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Slipped-strand Mispairing

Why is the synapsis of homologous chromosomes in prophase I a potential source of error? The key event of meiosis separating it from mitosis is the pairing and synapsis of homologous chromosomes during prophase I. Without the pairing of homologs the segregation of alleles would not be possible. Synapsis represents the one time outside of DNA replication and transcription that DNA sequences are actively interacting with other molecules. Each base in the DNA is pairing up with its counterpart on the homolog. Errors in pairing can lead to a wide variety of errors. If the two chromosomes are not properly aligned then crossing over will result in the exchange of unequal amounts of DNA or place DNA in a different location than it was originally.

View the animation below, then complete the quiz to test your knowledge of the concept.

1During slipped-strand mispairing, homologous chromosomes
A)do not pair up with each other.
B)pair up normally with each other.
C)pair up with each other, but then they are both degraded.
D)pair up with each other, but out of register.
E)pair up with different homologous chromosome pairs.

2Slipped-strand mispairing may cause deletions resulting in
B)inhibition of translation.
C)a frameshift mutation.
D)a single base substitution.
E)chromosome inactivation.

3Slipped-strand mispairing requires that a DNA sequence be present
A)in one copy on a homologous chromosome but no copies in the other.
B)in two copies on a homologous chromosome but no copies in the other.
C)in multiple copies on a homologous chromosome but no copies in the other.
D)in only a single copy on both homologous chromosomes.
E)in multiple copies on both homologous chromosomes.

4The loop formed during slipped strand mispairing may be excised by the cell’s repair enzymes.

5Some chemicals can stabilize these loops and thus decrease the time the loops are vulnerable to excision.

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