CIE A Level Biology (9700) 2019-2021

Revision Notes

16.1.4 Meiosis in Animal & Plant Cells

Meiosis in Animal & Plant Cells

  • Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells
  • It produces gametes in plants and animals that are used in sexual reproduction
  • It has many similarities to mitosis however it has two divisions: meiosis I and meiosis II
  • Within each division there are the following stages: prophase, metaphase, anaphase and telophase

Prophase I

  • DNA condenses and becomes visible as chromosomes
  • DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere
  • The chromosomes are arranged side by side in homologous pairs
    • A pair of homologous chromosomes is called a bivalent
  • As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)
  • In this stage centrioles migrate to opposite poles and the spindle is formed
  • The nuclear envelope breaks down and the nucleolus disintegrates

Metaphase I

  • The bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres

Anaphase I

  • The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle
  • The centromeres do not divide

Telophase I

  • The chromosomes arrive at opposite poles
  • Spindle fibres start to break down
  • Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
  • Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I

Cytokinesis

  • This is when the division of the cytoplasm occurs
  • Cell organelles also get distributed between the two developing cells
  • In animal cells: the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half
  • In plant cells, vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The vesicles merge with each other to form the new cell surface membrane and also secrete a layer of calcium pectate which becomes the middle lamella. Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell
  • The end product of cytokinesis in meiosis I is two complete haploid cells

Second division of Meiosis : Meiosis II

  • There is no interphase between meiosis I and meiosis II so the DNA is not replicated
  • The second division of meiosis is almost identical to the stages of mitosis
  • Prophase II
    • The nuclear envelope breaks down and chromosomes condense
    • A spindle forms at a right angle to the old one
  • Metaphase II
    • Chromosomes line up in a single file along the equator of the spindle
  • Anaphase II
    • Centromeres divide and individual chromatids are pulled to opposite poles
    • This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell
  • Telophase II
    • Nuclear membranes form around each group of chromosomes
  • Cytokinesis
    • Cytoplasm divides as new cell surface membranes are formed creating four haploid cells

Exam Tip

Understanding the difference between chromosomes and chromatids can be difficult. We count chromosomes by the number of centromeres present. So when the 46 chromosomes duplicate during interphase and the amount of DNA in the cell doubles there are still only 46 chromosomes present because there are still only 46 centromeres present. However, there are now 92 chromatids, which are strands of replicated chromosomes.

Identifying the Stages of Meiosis

  • Cells undergoing meiosis can be observed and photographed using specialised microscopes
  • The different stages of meiosis have distinctive characteristics meaning they can be identified from photomicrographs or diagrams

Meiosis I or Meiosis II

  • Homologous chromosomes pair up side by side in meiosis I only
  • This means if there are pairs of chromosomes in a diagram or photomicrograph meiosis I must be occurring
  • The number of cells forming can help distinguish between meiosis I and II
  • If there are two new cells forming it is meiosis I but if there are four new cells forming it is meiosis II

The distinguishing features at each stage of Meiosis I

  • Prophase I: Homologous pairs of chromosomes are visible
  • Metaphase I: Homologous pairs are lined up side by side along the equator of spindle
  • Anaphase I: Whole chromosomes are being pulled to opposite poles with centromeres intact
  • Telophase I: There are 2 groups of condensed chromosomes around which nuclei membranes are forming
  • Cytokinesis: Cytoplasm is dividing and cell membrane is pinching inwards to form two cells

The distinguishing features at each stage of Meiosis II

  • Prophase II: Single whole chromosomes are visible
  • Metaphase II: Single whole chromosomes are lined up along the equator of the spindle in single file (at 90 degree angle to the old spindle)
  • Anaphase II: Centromeres divide and chromatids are being pulled to opposite poles
  • Telophase II: Nuclei are forming around the 4 groups of condensed chromosomes
  • Cytokinesis: Cytoplasm is dividing and four haploid cells are forming

Identifying the stages of meiosis table

Identifying the Stages of Meiosis Table, downloadable AS & A Level Biology revision notes

Exam Tip

For metaphase remember M for the middle of the spindle and cell which is where the chromosomes will be lined up.

For anaphase remember A for away from the middle to the poles, which is where the chromosomes / chromatids are being pulled.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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