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DNA replication is not perfect. In addition, DNA may be altered or damaged by environmental factors. Repair mechanisms detect and repair mismatched or damaged DNA.
learning outcome
You should be able to:
Predict the consequences of DNA errors if not corrected.
From what you know about the role of DNA in the cell, what would be the genetic consequences of an unrepaired DNA change in S phase before meiosis?
If there were a mutation in S phase before meiosis (e.g., a T changed to a C), the replicated strand would be mispaired; e.g.,
| AAGGTT | -----→ | AAGGCT |
| TTCCAA | TTCCAA |
If this mispairing was not recognized and repaired, a gamete formed might have the mutation:
AAGGCT
TTCCAA
And after the cell containing the mutation divided, one of the daughters would have the mutation and the other would retain the normal sequence.
| Top strand replicated: | AAGGCT |
| TTAAGA Mutation |
| Bottom strand replicated: | AAGGTT |
| TTCCAA Normal |
One form of colon cancer is caused by a mutation in a gene encoding a protein involved in mismatch repair. What would be the consequences of such a mutation in cancer formation? Recall cell cycle control genes involved in cancer formation (see Figure 11.24).
If the colon cancer cell had a mutation in a tumor suppressor gene, it might not be repaired. This would lead to unregulated cell division.
Understanding how DNA is replicated and repaired has allowed scientists to develop techniques for studying genes. We’ll look at just one of those techniques next.