DNA replication is a process in which the genetic material of a cell, in this case, the DNA makes an exact copy of itself and the process is controlled by the enzyme DNA polymerase. In mammals, the rate of replication is around 50 nucleotides per second whereas, in bacteria, the rate is around 500 nucleotides per second.
In general, the hydrogen bonds between two complementary bases between two DNA strands break which helps the strand to unwind. Each strand acts as a template for synthesis of a new strand which is complementary to itself.
The entire process continues till all the nucleotides on the template have joined with the free nucleotides and two identical DNA strands are formed. This model of replication is known as the semiconservative model of replication.
In this topic, we will have a brief overview of both eukaryotic and prokaryotic replication and what are their differences.
Centring on the general principle of DNA replication, the prokaryotic DNA replication in prokaryotic cells takes place just before a cell divides in an organism and ensures both daughter cells receive an exact copy of the parent’s genetic material. The process uses the semi-conservative model of replication which results in a double-stranded DNA with one parental and one daughter strand.
The Steps of Prokaryotic DNA Replication are as follows:
The DNA replication process is bi-directional begins at a spot on the DNA molecule called the origin of replication.
At this spot, enzymes unwind the double helix structure of the DNA which makes its components accessible for replication.
The helix is unwound by the helicase enzyme to form a pair of replication forks, and the unwound helix is stabilised by SSB proteins and DNA isomerases.
Primase forms 10 base RNA primers which initiate the synthesis of the leading and the lagging strand.
The leading continues to synthesise in the 5’ to 3’ direction by DNAP III (DNA Polymerase III)
The lagging strand is also synthesised in the 5’ to 3’ direction but it is discontinued through the formation of Okazaki fragments.
DNA polymerase I removes the 10 base RNA primers and replaces the gap with deoxynucleotides.
Then DNA ligase seals the breaks between Okazaki fragments as well as around the primers to form continuous strands.
The entire process of replication takes place in the cell cytoplasm.
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Diagram of DNA replication in Prokaryotes
The eukaryotic DNA replication takes place in the cell nucleus and only occurs in the S phase at many chromosomal origins. Similar to prokaryotic DNA replication, eukaryotic cells also use the semi-conservative process of replication but there are multiple origins of replication.
The Steps of the Eukaryotic DNA Replication are as follow:
The replication process starts in a chromosome at multiple origins, with one origin being at 30-300 kb of DNA depending on the tissue and species.
A replication bubble of two forks forms at each origin. The DNA replicated under the control of a single origin is called a replicon. The synthesis proceeds until all bubbles merge together.
The process starts with the unwinding of DNA with the help of enzymes, which makes its components accessible for replication.
The unwound helix forms a pair of replication forks and is stabilised by DNA topoisomerases and SSB proteins.
The RNA primers required for the process are made by DNA polymerases α which initiates the synthesis of the lagging strand and makes the first primer. It then extends it with a short region of DNA.
The Okazaki fragments and the leading strand are synthesised by DNA polymerase δ.
The leading strand is synthesised continuously whilst the lagging strand is synthesised discontinuously. Both strands are synthesised in the 5′to 3′ direction.
At completion, DNA ligase seals the breaks around the primers and between the Okazaki fragments.
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Although there are some similarities between DNA replication in prokaryotes and eukaryotes, the differences are many. Here we will discuss the differences between prokaryotes’ and eukaryotes’ DNA replication process.
Q1. What are the similarities between Prokaryotic and Eukaryotic DNA Replication?
Ans. Some of the similarities between Prokaryotic and Eukaryotic DNA replication are:
DNA replication is carried out before nuclear division.
Both processes work with double-stranded DNA.
The DNA replication takes place in 5’ to 3’ direction.
In both processes, Single Strand Binding proteins unwind and stabilise the DNA strands.
Both cells follow the semi-conservative model of replication.
Both processes are bi-directional.
The manufacturing of DNA strands is controlled by polymerase enzymes.
The lagging strands are created by small fragments known as Okazaki fragments.
The DNA strands in both cells begin with a small RNA primer.
The ligase enzymes seal the breaks and the primers between the strands.
Q2. What are Okazaki Fragments?
Ans. Okazaki fragments are short DNA nucleotide sequences which are synthesised discontinuously in DNA replication and the later phase of the process are linked together by DNA ligase to form the lagging strand. During replication RNA and DNA, primers are removed so the Okazaki fragments can bind to the DNA molecule. This is a common process and Okazaki fragment maturation takes place around a million times during DNA replication. These fragments are located on each strand which dictates new DNA synthesis away from the direction of replication fork movement.