Replication of DNA: Know Process and Diagram

What Is DNA Replication?

DNA replication is a biological process by which a cell makes a duplicate copy of its DNA, essentially making two copies out of a single DNA molecule. This step is accomplished before cell division to ensure that the new cell produced at the end of the process has an exact, complete set of genetic material. Hence, DNA replication is the basis for life processes such as growth, development, and reproduction.

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DNA Structure

The structure of the DNA molecule was elucidated by James Watson and Francis Crick in 1953. It was described as being a double helix very similar in structure to a twisted ladder. The sides of this ladder were represented to be composed of a sugar-phosphate backbone, with the rungs represented by the nitrogenous base pairs: adenine with thymine (A-T) and cytosine with guanine (C-G).

The DNA strands are anti-parallel; they run in opposite directions. One runs 5' to 3', and another runs 3' to 5'. This orientation is important for replication, for the DNA polymerases can add nucleotides to the 3' end of the growing DNA strand only.

Enzymes Involved In DNA Replication

DNA replication is facilitated and accurately done simultaneously due to the various roles played by different enzymes. These enzymes include:

• Helicase: These open the double helix of DNA into two single-stranded templates.

• Primase: These enzymes synthesise short RNA primers required for DNA polymerases to initiate nucleotide incorporation.

• DNA Polymerase: Adds nucleotides to the growing DNA strand in a 5' to 3' direction. In prokaryotes, it is mainly DNA Polymerase III that synthesizes the new DNA while replacing RNA primers with DNA is done by DNA Polymerase

• Ligase: Seals nicks and joins Okazaki fragments on the lagging strand to provide a continuous DNA strand.

• Topoisomerase: Averts the DNA from supercoiling and tangling ahead of the replication fork.

• Single-Strand Binding Proteins (SSBs): Bind unwound DNA strands to prevent them from reannealing.

Steps Of DNA Replication

The entire process of DNA replication consists of three main stages: initiation, elongation, and termination.

Initiation

Replication begins at specific regions of the DNA molecule known as the origins of replication. In these regions, the helix is unwound initially by helicase to generate a replication fork. The binding of single-strand binding proteins would stabilize the unwound DNA while RNA primers are made by a primase which acts as a starting point for actual DNA polymerases.

Elongation

During the process of elongation, DNA polymerase III adds nucleotides to synthesise the new DNA strands. The leading strand is continuously synthesized toward the replication fork while the lagging strand is synthesised in short segments called Okazaki fragments. As described earlier, these RNA primers are later replaced with DNA by DNA polymerase I and the Okazaki fragments are joined into a continuous strand by DNA ligase.

Termination

Replication is completed when the entire DNA molecule has been duplicated. In eukaryotes, the ends of linear chromosomes are capped by telomeres, which also prevent the loss of genetic material during replication. Telomerase, an enzyme, lengthens the end telomeres and enables the complete replication of chromosome ends.

Diagram: DNA Replication

The diagram below shows the process of replication in DNA.

Semi-Conservative Nature Of DNA Replication

In the semi-conservative model for DNA replication put forward by Watson and Crick, each of the new DNA molecules contains one old strand and one newly synthesized strand. Experiments on isotopic cross-over by Meselson and Stahl supported this model by showing, after a single round of replication, that the DNA consisted of one old and one new strand associated with it, fitting the mechanism of replication as being semi-conservative.

Replication In Prokaryotes Vs Eukaryotes

The replication process of prokaryotes and eukaryotes are mentioned below

Prokaryotic Replication:

• Single origin of replication

• Faster replication due to a simpler structure

• Circular DNA

• Fewer proteins involved

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Replication In Eukaryotes:

• Multiple origins of replication

• Slower replication due to complex chromatin structure

• Linear DNA with telomeres

• More controlling elements and proteins involved in the process

Conclusion

DNA replication is the basis of life perpetuation and continuity of genetic material when cells divide. Further development in knowledge about DNA replication will have important ramifications in genetics, medicine, and biotechnology. Research in progress continually unravels the fine details and regulatory mechanisms of this most fundamental biological process.