DNA Replication Process: Helicase and Primase Functions

DNA Replication Process

• Helicase unwinds the DNA double helix by breaking the hydrogen bonds between complementary base pairs (A-T and G-C), creating a replication fork.

Helicase and Primase Function

• Primase synthesizes RNA primers, adding short RNA sequences to the DNA template strands, which are essential for DNA Polymerase to extend the existing strand.
• On the leading strand, primase synthesizes a single RNA primer at the 5' end, providing a starting point for DNA Polymerase.

DNA Polymerase III Function

• DNA Polymerase III is the primary enzyme responsible for DNA synthesis during replication, adding nucleotides to the growing DNA strand.
• On the leading strand, DNA Polymerase III synthesizes the new DNA strand in a continuous manner, extending from the 5' to 3' direction, using the parental DNA strand as a template.

Leading Strand Synthesis

• The leading strand is synthesized continuously, with DNA Polymerase III extending the new DNA strand by reading the template 3' to 5' and synthesizing in a 5' to 3' direction.
• Only one RNA primer is needed at the beginning of the leading strand, as DNA Polymerase can extend the new DNA strand by reading the template 3' to 5'.

Lagging Strand Synthesis

• The lagging strand is antiparallel and is read in a 5' to 3' direction, requiring multiple RNA primers to be added to the newly exposed bases.
• DNA synthesis occurs in fragments, known as Okazaki fragments, which are synthesized in a 5' to 3' direction.
• Exonucleases degrade the RNA primers, creating gaps in the DNA strands.

Exonuclease and DNA Polymerase I Function

• Exonucleases remove RNA primers from the DNA template, creating gaps in the DNA strands.
• DNA Polymerase I replaces the RNA nucleotides with DNA nucleotides, filling in the gaps created by the removal of RNA primers.

Ligase Function

• Ligase seals the nicks or gaps in the DNA backbone by catalyzing the formation of phosphodiester bonds between adjacent nucleotides.
• On the lagging strand, ligase joins the Okazaki fragments together, ensuring the integrity and continuity of the newly synthesized DNA strand.

Termination of DNA Replication

• The replication process continues until the end of the chromosome is reached or two replication forks meet and terminate.
• The meeting of two replication forks is not regulated and happens randomly along the course of the chromosome.