Question

Describe the discontinuous synthesis of the lagging strand in DNA replication.

Short answer

The lagging strand is synthesized discontinuously through Okazaki fragments, followed by primer removal and fragment joining.

Step-by-step solution

Initiation

The replication process begins at the origin of replication. Helicase unwinds the double helix, creating a replication fork with two single strands of DNA.

Primer Synthesis

Primase synthesizes short RNA primers on the lagging strand template to provide a starting point for DNA synthesis.

Synthesis of Okazaki Fragments

DNA polymerase III extends the RNA primers by adding DNA nucleotides to form short DNA segments called Okazaki fragments.

Primer Removal and Gap Filling

DNA polymerase I removes the RNA primers and replaces them with DNA nucleotides.

Fragment Joining

DNA ligase seals the gaps between the Okazaki fragments to form a continuous DNA strand.

Key concepts

Origin of Replication

The replication process of DNA begins at a special site called the 'origin of replication'. This is the specific location where the double helix of DNA starts to unwind.
The enzyme helicase plays a crucial role here as it unzips the DNA, creating two single strands forming a Y-shaped structure known as the replication fork.
Each replication fork has a leading and a lagging strand, which are synthesized differently due to their antiparallel nature.

Okazaki Fragments

During the replication of the lagging strand, DNA synthesis happens in a discontinuous manner.
The lagging strand is synthesized in short segments called Okazaki fragments. These fragments are typically about 100-200 nucleotides long in eukaryotes.
Each fragment starts with an RNA primer and is later extended by DNA polymerase III to form small DNA pieces.
This fragmented approach is necessary because the lagging strand runs in the opposite direction to the movement of the replication fork.

DNA Ligase

DNA ligase is the enzyme responsible for joining the Okazaki fragments into a single continuous strand.
After the RNA primers are replaced with DNA, there still remain small nicks or gaps between the Okazaki fragments. DNA ligase seals these nicks by forming phosphodiester bonds between adjacent DNA nucleotides.
This process ensures that the lagging strand is a continuous and stable DNA sequence.

DNA Polymerase III

DNA polymerase III is the primary enzyme involved in the synthesis of new DNA strands.
On the lagging strand, it extends the RNA primers by adding complementary DNA nucleotides to form Okazaki fragments.
It works in the 5' to 3' direction, which is why it synthesizes discontinuous segments on the lagging strand.
Besides its role in elongation, DNA polymerase III also has a proofreading function to ensure high fidelity during DNA replication.

Primase

Primase is an enzyme that synthesizes short RNA primers, which are essential starting points for DNA synthesis.
On the lagging strand, primase periodically attaches and synthesizes a new primer for each Okazaki fragment.
These RNA primers provide the necessary 3' end for DNA polymerase III to attach DNA nucleotides.
Without primase’s activity, the lagging strand synthesis would not be possible because DNA polymerase III cannot initiate synthesis on its own.