DNA Replication Revision notes | A-Level Biology CIE

DNA Replication

This lesson covers:

The process of DNA replication

DNA replication

DNA is copied via a process known as semi-conservative replication.

This produces DNA molecules consisting of one original DNA strand and one newly synthesised DNA strand.

Diagram showing the process of DNA replication with adenine, thymine, guanine, and cytosine base pairs.

A DNA molecule ready to be replicated.

Diagram showing the process of DNA replication with hydrogen bonds broken and nucleotides adenine, thymine, guanine, and cytosine.

2. The hydrogen bonds between complementary bases are broken. This unwinds the double helix and separates the strands.

Diagram showing DNA replication with original strands acting as templates and free nucleotides forming hydrogen bonds.

3. Each strand acts as a template as activated nucleotides attach to their complementary bases. Adenine pairs with thymine, while cytosine pairs with guanine.

Diagram showing the process of DNA replication with DNA polymerase, DNA ligase, and phosphodiester bonds.

4. The enzyme DNA polymerase joins the new nucleotides via phosphodiester bonds in the 5' to 3' direction. This forms the sugar-phosphate backbone of the new DNA strand. The enzyme DNA ligase is also used to join Okazaki fragments.

Diagram illustrating the process of DNA replication showing original and new DNA strands with adenine, thymine, guanine, and cytosine.

5. Two identical copies of DNA are made. Each copy is made up of one original DNA strand and one new DNA strand (semi-conservative replication).

Illustration showing the process of DNA replication with steps including strand separation, base pairing, and formation of new strands.

Leading and lagging strands

DNA polymerase can only build new DNA strands in the 5' to 3' direction. This affects the process by which each new DNA strand is made.

We call the two newly synthesised strands the leading strand and the lagging strand.

Leading strand:

This is synthesised in the same direction as the unwinding of the double helix.
This means that DNA polymerase can synthesise it continuously.

Lagging strand:

This is synthesised in the opposite direction to the unwinding of the double helix.
This means that DNA polymerase has to synthesise a short fragment of DNA and then go backwards to synthesise the next piece of unwound DNA.
DNA polymerase continues this process to create a series of copied DNA fragments known as Okazaki fragments.
The Okazaki fragments can then be joined together by DNA ligase.