DNA Replication in Prokaryotes and Eukaryotes
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 their differences are.
DNA Replication in Prokaryotes
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 semiconservative 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.
DNA Replication in Eukaryotes
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.
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.
Difference Between Prokaryotic and Eukaryotic Replication: Tabular Form
FAQs on Difference Between Prokaryotic and Eukaryotic DNA Replication
1. What is the main difference in how prokaryotic and eukaryotic cells replicate their DNA?
The main difference lies in the complexity and location. Eukaryotic replication happens inside the nucleus, involves multiple starting points (origins of replication), and uses several types of DNA polymerase enzymes. In contrast, prokaryotic replication occurs in the cytoplasm, starts from a single origin of replication, and is a much faster process.
2. Why do eukaryotes need multiple origins of replication, while prokaryotes only have one?
Eukaryotic genomes are much larger and more complex than prokaryotic ones. Having multiple origins of replication allows the cell to copy its vast amount of DNA in a reasonable amount of time. If it started from only one point, the process would take too long. Prokaryotes have a smaller, circular chromosome, so a single origin is efficient enough to replicate their entire genome quickly.
3. What are Okazaki fragments and why do they form during DNA replication?
Okazaki fragments are short, newly synthesised DNA pieces that are formed on the lagging strand during replication. DNA polymerase can only build a new strand in the 5' to 3' direction. Since the two DNA strands are anti-parallel, one strand (the lagging strand) must be synthesised discontinuously in small chunks. These chunks are the Okazaki fragments, which are later joined together by the enzyme DNA ligase.
4. How does the location of DNA replication differ in prokaryotes and eukaryotes, and why is this important?
In prokaryotic cells, which lack a distinct nucleus, DNA replication takes place directly in the cytoplasm. In eukaryotic cells, the genetic material is protected within a nucleus. Therefore, DNA replication must occur inside the nucleus before the cell can divide, keeping the process separate from other cellular activities in the cytoplasm.
5. What is the role of telomerase in eukaryotic replication, and why don't prokaryotes need it?
Eukaryotes have linear (straight) chromosomes. With each replication cycle, a small piece of DNA at the very end can be lost. The enzyme telomerase adds protective caps (telomeres) to the ends of the chromosomes, preventing the loss of important genetic information. Prokaryotes have circular chromosomes, which have no ends, so they do not face this problem and do not need telomerase.
6. Are there any similarities in how DNA replication happens in both prokaryotic and eukaryotic cells?
Yes, several fundamental steps are the same. In both cell types, DNA replication is:
- Semiconservative, meaning each new DNA molecule has one old and one new strand.
- Carried out in the 5' to 3' direction.
- Initiated by an RNA primer to start the synthesis.
- Reliant on key enzymes like helicase to unwind the DNA and DNA polymerase to build the new strands.
