As DNA polymerase adds nucleotides to a growing DNA strand, it checks whether each newly added base pairs correctly with the corresponding base in the template strand. If the pairing is correct, synthesis continues. If a mismatch is detected, the polymerase removes the incorrect nucleotide and replaces it with the correct one.
This correction uses the enzyme’s 3′ exonuclease activity. The enzyme cuts the phosphodiester bond holding the incorrect nucleotide at the growing strand’s end, releases that nucleotide, and then resumes DNA synthesis. Proofreading corrects many replication errors before they become permanent changes in the DNA sequence.
All flashcards
How does mismatch repair correct a replication error?
It removes a strand segment containing a mispaired nucleotide and resynthesizes that region using the opposite strand as a template.
What distinguishes nucleotide excision repair from mismatch repair?
Nucleotide excision repair removes a segment containing damaged bases; mismatch repair corrects mispaired bases that escaped replication proofreading.
What is a silent mutation?
A mutation that does not change the amino acid in the resulting protein.
What is a trinucleotide repeat expansion?
An increase in the number of copies of a three-nucleotide sequence.
How do somatic-cell and germ-cell mutations differ in inheritance?
Somatic mutations affect the individual and are not ordinarily inherited; germ-cell mutations can be passed to offspring.
Quiz questions
Which activity allows DNA polymerase to remove an incorrectly paired nucleotide during proofreading?Answer: 3′ exonuclease activity
How does methylation help mismatch repair identify the newly synthesized strand in E. coli?Answer: The parental strand is methylated while the new strand is temporarily unmethylated.
Which sequence describes nucleotide excision repair of a damaged DNA segment?Answer: The segment is cut out, DNA polymerase fills the gap, and DNA ligase seals the break.
Why does defective nucleotide excision repair in xeroderma pigmentosum increase skin cancer risk?Answer: UV-induced pyrimidine dimers can persist and contribute to mutations.
Which substitution is a transition?Answer: Replacing adenine with guanine
Based on Biology 2e by Mary Ann Clark, Matthew Douglas, Jung Choi (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.