D1.1 DNA Replication
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Purpose of DNA replication?
Produce two identical DNA molecules; ensures each daughter cell gets full genetic info.
What is semi-conservative replication?
Each new DNA molecule contains one original and one new strand.
Why is DNA replication essential before cell division?
Without it, daughter cells would be incomplete → cell death or dysfunction.
Which enzyme unzips DNA?
Helicase (breaks hydrogen bonds).
Which enzyme relieves supercoiling and tension?
Topoisomerase.
Which enzyme adds nucleotides to a new strand?
DNA polymerase.
Which enzyme joins Okazaki fragments?
DNA ligase.
Difference between leading and lagging strands?
Leading = continuous; Lagging = discontinuous (Okazaki fragments).
Which enzyme synthesizes RNA primers?
DNA primase.
Direction of DNA synthesis?
5’ → 3’ on new strand; polymerase reads 3’ → 5’ on template.
Explain complementary base pairing in replication.
A pairs with T, C pairs with G → ensures accurate replication.
What ensures high fidelity in replication?
Proofreading by DNA polymerase and mismatch repair.
What did Meselson-Stahl experiment show?
DNA replication is semi-conservative (0 gen: heavy; 1 gen: intermediate; 2 gen: 50% light, 50% intermediate) .
Which strand is continuous, and why?
Leading strand — synthesis in same direction as fork.
Which strand is discontinuous, and why?
Lagging strand — synthesized in Okazaki fragments because polymerase can only add 5′ → 3′.
How does DNA polymerase ensure replication fidelity?
Proofreading activity → detects mismatches, removes incorrect nucleotide, restores correct base.
Keywords for General process:
Semi-conservative replication Meselson-Stahl experiment ¹⁵N / ¹⁴N isotopes Intermediate band Template strand New strand Accuracy / fidelity
Key words for Enzymes & Steps:
Helicase Hydrogen bonds Unwinding Topoisomerase (gyrase) Relieves supercoiling / torsional strain DNA primase RNA primer DNA polymerase III 5′ → 3′ direction DNA polymerase I Primer removal DNA ligase Phosphodiester bonds
Key words for Strand synthesis:
Leading strand Continuous synthesis Lagging strand Discontinuous synthesis Okazaki fragments Replication fork
Key words for Error-checking:
Complementary base pairing Proofreading Mismatch repair
Features of DNA structure that suggest a mechanism for DNA replication
The **double-helical ** structure of DNA suggests a semiconservative replication mechanism, where each of the two complementary strands separates and serves as a template for synthesizing a new strand. Complementary base pairing (A with T, and G with C) ensures that the new strands are exact copies of the original DNA. The double-stranded nature of the molecule allows for the unwinding and separation of the two strands by enzymes like helicase to begin the process.
Complementary base pairing in DNA role
the nucleotides need complementary base pairs –> ensuring accurate and identical new DNA strand to template.
DNA strands must be sepearted prior to replication
to allow enzymes to access individual strands.