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3.1 Essential ideas

3.1.3 Meiosis

  • Meiosis is a form of nuclear division in which one diploid nucleus divides to produce four genetically different haploid nuclei.

Figure 3.1.3a – Comparing meiosis (left) and mitosis (right)Figure 3.1.3a – Comparing meiosis (left) and mitosis (right)

  • Mature haploid cells are called gametes. When gametes fuse during sexual reproduction, the result is a diploid zygote.
  • Halving the number of chromosomes through meiosis allows organisms to have a sexual life cycle.

Figure 3.1.3b – Meiosis is necessary to conserve chromosome number in a sexual life cycleFigure 3.1.3b – Meiosis is necessary to conserve chromosome number in a sexual life cycle

  • Sexual reproduction, via the fusion of gametes from different parents, promotes genetic variation.

Meiosis results in genetic variation

  • Meiosis is a single process. However, there are two separate division events: meiosis i and meiosis II.
  • During the first division, the chromosome number is halved and homologous chromosomes separate randomly.
  • During the second division, sister chromatids are separated, resulting in four genetically unique haploid daughter cells.

Crossing over and random orientation

  • DNA replication occurs before the start of meiosis, so that all chromosomes consist of two sister chromatids.

Figure 3.1.3c – Sister chromatids and homologous chromosomesFigure 3.1.3c – Sister chromatids and homologous chromosomes

Figure 3.1.3d – Crossing over occurs between non-sister chromatids during prophase I of meiosisFigure 3.1.3d – Crossing over occurs between non-sister chromatids during prophase I of meiosis

  • Crossing over occurs at points called chiasmata (singular chiasma).
  • The result of crossing over is that sister chromatids can have different combinations of alleles. Some of the alleles from the maternal chromosome become part of the paternal chromosome and vice versa.
  • Crossing over results in genetic recombination.
  • During metaphase I, homologous chromosomes line up as bivalents at the equator of the cell.
  • The pairs line up randomly, meaning that each chromosome in the pair has an equal chance of travelling to either pole during anaphase I.

Figure 3.1.3e – Four possible orientation patterns for three pairs of homologous chromosomesFigure 3.1.3e – Four possible orientation patterns for three pairs of homologous chromosomes

  • The result of random orientation is that each daughter cell will inherit an unpredictable combination of paternal and maternal chromosomes.
  • Crossing over and random orientation of homologous chromosomes prior to separation ensure that there is genetic variation in the cells resulting from meiosis.

Skill: The stages of meiosis

Figure 3.1.3f – The stages of meiosis I (images 1–5), and meiosis II (images 6–10) in a plant cellFigure 3.1.3f – The stages of meiosis I (images 1–5), and meiosis II (images 6–10) in a plant cell
Source: biologyforhighschool.net

 

 

Image

Description

Meiosis I

 

Prophase I

1

Metaphase I

2

Anaphase I

3 + 4

  • Homologous pairs are separated, one of each pair moves to opposite poles

Telophase I

5

  • Nuclear membrane reforms around two haploid nuclei

Meiosis II

Prophase II

6

Metaphase II

7

Anaphase II

8 + 9

  • Sister chromatids separate and move to opposite poles

Telophase II

10

  • Nuclear membrane reforms around four haploid nuclei


Figure 3.1.3g – Stages of meiosis in an animal cellFigure 3.1.3g – Stages of meiosis in an animal cell

Watch the video to review how a diploid cell divides to form four genetically unique haploid cells during meiosis.

This video will help you understand how random orientation of chromosomes in metaphase I leads to genetic variation in daughter cells.

Essential idea

New genetic combinations result from segregation of alleles during meiosis.

Food for thought

  • Without meiosis, the chromosome number would double with each generation!
  • Why is meiosis I sometimes called the ‘reduction division’?
  • Which of the two divisions is most like mitosis, meiosis i or meiosis II?
  • Why is genetic variation useful for the survival and success of a species?

Concept help

DNA replication occurs during interphase before prophase of meiosis I. There is a short interphase between meiosis I and meiosis II, but there is no further DNA replication.

Course link

Review the stages of mitosis in 1.1.6.