Module 2: DNA Replication

These free OCR A Level Biology DNA Replication revision notes have been written for specification points 2.1.3(b), 2.1.3(c), 2.1.3(d.i) and 2.1.3(e).

DNA Replication

DNA synthesis refers to the assembly of a new DNA polymer from nucleotides.

Nucleotides are joined together by DNA polymerase, which catalyses condensation reactions between the phosphate groups of adjacent nucleotides in the 5′ → 3′ direction.

This forms a phosphodiester bond, building the sugar-phosphate backbone of the DNA strand.

“5′ → 3′” refers to the orientation of a nucleic acid strand in DNA. The phosphate group on the 5′ carbon of one nucleotide is bonded to the hydroxyl group of the 3′ carbon of the next nucleotide, so the strand is built up from the 5′ end towards the 3′ end.

DNA replication is the process by which an accurate copy of the DNA molecule is made during the S phase of the cell cycle. 

Step-By-Step: DNA Synthesis

1. Nucleotide Formation

Each DNA nucleotide is formed by a condensation reaction between a deoxyribose sugar, a phosphate group, and a nitrogenous base (A, T, C, or G).

2. DNA Unwinding

DNA helicase breaks hydrogen bonds between bases, separating the two strands of the helix.

3. Primer Binding

RNA primers are added to both strands by primase, providing a starting point for DNA polymerase to begin synthesis.

4. Complementary Base Pairing

Free activated nucleotides pair with exposed bases on each template strand:

• A pairs with T
• C pairs with G

5. DNA Polymerase Activity

DNA polymerase adds nucleotides to the 3′ end of the new strand, synthesising in the 5′ → 3′ direction.

It forms phosphodiester bonds between adjacent nucleotides using energy from hydrolysing the extra phosphate groups.

6. Leading Strand Synthesis

On the leading strand (template runs 3′ → 5′), DNA polymerase works continuously in the same direction as the replication fork.

7. Lagging Strand Synthesis

On the lagging strand (template runs 5′ → 3′), DNA polymerase works discontinuously away from the fork, producing short sections called Okazaki fragments.

Each fragment requires a new primer.

8. Joining of Fragments

DNA ligase joins Okazaki fragments by forming phosphodiester bonds, creating a complete and continuous strand.

9. Semi-Conservative Replication

Each resulting DNA molecule contains one original strand and one newly synthesised strand – this is semi-conservative replication.