D1.1.8 DNA primase, DNA polymerase I, DNA polymerase III and DNA ligase in replication (HL) Notes

Functions of DNA Primase, DNA Polymerase I, DNA Polymerase III, and DNA Ligase in Replication

1. DNA replication is a highly coordinated process involving multiple enzymes, each with a specific role.

DNA Primase: Laying the Foundation

1. DNA polymerases can’t start synthesizing a new strand on their own.
2. They need a starting point, a short segment of RNA called a primer.

Key Functions of DNA Primase

1. Synthesizes RNA Primers: DNA primase creates a short RNA primer (about 10 nucleotides long) on the template strand.
2. Provides a 3′-OH Group: The RNA primer offers a free 3′-OH group, which is essential for DNA polymerase III to begin adding DNA nucleotides.
3. Multiple Primers on the Lagging Strand: On the lagging strand, DNA primase adds a new primer for each Okazaki fragment.

DNA Polymerase III: The Main Builder

1. DNA polymerase III is the workhorse of DNA replication, responsible for assembling the majority of the new DNA strand.
2. It is the primary enzyme responsible for synthesizing new DNA strands during replication in prokaryotes.

Key Functions of DNA Polymerase III

1. Synthesizes DNA in the 5′ to 3′ Direction: Adds nucleotides to the growing DNA strand, using the template strand as a guide.
2. High Processivity: Can add thousands of nucleotides without detaching, ensuring efficient replication.
3. Proofreading Ability: Checks each nucleotide after adding it, removing and replacing incorrect ones to minimize errors.

DNA Polymerase I: The Cleaner

1. Once DNA polymerase III has synthesized the new DNA, the RNA primers need to be removed and replaced with DNA.
2. This is where DNA polymerase I comes in.
3. An enzyme that removes RNA primers and replaces them with DNA nucleotides during replication.

Key Functions of DNA Polymerase I

1. Removes RNA Primers: Acts as an exonuclease to remove RNA primers from the newly synthesized strand.
2. Replaces RNA with DNA: Fills in the gaps left by the removed primers with DNA nucleotides.
3. Works in the 5′ to 3′ Direction: Like DNA polymerase III, it adds nucleotides in the 5′ to 3′ direction.

DNA Ligase: The Final Connector

1. After DNA polymerase I replaces the RNA primers with DNA, there are still small gaps in the sugar-phosphate backbone of the DNA strand.
2. DNA ligase seals these gaps.

Key Functions of DNA Ligase

1. Seals Nicks in the DNA Backbone: Forms phosphodiester bonds between adjacent DNA nucleotides, completing the sugar-phosphate backbone.
2. Joins Okazaki Fragments: Ensures that the short DNA fragments on the lagging strand become a continuous strand.
3. Requires Energy: Uses ATP to catalyze the formation of these bonds.

Coordination of Enzymes in DNA Replication

1. Primase creates RNA primers to initiate synthesis.
2. DNA polymerase III extends the DNA strand from the primer.
3. DNA polymerase I removes RNA primers and replaces them with DNA.
4. DNA ligase seals the gaps, forming a continuous DNA strand.

Why These Enzymes Matter

1. Understanding the roles of these enzymes is crucial for grasping how DNA replication maintains genetic continuity.
2. These enzymes ensure that DNA replication is accurate and efficient, allowing life to persist across generations.