Prophase 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.
• As in mitosis, the nucleolus disappears, the nuclear envelope breaks down and a spindle forms (from centrioles that move to opposite poles, in animal cells).Crossing over produces new combinations of alleles on the chromatids. For example, if one chromosome carries alleles A and B and its homologue carries a and b, crossing over between the two gene loci can produce chromatids carrying A with b, and a with B. These are called recombinant chromatids.The further apart two genes are on a chromosome, the more likely a chiasma is to form between them, so the more often they are separated by crossing over. Genes very close together are usually inherited together: they are linked.Because crossing over mixes maternal and paternal alleles, the four chromatids in a bivalent are no longer identical, which increases genetic variation among the gametes.