DNA Replication Cheat Sheet

The core ideas of DNA Replication distilled into a single, scannable reference — perfect for review or quick lookup.

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Quick Reference

Semiconservative Replication

DNA replication produces two identical double helices, each containing one original (parent) strand and one newly synthesized strand. This was demonstrated by the Meselson-Stahl experiment.

Helicase and the Replication Fork

Helicase is the enzyme that unwinds the double helix by breaking hydrogen bonds between base pairs, creating a Y-shaped replication fork where new strands are synthesized.

DNA Polymerase

The primary enzyme that synthesizes new DNA strands by adding complementary nucleotides to the 3' end of a growing strand, reading the template in the 3' to 5' direction. It requires an RNA primer to begin synthesis.

Leading and Lagging Strands

The leading strand is synthesized continuously in the 5' to 3' direction toward the replication fork. The lagging strand is synthesized discontinuously as Okazaki fragments in the 5' to 3' direction away from the fork, then joined by DNA ligase.

Proofreading and Error Correction

DNA polymerase has 3' to 5' exonuclease activity that allows it to detect and remove mismatched nucleotides immediately after they are added. Additional mismatch repair systems correct errors that escape proofreading.

Key Terms at a Glance

Semiconservative Replication:The mode of DNA replication in which each daughter molecule contains one original strand and one newly synthesized strand.

Replication Fork:The Y-shaped region where the two parental DNA strands are separated and new strands are being synthesized.

Helicase:An enzyme that unwinds the DNA double helix by breaking hydrogen bonds between base pairs, using energy from ATP hydrolysis.

DNA Polymerase:The enzyme that synthesizes new DNA by adding complementary nucleotides to the 3-prime end of a growing strand. It also has proofreading (exonuclease) activity.

Primase:An RNA polymerase that synthesizes short RNA primers on the template strand, providing a 3-prime OH group for DNA polymerase to begin synthesis.

Okazaki Fragments:Short DNA segments synthesized on the lagging strand during replication, later joined by DNA ligase into a continuous strand.

DNA Ligase:An enzyme that joins Okazaki fragments by catalyzing phosphodiester bonds between adjacent DNA segments, sealing nicks in the backbone.

Topoisomerase:An enzyme that relieves supercoiling tension ahead of the replication fork by cutting, rotating, and resealing the DNA strands.

Single-Strand Binding Proteins:Proteins that bind to and stabilize single-stranded DNA at the replication fork, preventing re-annealing and degradation.

Telomere:Repetitive non-coding DNA sequences at chromosome ends that protect against degradation and shorten with each replication cycle.

Telomerase:A reverse transcriptase enzyme that extends telomeres using its own RNA template, counteracting chromosome shortening.

Origin of Replication:A specific DNA sequence where replication is initiated. Prokaryotes typically have one; eukaryotes have thousands.

Proofreading:The 3-prime to 5-prime exonuclease activity of DNA polymerase that detects and removes mismatched nucleotides immediately after incorporation.

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