Prophase 1

• what do the paternal and maternal chromosomes do in prophase 1?

  they come together in homologous pairs - this pairing is called synapsis, and each homologous chromosomes pair is a bivalent
• what is a bivalent?

  the association of the 2 chromosomes of a homologous pair at prophase 1 of meiosis 1
• what do the chromosomes do in prophase 1?

  they coil up, condensing to become shorter and thicker, visible as 2 chromatids
• what do the centrioles (only in animals and lower plants) do in prophase?

  • where centrioles are present, the centrioles separate and move to the poles of the cells
  • the centrioles organise the polymerisation of microtubules, which radiate out from them, and the spindle fibres form
• how does prophase 1 differ from prophase of mitosis?

  in prophase 1, the homologous chromosomes associate in their pairs, the bivalents
• what do the chromatids do in prophase 1?

  they wrap around each other and then partially repel each other but remain joined at points called chiasmata
• what happens at a chiasma?

  a segment of DNA from one chromatid may be exchanged with the equivalent part from a chromatid of the homologous chromosome - this swapping is called crossing over
• what is chiasma (plural=chiasmata)?

  the site as seen in the light microscope, at which chromosomes exchange DNA in genetic crossing over
• what is crossing over?

  the reciprocal exchange of genetic material between the chromatids of homologous chromosomes during synapsis in prophase 1 of meiosis
• why is crossing over a source of genetic variation?

  as it mixes genes from the 2 parents in one chromosome - this is called the ‘genetic recombination’
• what does the genetic recombination produce?

  it produces new combinations of alleles
• what can a single cross over occurring during meiosis 1 cause?

  it results in 4 haploid gametes having a different genetic composition
• where can crossing over occur?

  it can happen at several places along the length of the chromatid and so there are huge numbers of different genetic combinations made
• what happens by the end of prophase 1?

  the nuclear envelope has disintegrated and the nucleolus has disappeared