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Crossing Over in Prophase I
Age 17 · Biology
During prophase I of meiosis, paired homologous chromosomes lie close together. At one point, a non-sister chromatid from each chromosome breaks and rejoins with a segment from the other, so the resulting chromatids carry a new mix of alleles.
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In the picture
- Centromere: the narrow waist that joins the two sister chromatids of one chromosome.
- Crossing point: the single contact where one chromatid from each paired chromosome breaks and rejoins.
- Swapped red segment: a piece of the red chromatid now attached to the blue chromatid.
- Swapped blue segment: a piece of the blue chromatid now attached to the red chromatid.
Examples
- Two neighbours swap a short strip of fence at one point; each fence still stands but now has a piece of the other’s colour.
- Two friends trade a single sticker between notebooks, so each page ends up with a slightly mixed set.
Key takeaways
- Crossing over happens between non-sister chromatids of paired homologous chromosomes.
- It creates new allele combinations in the gametes.
Quiz: Crossing Over in Prophase I (Age 17)
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When during meiosis does crossing over take place?
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Crossing over happens between which chromatids?
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What does crossing over create in the gametes?
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