Many different enzymes work together to catalyze (speed up) the distinct steps of DNA replication.
The diagram below summarizes the enzymes involved in prokaryotic DNA replication, which is very similar to eukaryotic replication:
Helicase opens up the DNA at the replication fork by breaking hydrogen bonds between the nucleotide base pairs. Single-strand binding proteins (SSBs) coat the DNA around the replication fork to prevent rewinding of the DNA. Topoisomerase works at the region ahead of the replication fork to prevent supercoiling by introducing breaks in the DNA and then resealing them. The sliding clamp helps to hold the DNA polymerase in place when nucleotides are being added. Primase synthesizes RNA primers complementary to the DNA strand. DNA polymerase III extends the primers, adding nucleotides in the 5′ to 3′ direction, to make the bulk of the new DNA. RNA primers are removed (by exonuclease activity) and replaced with newly synthesized DNA by DNA polymerase I. The gaps between the DNA fragments on the lagging strand, known as Okazaki fragments, are sealed by DNA ligase. There is a third polymerase involved, called DNA polymerase II, which has a DNA proofreading ability and serves a repair function.
The enzymes involved in eukaryotic replication are similar to those involved in prokaryotic replication. However, a single enzyme in prokaryotes may be represented by multiple enzymes in eukaryotes. For instance, there are five human DNA polymerases with important roles in replication. In addition, a separate RNase enzyme helps in the removal of RNA primers instead of DNA polymerase I.
Practice Questions
Khan Academy
MCAT Official Prep (AAMC)
Practice Exam 3 B/B Section Question 13
Key Points
• The enzymes involved in the replication of prokaryotic DNA are DNA polymerase I to III, helicase, ligase, primase, sliding clamp, topoisomerase, and single-strand binding proteins (SSBs).
• The basics of DNA replication are similar in prokaryotes and eukaryotes, but eukaryotes have many more enzymes involved.
Key Terms
Catalysis: The speeding up of a chemical reaction, in this case by an enzyme.
DNA replication: The process by which DNA is copied.
Prokaryote: Single-celled organisms without membrane-bound organelles.
Eukaryote: Organisms with membrane-bound organelles.
Helicase: An enzyme that unwinds the DNA helix ahead of the replication machinery by breaking hydrogen bonds between nucleotide base pairs.
Single-strand binding proteins (SSBs): An enzyme that coats the separated DNA strands around the replication fork to prevent rewinding of the DNA.
Replication fork: A Y-shaped structure that forms as the double-stranded DNA helix is separated by helicase.
Topoisomerase: An enzyme that functions ahead of the replication fork to prevent supercoiling of the DNA by introducing breaks and then sealing them.
Supercoiling: Over or underwinding of the DNA helix that occurs as the double-stranded helix is separated during replication; this puts strain on the DNA and can inhibit replication.
Sliding clamp: An enzyme that holds the DNA polymerase in place when nucleotides are being added.
Primase: An enzyme that synthesizes RNA primers complementary to the DNA strand.
DNA polymerase III: An enzyme that extends the RNA primers by adding nucleotides in the 5′ to 3′ direction; the main factor that synthesizes new DNA.
Exonuclease: An enzyme that removes nucleotides from the end of a DNA or RNA molecule.
DNA polymerase I: An enzyme that removes the RNA primers and replaces them with newly synthesized DNA.
Lagging strand: One of the two newly synthesized DNA strands that must be synthesized in the opposite direction from the replication fork.
Okazaki fragments: Fragments of newly synthesized DNA formed on the lagging strand.
DNA ligase: An enzyme that seals gaps between Okazaki fragments on the lagging strand.
DNA proofreading: An enzymatic function that some polymerases have to recognize and repair incorrectly added nucleotides during replication.
