DNA Replication
Base pairing allows each strand to serve as a template for a new strand. The new strand is half parent template and half new DNA. This is known as semi-conservative replication.
Stage 1: Unwind and separate the DNA
A replication fork forms where the DNA is being separated and copied.
Enzyme: helicase
Helicase moves along the DNA molecule and breaks the hydrogen bonds between complementary bases.
This causes the DNA double helix to unwind.
The two DNA strands separate, exposing the bases.
Each original DNA strand now acts as a template for the formation of a new complementary strand.
Stage 2: Add RNA primers
Enzyme: primase
DNA polymerase cannot begin synthesising a new DNA strand from nothing.
It requires a pre-existing 3′ end to which it can add nucleotides.
Primase produces a short sequence of RNA nucleotides called an RNA primer.
The RNA primer is complementary to the DNA template.
The primer provides the starting point for DNA polymerase.
A single primer is generally required to initiate synthesis on the leading strand.
The lagging strand requires multiple RNA primers because it is synthesised in separate sections.
Stage 3: Build daughter DNA strand
Enzyme: DNA polymerase
DNA polymerase attaches to the RNA primer. It adds DNA nucleotides that are complementary to the exposed bases on the template strand. Base pairing occurs according to the complementary base-pairing rules:
- A pairs with T
- C pairs with G
DNA polymerase forms phosphodiester bonds between adjacent nucleotides in the new DNA strand. DNA polymerase reads the template strand 3′ → 5′.
It can only add a nucleotide to the 3′ end of the growing strand. Therefore, the new DNA strand is always synthesised 5′ → 3′.
The two strands are synthesised differently:
Leading strand
- Synthesised continuously.
- DNA polymerase follows the replication fork.
- Only one RNA primer is needed to initiate synthesis.
Lagging strand
- Synthesised discontinuously.
- DNA polymerase works away from the replication fork.
- Short sections called Okazaki fragments are produced.
- Each Okazaki fragment requires its own RNA primer.
Stage 4: Remove the RNA primers and replace them with DNA
- The RNA primers are removed after the new DNA has been synthesised.
- The RNA primers leave short gaps in the newly synthesised DNA.
- These gaps are filled with DNA nucleotides.
- The result is a DNA strand in which the RNA primers have been replaced by DNA.
- On the lagging strand, the Okazaki fragments are still separate sections at this point.
Stage 5: Join the DNA fragments
- Enzyme: DNA ligase
- DNA ligase joins adjacent sections of DNA by forming phosphodiester bonds.
- On the lagging strand, it joins the Okazaki fragments into one continuous DNA strand.
- The completed DNA molecules are now double-stranded.
- Each DNA molecule contains:
- one original DNA strand
- one newly synthesised DNA strand
- This is why DNA replication is described as semi-conservative.