BRAINCAKEProphase 1
Prophase I is the first and longest stage of meiosis. The DNA has already been replicated during interphase, so each chromosome consists of two sister chromatids joined at the centromere.
Events of prophase I:
• The chromosomes condense (supercoil), becoming shorter and thicker and visible with a light microscope.
• Homologous chromosomes pair up side by side. This pairing is called synapsis, and each pair is called a bivalent (four chromatids in total).
• Non-sister chromatids of the homologous chromosomes wrap around each other, break and rejoin at points called chiasmata (singular chiasma). Sections of chromatid, and the alleles they carry, are exchanged. This is crossing over.
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Key terms in this lesson
- homologous chromosomes
- A pair of chromosomes, one from each parent, that are the same size and shape and carry the same genes at the same loci.
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