An origin of replication is the specific site in DNA where replication begins; while there is only one origin of replication in circular prokaryotic DNA, linear eukaryotic DNA has many origins of replication.
Replication always starts at specific locations in DNA, which are called origins of replication. In the prokaryotic genome, the single origin of replication has many A-T base pairs, which have weaker hydrogen bonding than G-C base pairs, and make it easier for the DNA strands to separate. An enzyme called helicase unwinds the DNA by breaking the hydrogen bonds between the nitrogenous base pairs. As the DNA opens up, two Y-shaped structures called replication forks are formed. Together these structures form what’s known as a replication bubble. Proteins called single-strand binding proteins coat the separated strands of DNA near the replication forks, keeping them from coming back together into a double helix. Take a look at the diagram below, which shows an example of an origin of replication and the resulting replication bubble.
Because eukaryotic chromosomes are linear and much larger than prokaryotic ones, there are multiple origins of replication in the eukaryotic genome during replication. This means that replication can occur simultaneously in hundreds to thousands of locations along each chromosome.
Key Points
• Origins of replication are the sites in DNA where the process of DNA replication begins.
• Helicase enzymes separate the two strands of a DNA helix bi-directionally, creating structures known as a replication forks at the sites where the strands separate.
• The area within the separating DNA strands is known as the replication bubble.
• While prokaryotic genomes only have one origin of replication, multiple origins are present simultaneously in the eukaryotic genome to allow for faster replication.
Key Terms
Origin of replication: A particular sequence in a genome at which replication is initiated.
DNA replication: The process by which DNA is copied.
Prokaryote: Single-celled organisms that lack membrane-bound organelles.
Eukaryote: Organisms with membrane-bound organelles.
Helicase: an enzyme that unwinds the DNA helix ahead of the replication machinery
Replication fork: It is a Y-shape structure formed due to the unwinding of the DNA at the origin of replication.
Replication bubble: The structure formed as helicase enzymes separate the DNA strands at origins of replication.
Single-strand binding proteins: Proteins that bind to the separated DNA strands during replication, preventing the strands from reforming the DNA helix.