Fidelity of DNA Replication

Fidelity of replication means  replication of DNA and the production of accurate daughter DNA using the parental DNA as a template.   Cell reproduction depends on the precision of DNA replication. 

The process of DNA replication would have to be entirely faithful to produce a sequence of bases in the DNA of a daughter cell that is identical to that of the parent.

This is clearly not the case, as mutants do appear on their own. 

During replication, errors are caused by mutation. This mistake frequency is far lower than would be expected based just on complementary base pairing.

Replication Fidelity:

Replicative polymerases use a variety of ways to accomplish high fidelity DNA replication:

(1) detecting the correct base pair’s suitable geometry,

(2)slowing down catalysis when a mismatch occurs, and

(3)partitioning the mismatched primer to the active site of the exonuclease.

What is the most Important Contributor of Fidelity during DNA Replication?

The contribution of the DNA polymerase (nucleotide selectivity and proofreading), mismatch repair, a balanced supply of nucleotides, and the state of the DNA template are all variables that influence the fidelity of DNA replication (both in terms of sequence context and the presence of DNA lesions). The contribution and interaction of these parameters to the overall fidelity of DNA replication is discussed below.

Process of DNA Replication:

DNA replication follows a semiconservative pattern. Each of the double helix’s strands serves as a template for the creation of a new, complementary strand.

DNA polymerases make new DNA by synthesising DNA in the 5′ to 3′ direction using a template and a primer .

During DNA replication, one new strand (the leading strand) is generated as a continuous length. On the other hand, the lagging strand is made up of small pieces.

In addition to DNA polymerase, other enzymes such as DNA primase, DNA helicase, DNA ligase and topoisomerase are required for DNA replication.

DNA polymerase is a type of enzyme that helps to make DNA.

The enzyme is a kind of protein. One of the most critical molecules in DNA replication is DNA polymerase. 

Polymerases:

A DNA strand can only have nucleotides inserted to the 3′ end.

They can’t start producing a DNA chain from scratch.For that they need a primer, which is a pre-existing chain or short stretch of nucleotides.

They proofread or double-check their work, deleting the vast majority of “wrong” nucleotides that are added to the chain by accident.

When it comes to Replication, How do DNA Polymerases and other Components know where to Start?

Replication always begins at specified sites on the DNA known as replication origins, which are identified by their sequence.

On its chromosome, E.coli, like most bacteria, has a single replication origin.

The origin is recognised by specialised proteins, which attach to this location and open up the DNA. Two Y-shaped structures called replication forks form as the DNA expands, forming what’s known as a replication bubble.

As replication progresses, the replication forks start moving in different directions.

Primers and Primase are two Types of Primers:

Only the 3′ end of an existing DNA strand can be added to by DNA polymerases.

It can’t do it on its own. Primase, a type of enzyme, is used to remedy the problem. Primase creates an RNA primer, which is a brief stretch of complementary nucleic acid that serves as a 3′ end for DNA polymerase to act on.

The length of a primer is usually between five and 10 nucleotides. The primer initiates DNA synthesis by priming it.

DNA polymerase “extends” the RNA primer by adding nucleotides one by one to create a new DNA strand that is complementary to the template strand.

Strands that are leading and lagging:

DNA polymerase III is the DNA polymerase in E. coli that does the majority of the synthesis. At a replication fork, two molecules of DNA polymerase III are hard at work on one of the two new DNA strands.

The fact that DNA polymerases can only synthesise DNA in the 5′ to 3′ orientation causes problems during replication. A DNA double helix is never parallel; it is always anti-parallel.

In eukaryotes, DNA replication takes place in two ways:

Although the fundamentals of DNA replication are same in bacteria and eukaryotes like humans, there are some differences:

Numerous linear chromosomes, each with multiple replication origins, are common in eukaryotes. Humans can have up to 100,000 replication sources!

Most E. coli enzymes have eukaryotic DNA replication equivalents, however a single E. coli enzyme may be represented by many eukaryotic enzymes. Five human DNA polymerases, for example, play critical roles in replication5.

The chromosomes of most eukaryotes are linear. Each cycle of replication results in some DNA being lost from the ends of linear chromosomes (telomeres) due to the way the lagging strand is constructed.

Conclusion:

Helicase opens the DNA at the replication fork.

Single-strand binding proteins wrap the DNA around the replication fork to prevent it from rewinding.

Topoisomerase works in the zone ahead of the replication fork to prevent supercoiling.

Primase is a protein that produces complementary RNA primers for DNA strands.

DNA polymerase III adds to the 3′ end of the primers to make the bulk of the new DNA.

RNA primers are removed by DNA polymerase and further replaced with DNA.

DNA ligase is a protein that bridges the gaps between DNA segments.