Can someone explain crossing over?

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Can someone explain what crossing over means in terms of chromosomes. (OCR biology F215)

Thank you so much in advance

Posted Sat 5th January, 2013 @ 14:49 by Hannah

3 Answers

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Its a way of increasing genetic variation during meiosis. It happens during prophase I when the homologous chromosomes form bivalents (these homologous chromosomes consist of one maternal (from your mother) and one paternal (from your father)). Each chromosome has already undergone semiconservative replication so consists of two chromatids joined at the centromere. The chromatid from say the maternal chrmosome may 'overlap' with a chromatid from the paternal chromosome. The chromatids wrap around each other at the chiasmata. During anaphase I, the two chromosomes are pulled apart. They are however joined at this chiasmata. When the chiasmata breaks to separate the chromosomes, the parts of the chromosome beneath the chiasmata may swap around, so that a section of the maternal chromosome swaps with that SAME section on the paternal chromosome. Because the maternal and paternal chrmosomes may contain different alleles and slightly different base sequences, this results in two new chromosomes which have a new combination of alleles

Answered Sat 5th January, 2013 @ 15:09 by Ross Edwards
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Thank you so muh that is really helpful

Answered Sat 5th January, 2013 @ 16:39 by Hannah
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no problem, good luck with the exam!

Answered Sat 5th January, 2013 @ 16:45 by Ross Edwards