although DNA replication in eukaryotes is closely resembles with prokaryotes, but there is some differences that makes the replication process in eukaryotes must complex. In prokaryotes, we have seen that DNA polymerase I, II and III plays important roles in synthesis of of both the strands, but in eukaryotes, there are five enzymes involved in synthesis of new DNA molecules in separately in both leading and lagging strands. Let us briefly discuss about various enzymes involved in eukaryotic DNA replication process-
DNA polymerase α: This enzyme is responsible for the synthesis of RNA primer for both leading and lagging strands of DNA
DNA polymerase β: This enzyme is involved in the repair of DNA. Its function is comparable with DNA polymerase I found in prokaryotes.
DNA polymerase γ: This enzyme involved in the synthesis of mitochindrial DNA.
DNA polymerase δ: This enzyme is responsible for the replication on the leading strand of DNA. It also possesses proof-reading activity.
DNA Polymerase ε: This enzyme is involved in DNA synthesis on the lagging strand and possesses proof-reading ability.
Process of replication in eukaryotes
Replication on the leading (continuous strand)
References
Satyanarayana, U and U. Chakrapani (2006). In: Biochemistry, 3rd revised edition, Publisher- Arunabh Sen, Books & Allied (P) Ltd.