DNA replication is a process of making a copy of DNA before cell division so that all the daughter cells can have an adequate amount of DNA. It uses a semi-conservative method that results in the formation of two double-stranded DNAs each with one parental strand and a new daughter strand.

DNA replication is considered as semiconservative which was proved by Meselson and Stahl. During replication of the DNA, the double-stranded coiled DNA unwinds with the help of enzyme helicase, and both the strands get separated. Both these strands act as a template strand for replication, and the new DNA strands are formed complementary to each of these template strands. After the formation of two new strands, each new strand winds with the old complementary strands, which were used as a template. So, after replication of the DNA, two DNAs are formed, and each of them contains one old or parent strands and one new or daughter strands. Thus, the process is semiconservative as half of the DNA is the old one and a half is the new one.

There are specific nucleotide sequences called origins of replication where replication begins. The origin of replication is recognized by certain proteins that bind to this site. An enzyme called helicase unwinds the DNA by breaking the hydrogen bonds between the nitrogenous base pairs. ATP hydrolysis is required for this process.

As the DNA opens up, Y-shaped structures called replication forks are formed. Two replication forks at the origin of replication are extended bi-directionally as replication proceeds. Single-strand binding proteins coat the strands of DNA near the replication fork to prevent the single-stranded DNA from winding back into a double helix. DNA polymerase is able to add nucleotides only in the 5′ to 3′ direction (a new DNA strand can be extended only in this direction). It also requires a free 3′-OH group to which it can add nucleotides by forming a phosphodiester bond between the 3′-OH end and the 5′ phosphate of the next nucleotide. This means that it cannot add nucleotides if a free 3′-OH group is not available. Another enzyme, RNA primase, synthesizes an RNA primer that is about five to ten nucleotides long and complementary to the DNA, priming DNA synthesis. A primer provides the free 3′-OH end to start replication. The DNA polymerase then extends this RNA primer, adding nucleotides one by one that is complementary to the template strand. This ensures specific coupling of the free nucleotides during the process of replication.

This process starts at a single side in the DNA of a prokaryotic cell while it starts at multiple sites in the DNA of a eukaryotic cell.

The specific enzymes are involved in the DNA replication of both prokaryotic and eukaryotic DNA.

Enzymes involved in prokaryotic DNA replication are:

Enzymes involved in the replication of eukaryotic DNA are:

The enzymes are similar to the enzymes involved in the replication of prokaryotic DNA except-

1. RNase helps in the removal of RNA primer instead of DNA polymerase I.

2. DNA polymerase α, δ, ε help in adding the nucleotides instead of DNA polymerase III.

While the replication of the DNA occurs at the leading strand, the replication completes until the end of the DNA molecule as the “leading strand” is synthesized continuously. The lagging strand is synthesized in pieces because the DNA polymerase can only synthesize in the 5′ to 3′ direction, and so it constantly encounters the previously-synthesized new strand. The pieces are called Okazaki fragments, and each fragment begins with its own RNA primer. So, when the replication reaches at the end of the lagging strand, the RNA primer cannot be replaced at the end. So, the end of the DNA cannot be replicated. This will result in a shorter lagging strand.

The telomerase contains an RNA template. DNA nucleotides complementary to this RNA template is formed at the 3′ end of the DNA where RNA primer cannot be replaced. The telomerase then shifts forwards. This increases the length of the DNA at this strand and the DNA polymerase can now replace RNA primer and add complementary nucleotides to complete the replication. The added sequence at the telomere acts as protective caps for the DNA.

Key Points

• DNA replication is a semiconservative process.
• The new DNA formed after replication contains one “old or parent strand” and one “new” strand.
• The semiconservative nature of DNA replication was proved by Meselson and Stahl.
• Origins of replication are the sites of the DNA where the process of replication of DNA begins.
• Helicase separates the DNA to form a replication fork at the origin of replication where DNA replication begins.
• Replication forks extend bi-directionally as replication continues.
• Multiple origins are present in the eukaryotic DNA to allow faster replication by starting the process at multiple sites together.
• Replication forks extend bi-directionally as replication continues.
• The DNA polymerase can add nucleotides only in 5′ to 3′ direction and where a free 3′-OH group is available. This ensures the selective coupling of free nucleotides.
• The enzymes involved in the replication of prokaryotic DNA are DNA polymerase I to III, helicase, ligase, primase, sliding clamp, topoisomerase, and SSB.
• The enzymes involved in the replication of eukaryotic DNA are DNA polymerase α, δ, ε; helicase, ligase, primase, sliding clamp, topoisomerase, and SSB.
• The replication of the DNA takes place in the form of fragments on the lagging strand which are joined to each other.
• At the end of the lagging strand i.e. the 3′, the RNA primer cannot be replaced as there is no space for attachment.
• The telomerase adds some nucleotides at the 3′ end that increase the length of the DAN and provides space for attachment of RNA primer that can be replaced the nucleotides with the help of enzyme DNA polymerase.

Key Terms

• DNA replication: a biological process occurring in all living organisms that is the basis for biological inheritance.
• Semiconservative: It is a type of replication in which the newly formed DNA molecules after replication contains one old strand and one new strand.
• helicase: an enzyme that unwinds the DNA helix ahead of the replication machinery
• origin of replication: a particular sequence in a genome at which replication is initiated
• nucleotides: These are the structural units of DNA.
• Replication fork:  It is a Y-shape structure formed due to the unwinding of the DNA at the origin of replication.
• Okazaki fragments: These are fragments of newly synthesized DNA formed at the lagging strand.
• leading strand: the template strand of the DNA double helix that is oriented so that the replication fork moves along it in the 3′ to 5′ direction
• lagging strand: the strand of the template DNA double helix that is oriented so that the replication fork moves along it in a 5′ to 3′ manner
• telomere: either of the repetitive nucleotide sequences at each end of a eukaryotic chromosome, which protects the chromosome from degradation
• telomerase: an enzyme in eukaryotic cells that adds a specific sequence of DNA to the telomeres of chromosomes after they divide, giving the stability of the chromosomes over time