BIOB 160 Principles of Living Systems · Unit 4: Molecular genetics · on Final exam
4.2 DNA replication
Study notes for DNA replication in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in Mathub.
Big ideas
- Replication is semiconservative: each new double helix has one old and one new strand (shown by Meselson and Stahl with ¹⁵N and ¹⁴N).
- At each origin, helicase unwinds the helix, single-strand binding proteins hold the strands apart and topoisomerase relieves twisting ahead of the fork. Primase lays down a short RNA primer.
- DNA polymerase III adds nucleotides only to a 3′ end, so new DNA grows 5′ → 3′. The leading strand is made continuously toward the fork; the lagging strand is made away from it in Okazaki fragments.
- DNA polymerase I replaces RNA primers with DNA, and DNA ligase joins the fragments.
- DNA polymerases proofread; mismatch repair and nucleotide excision repair fix errors that remain. Linear chromosomes shorten at their ends each round; telomerase extends telomeres in germ cells and many cancer cells.
Worked example
Bacteria grown in ¹⁵N are moved to ¹⁴N. After one round of replication, what DNA densities appear? After two?
Show the solution
After one round: all DNA is hybrid (one ¹⁵N strand, one ¹⁴N strand). After two: half hybrid, half light (¹⁴N/¹⁴N). This pattern is what semiconservative replication predicts.
Common mistakes
- Saying polymerase builds 3′ → 5′. New strands always grow 5′ → 3′.
- Mixing up leading (continuous) and lagging (Okazaki fragments) strands.
- Forgetting that primers are RNA and must be replaced.
Exam tipReading: Campbell ch. 16 (pp. 320–329). Draw a replication fork with every enzyme labeled; practise it until you can do it from memory.
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