Study guide
DNA Replication: A Study Guide That Actually Explains It
Semi-conservative, bidirectional, and antiparallel. Read the guide, then lock it in with flashcards.
DNA replication is on every biology and pre-med exam. Meselson and Stahl showed it is semi-conservative in 1958 — each daughter double helix contains one parent strand and one new strand. The complexity comes from the enzymes: about a dozen proteins work in concert at every replication fork.
The key enzymes in order
Learn them in the sequence they act at a fork.
- Helicase — unwinds the parent double helix
- Single-strand binding proteins (SSB) — hold strands apart
- Topoisomerase — relieves supercoiling ahead of the fork
- Primase — lays down short RNA primers (needed because DNA polymerase cannot start from scratch)
- DNA polymerase III — extends primers, synthesizes 5' to 3'
- DNA polymerase I — replaces RNA primers with DNA (in prokaryotes)
- Ligase — seals nicks between fragments
Leading vs lagging strand
DNA polymerase only synthesizes 5' to 3'. Since the two parent strands are antiparallel, only one daughter strand can grow continuously (leading). The other grows in short backwards pieces called Okazaki fragments, each requiring its own primer. Ligase joins them.
Bidirectional replication
At each origin of replication, two forks move in opposite directions. Prokaryotes have one origin per circular chromosome; eukaryotes have thousands per linear chromosome.
Telomere problem in eukaryotes
The lagging strand cannot fully replicate the very end of a linear chromosome — the last primer's position leaves a gap. Telomerase (present in germ cells and cancer, largely absent in somatic cells) extends telomeres to compensate. Absence in somatic cells is one theory of aging.
Frequently asked questions
Is human DNA replication the same as bacterial?
Mechanism is conserved (antiparallel, semi-conservative, requires primers). Prokaryotes use DNA polymerase III; eukaryotes use pol α (primer), δ (lagging), ε (leading). Origins and licensing are more complex in eukaryotes.
Why RNA primers instead of DNA?
DNA polymerase cannot initiate — it can only extend an existing strand. RNA primase can initiate. Primers are removed and filled with DNA later.
What is proofreading?
DNA polymerase has 3'→5' exonuclease activity that removes mispaired nucleotides. Combined with mismatch repair later, error rate drops to about 1 in 10⁹.
How fast is human replication?
About 50 nucleotides per second per fork in humans (vs ~1000 in E. coli). Multiple origins compensate.
What's the difference between replication and transcription?
Replication makes a full DNA copy; happens once per cell cycle. Transcription makes an RNA copy of a specific gene; happens continuously.
Keep exploring
Made for exam season
Pass that exam.
Turn your notes into flashcards and quizzes in seconds. Study smarter — start free today.