20 June 2016

#130 Meiosis

Meiosis involves two divisions, called meiosis I and meiosis II.

Meiosis I:

  • reduction division 
  • resulting in 2 daughter nuclei (haploid)  half the number of chromosomes of the parent nucleus

Meiosis II:

  • behaves like mitosis
  • results in total of 4 haploid nuclei

Meiosis I
1. Prophase I
  • centrosomes divide and move to opposite poles of nucleus --> spindle formation
  • homologous chromosomes pair up --> crossing over and form bivalents
  • nucleus disappears
  • nuclear envelope breaks down
2. Metaphase I
  • bivalents line up at equator
  • spindles attach to centromere
3. Anaphase I
  • whole chromosomes move to opposite poles of spindle
  • pulled by microtubules
4. Telophase I
  • nuclear envelope reforms
  • nucleolus reform

Meiosis II
5. Prophase II
  • nuclear envelope breaks down
  • nucleolus disappears
  • centrosomes and centrioles replicate; move to opposite poles

6. Metaphase II
  • chromosomes line up separately across equator of spindle

7. Anaphase II
  • centromeres divide
  • spindle microtubules pull chromatids to opposite poles

8. Telophase II
  • 4 haploid daughter cells

How meiosis causes variation
Crossing over:
  • prophase of meoisis I
  • chromatids of 2 homologous chromosomes break and rejoin --> part of one chromatid swaps places with the same part of the other (exchange of gene loci between a maternal and paternal chromatid)
  • chiasmata: point where crossing over occurs
Recombinant organisms result from crossing over and so 'recombines' the characteristics of the parent organisms. The cross over value is the percentage of offspring that belong to the recombinant class.

Independent assortment
  • random alignment of bivalents on the equator during meiosis I
  • different alleles of genes on different chromosomes may end up in any combination in gametes

  Syllabus 2016-2018

16.1 Passage of information from parent to offspring 

Diploid organisms contain pairs of homologous chromosomes. The behaviour of maternal and paternal chromosomes during meiosis generates much variation amongst individuals of the next generation. 

a) explain what is meant by homologous pairs of chromosomes 

b) explain the meanings of the terms haploid and diploid and the need for a reduction division (meiosis) prior to fertilisation in sexual reproduction 

c) outline the role of meiosis in gametogenesis in humans and in the formation of pollen grains and embryo sacs in flowering plants 

d) describe, with the aid of photomicrographs and diagrams, the behaviour of chromosomes in plant and animal cells during meiosis, and the associated behaviour of the nuclear envelope, cell surface membrane and the spindle (names of the main stages are expected, but not the sub-divisions of prophase) 

e) explain how crossing over and random assortment of homologous chromosomes during meiosis and random fusion of gametes at fertilisation lead to genetic variation including the expression of rare, recessive alleles


  1. In prophase the nucleolus disappears not the nucleus.

  2. It's true that the nucleolus disappears, but by definition if the whole nuclear envelope breaks down, and the chromatin has condensed into shortened chromosomes, there is no nucleus.