Replication of DNA-DNA Polymerase & Primers

Introduction to DNA Replication

DNA Replication

DNA replication is the process by which a cell makes an identical copy of its DNA before cell division. This ensures that each daughter cell receives a complete set of genetic information.

DNA Polymerase

DNA replication is carried out by enzymes called DNA polymerases, which synthesise new DNA strands using the existing DNA as a template.

Role of DNA Polymerase

Adding DNA Nucleotides

DNA polymerase adds DNA nucleotides to the 3' end of the new DNA strand being formed. This involves complementary base pairing, where adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C).

Complementary Base Pairing

Complementary base pairing ensures that the new DNA strand is an exact copy of the original strand. For example, if the template strand has the sequence ATCG, the complementary strand will have the sequence TAGC.

Use of Primers

1. Primer Definition

A primer is a short strand of nucleotides that binds to the 3' end of the template DNA strand before DNA polymerase can start replication. Primers are essential because DNA polymerase requires a starting point.

2. Role of Primers

Primers serve as a starting point for DNA polymerase to add DNA nucleotides. They provide a free 3' end to which the new nucleotides can be added.

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Replication of DNA by DNA Polymerase & Primers

3. Primer Sequence

The sequence of the primer is complementary to the template DNA strand and allows for accurate replication to occur.

DNA Unwinding and Base Pair Separation

Unwinding DNA

Before DNA replication can occur, the DNA double helix must be unwound. This process is carried out by enzymes called helicases, which separate the two DNA strands by breaking the hydrogen bonds between the bases.

Formation of Template Strands

Once the DNA is unwound, two template strands are exposed, each serving as a template for the synthesis of a new complementary strand.

Direction of DNA Polymerase

Leading Strand

DNA polymerase can only add DNA nucleotides in one direction, which is from the 5' end to the 3' end of the new strand. As a result, one of the template strands can be replicated continuously in this direction and is called the leading strand.

Lagging Strand

The other template strand is replicated in the opposite direction and is called the lagging strand. DNA polymerase synthesises the lagging strand in short fragments.

Replication of the Leading Strand

Leading Strand Synthesis

For the leading strand, DNA polymerase continuously adds DNA nucleotides in the 5' to 3' direction, moving towards the replication fork (the point where DNA unwinding occurs).

Continuous Replication

This continuous replication of the leading strand allows for efficient and rapid synthesis of the new DNA strand.

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Replication of DNA by DNA Polymerase & Primers

Replication of the Lagging Strand

1. Lagging Strand Synthesis

The lagging strand is synthesised in short fragments called Okazaki fragments. These fragments are synthesised in the 5' to 3' direction away from the replication fork.

2. Primase's Role

Before each Okazaki fragment is synthesised, a small RNA primer is laid down by an enzyme called primase. This primer provides the starting point for DNA polymerase to add nucleotides.

3. Fragment Joining

After DNA polymerase synthesises an Okazaki fragment, another enzyme called ligase joins the fragments together by forming covalent bonds. This creates a continuous lagging strand.

Summary

In summary, DNA replication is a crucial process in which DNA polymerase adds DNA nucleotides, following complementary base pairing, to the 3' end of a new DNA strand. Primers are required to initiate replication by providing a starting point for DNA polymerase. DNA is unwound, and the hydrogen bonds between bases are broken to expose two template strands. DNA polymerase can only add nucleotides in one direction, resulting in the leading strand being replicated continuously and the lagging strand being replicated in fragments. The lagging strand's fragments are joined together by ligase to create a complete, double-stranded DNA molecule. This process ensures accurate duplication of genetic information before cell division.