10.1 Essential ideas

10.1.1 Meiosis: a closer look

  • You have already learned in 3.1.3 of the two functions of meiosis:
    • To halve the chromosome number – during meiosis diploid cells divide to form haploid cells. The reduction of chromosome number occurs in meiosis I.
    • To produce genetic variety – bits of DNA are exchanged between non-sister chromatids in prophase I, through a process called crossing over. Furthermore, there is random assortment of chromosomes during metaphase I. 

Behaviour of chromosomes during prophase I

  • DNA replication occurs during the S-phase of interphase (before the start of meiosis).
  • In the earliest stage of prophase I, the duplicated chromosomes condense, shorten and coil to form a single body composed of two sister chromatids.
  • As prophase I progresses, homologous chromosomes migrate towards each other and line up in pairs. This pairing is called synapsis.

Figure 10.1.1a – BivalentFigure 10.1.1a – Bivalent
Homologous chromosomes during synapsis.

  • A group of four chromatids, or a tetrad, results from synapsis. This is also known as a bivalent because it is a pair of homologous chromosomes.
  • In some cells, a protein complex, called the synaptonemal complex, forms between the homologous pairs.

Crossing over and genetic recombination

  • Crossing over is the exchange of DNA material between non-sister homologous chromatids during prophase I.
  • Crossing over occurs when there are breaks in the DNA structure at identical locations on sister chromatids.
  • Non-sister chromatids invade homologous sequences and bind to the region of the break.

Figure 10.1.1b – Crossing over in non-sister chromatidsFigure 10.1.1b – Crossing over in non-sister chromatids

  • The crossover points are called chiasmata (singular: chiasma).
  • Chiasmata formation between non-sister chromatids can result in an exchange of alleles.

Figure 10.1.1c – ChiasmataFigure 10.1.1c – Chiasmata
TEM micrograph (top) and diagram (bottom) showing locations of chiasmata

Skill: Drawing chiasmata formed by crossing over

Figure 10.1.1d – Diagram of chiasmataFigure 10.1.1d – Diagram of chiasmata
Drawing a chiasma formed by crossing over.

  1. Use different colours to represent homologous chromosomes.
  2. Draw chiasma formation as a series of small diagrams.
  3. The chromosomes should be shown very close together on the page. They should be arranged in a tetrad both before and after formation of the chiasma.
  4. Make sure only non-sister chromatids cross over.
  5. Labels may not be necessary. If you are asked for diagrams, label the centromere, bivalent, sister chromatids, homologous chromosomes, and chiasma only once.

Distinguishing meiosis I and meiosis II

  • The main difference between the first and second meiotic divisions is that homologous pairs separate during meiosis I, and sister chromatids separate in meiosis II.

Figure 10.1.1e – Summary of meiosis I and meiosis IIFigure 10.1.1e – Summary of meiosis I and meiosis II

  • The genetic variety that is found in daughter cells at the end of meiosis can be traced back to meiosis I:
    • During prophase I, genetic variety is introduced via crossing over.
    • During metaphase I, tetrads line up at the equator of the cell. Homologous chromosomes orientate randomly, so that when they separate (during anaphase I) they have an equal chance of ending up at either pole.

Random orientation and independent assortment

  • One of the principles that Mendel discovered in his pea-breeding experiments was the ‘principle of independent assortment’.
  • Independent assortment means that how the alleles of one gene separate does not depend on how the alleles of another gene separate.
  • Assuming there is independent assortment, for every number (n) of chromosomal pairs, there should be 2n different chromosome combinations in the daughter cells.

Figure 10.1.1f – Principle of independent assortmentFigure 10.1.1f – Principle of independent assortment

Essential idea:

Meiosis leads to unique allele combinations and independent assortment of chromosomes.

Figure 10.1.1g – Lots of crossing overFigure 10.1.1g – Lots of crossing over

Did you know?

DNA is also exchanged between sister chromatids, but these exchanges do not result in genetic recombination and so are not classified as crossing over.

Figure 10.1.1h – Siblings Figure 10.1.1h – SiblingsFigure 10.1.1h – Siblings

Food for thought

Random assortment of homologues produces 8,388,608 (n = 23, so 223) possible chromosome combinations in humans!

Figure 10.1.1i – Cell cycleFigure 10.1.1i – Cell cycle

Course links

  • Interphase is an active part of the cell cycle 1.2.6.
  • Review the basics of meiosis in 3.1.3.
  • Learn the specifics of animal gametogenesis in 11.1.4a.