The DNA Replication

What is DNA Replication?

DNA replication also referred to as semi-conservative replication, is the procedure by which DNA is basically duplicated. It is a crucial process that occurs within the dividing cell. DNA, discovered within the nucleus, should be replicated in order to ensure that each new cell receives the correct number of chromosomes.

In eukaryotic cells, such as animal cells and plant cells, DNA replication occurs in the S stage of interphase throughout the cell cycle. The process of DNA duplication is vital for cell growth, repair, and reproduction in organisms.


Steps of DNA Replication

Replication follows numerous steps that include multiple proteins called replication enzymes and RNA. These steps are as follows.

Step 1: Replication Fork Formation

Before DNA can be replicated, the double-stranded molecule must be “unzipped” into two single strands. DNA has actually four bases called adenine (A), thymine (T), cytosine (C), and guanine (G) that form sets in between the two strands. Adenine just couple with thymine and cytosine only binds with guanine. In order to unwind DNA, these interactions between base pairs need to be broken.

This is performed by an enzyme called DNA helicase. DNA helicase interferes with the hydrogen bonding between base pairs to separate the strands into a Y shape referred to as the replication fork. This area will be the template for replication to begin.


DNA is directional in both strands, signified by a 5′ and 3′ end. This notation signifies which side group is attached to the DNA backbone. The 5′ end has a phosphate (P) group attached, while the 3′ end has a hydroxyl (OH) group attached.

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This directionality is important for replication as it just progresses in the 5′ to 3′ direction. However, the replication fork is bi-directional; one strand is oriented in the 3′ to 5′ direction (leading hair) while the other is oriented 5′ to 3′ (lagging strand). The two sides are therefore reproduced with 2 different procedures to accommodate the directional difference.

Step 2: Primer Binding

The leading strand is the easiest to reproduce. Once the DNA strands have actually been separated, a brief piece of RNA called a primer binds to the 3′ end of the strand. The primer always binds as the beginning point for replication. Primers are produced by the enzyme DNA primase.

Step 3: Elongation

Once the DNA Polymerase has actually connected to the initial, unzipped two strands of DNA (i.e., the template strands), it has the ability to start synthesizing the new DNA to match the design templates. It is essential to note that DNA polymerase is only able to extend the primer by adding complementary nucleotides to the 3′ end. One of the model templates reads in a 3′ to 5′ direction, which means that the new strand will be formed in a 5′ to 3′ direction.

This newly formed strand is referred to as the Leading strand. Along this strand, DNA Primase just needs to synthesize an RNA primer when, at the start, to start DNA Polymerase. This is because DNA Polymerase has the ability to extend the new DNA strand by checking out the template 3 ′ to 5 ′, synthesizing in a 5 ′ to 3 ′ direction as kept in mind above.

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However, the other template strand (the lagging strand) is antiparallel and is for that reason checked out in a 5′ to 3′ direction. Constant DNA synthesis, as in the leading strand, would require to be in the 3 ′ to 5 ′ direction, which is impossible as we can not include bases to the 5 ′ end.

Rather, as the helix unwinds, RNA primers are contributed to the recently exposed bases on the delayed strand and DNA synthesis happens in pieces, however still in the 5 ′ to 3 ′ direction as before. These fragments are known as Okazaki fragments.


Step 4: Termination

The process of expanding the new DNA strands continues up until there is either say goodbye to the DNA template delegated reproduction (i.e., at the end of the chromosome), or 2 replication forks satisfy and consequently terminate. The conference of two duplication forks is not managed and takes place arbitrarily along the course of the chromosome.

When DNA synthesis has actually ended up, it is very important that the freshly synthesized strands are bound and supported. With regards to the delayed strand, 2 enzymes are needed to accomplish this; RNAase H eliminates the RNA primer that is at the start of each Okazaki piece, and DNA Ligase signs up with pieces together to produce one total strand.


Summary of Enzymes involved in DNA replication

DNA replication would not take place without enzymes that catalyze numerous steps in the process. Enzymes that take part in the eukaryotic DNA replication process include:

DNA helicase – unwinds and separates double-stranded DNA as it moves along the DNA. It forms the replication fork by breaking hydrogen bonds between nucleotide pairs in DNA.

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DNA primase – a kind of RNA polymerase that generates RNA primers. Primers are short RNA molecules that act as templates for the beginning point of DNA replication.

DNA polymerases – manufacture new DNA molecules by adding nucleotides to leading and lagging DNA strands.

Topoisomerase or DNA Gyrase – unwinds and rewinds DNA strands to prevent the DNA from becoming tangled or supercoiled.

Exonucleases – a group of enzymes that eliminate nucleotide bases from completion of a DNA chain.

DNA ligase – joins DNA fragments together by forming phosphodiester bonds in between nucleotides.

Key Points!!!
  • The semi-conservative method was confirmed by Meselson and Stahl.
  • In this type, the sequence of an original duplex is conserved after one round of replication.
  • According to conservative method, parental double helix would remain intact and generate DNA copies consisting of entirely new molecule.
  • According to dispersive model, DNA would become completely dispersed and each strand of all daughter molecules would be the mixture of old and new DNA.
  • DNA polymerase catalyze addition of nucleotides to the complementary strand of DNA.
  • Primase constructs primer which is a sequence of about 10 RNA nucleotides complementary to the parent DNA template.
  • Replication always proceeds from 5’ to 3’ direction on growing strand.
  • The leading strand elongates towards the replication fork and lagging strand away from replication fork.
  • Okazaki fragments are 100 – 200 nucleotides long in eukaryotes and 1000 – 2000 nucleotides long in prokaryotes.