32.3

During DNA replication, one parental DNA molecule produces two identical daughter molecules.
The fidelity of this replication process is crucial to prevent mutations, which are changes in genetic information.

Mistakes in DNA replication can have harmful effects on cells or organisms.
Example: A mutation can differentiate virulent bacteria from nonvirulent strains.

Most DNA polymerases exhibit a proofreading function, allowing for immediate correction of errors during replication.
Mechanism:
- When a new nucleotide is added, it forms hydrogen bonds with the complementary base of the parental strand (A with T and G with C).
- Occasionally, an incorrect nucleotide may bind to the new DNA strand.
- DNA polymerase detects mispairing between the parent strand and the newly added nucleotide.
- Upon detecting a mismatch, it removes the incorrect nucleotide and replaces it with the correct one.

While proofreading reduces the occurrence of mutations, errors still occur.
In Escherichia coli, about 99% of incorrect nucleotides during replication are corrected by DNA polymerase.
Mutations that slip past proofreading can be harmful, neutral, or beneficial, and they contribute to genetic variation and are critical for evolution.

Mutations:
- May lead to harmful changes, but can also be neutral or beneficial.
- Serve as the source of genetic variation within species and across species.
Evolution:
- Mutations allow populations to adapt and evolve over time.

Students should understand the functions of these enzymes:
- DNA Polymerase: Responsible for synthesizing new DNA and proofreading.
- DNA Ligase: Joins Okazaki fragments and seals nicks in the DNA.
- Topoisomerase: Relieves the tension in DNA during replication.
- Helicase: Unwinds the DNA double helix.