DNA Replication and Repair
Copying the Genome
Semiconservative Replication
DNA replication is semiconservative: each daughter molecule contains one parental strand and one newly synthesized strand. DNA polymerases synthesize DNA only in the direction.
Key Enzymes
Helicase unwinds the helix, primase lays down RNA primers, DNA polymerase extends the new strand, and ligase seals fragments on the lagging strand.
Replication Roles
| Protein | Function |
|---|---|
| Helicase | Separates DNA strands |
| Primase | Synthesizes primers |
| DNA polymerase | Adds nucleotides |
| Ligase | Joins DNA fragments |
Leading vs Lagging
Because polymerase works only , one strand is made continuously while the other is made in Okazaki fragments.
What is the primary role of DNA ligase?
Ligase seals gaps between fragments, especially on the lagging strand.
Correct answer: Joins DNA fragments
Why does DNA polymerase require a primer?
A primer provides a free -OH for extension.
Correct answer: It can only extend an existing nucleic acid strand and cannot start synthesis from scratch.
Replication is called semiconservative because
Each daughter duplex contains one parental and one newly synthesized strand.
Correct answer: Each daughter DNA has one old strand and one new strand
Repair Systems
Cells repair mismatches, base damage, and double-strand breaks using pathways such as mismatch repair, base excision repair, nucleotide excision repair, and homologous recombination.
Name one DNA repair pathway and the type of damage it helps fix.
Many valid examples are acceptable.
Correct answer: Nucleotide excision repair fixes bulky DNA lesions such as UV-induced thymine dimers.