The role of meiosis, errors in meiosis, and mitosis compared

25 min

A fruit without seeds

Cut open a banana from a Rwandan farm and you will find no hard seeds, only tiny dark specks. Farmers never plant banana seeds; they dig up the suckers that grow from the base of a mature plant and replant them. The reason lies in meiosis. The bananas we eat are triploid: their cells have three sets of chromosomes. Three sets cannot be shared evenly between two cells, so meiosis in the flowers goes wrong and almost no working gametes are made. This lesson asks what meiosis normally achieves, what happens when it fails, and how it compares with mitosis.

The role of meiosis

It halves the chromosome number. Gametes are haploid, so when two fuse at fertilisation the zygote is diploid again. Without meiosis the chromosome number would double in every generation.
It creates genetic variation in three ways:
- Independent assortment in metaphase I: each bivalent orients at random, so with $n$ pairs of chromosomes there are $2^{n}$ possible combinations of maternal and paternal chromosomes in a gamete.
- Crossing over in prophase I: new combinations of alleles on the same chromosome (Lesson 3).
- Random fertilisation: any one of a vast number of different sperm can fuse with any one of a vast number of different eggs.
Variation is the raw material for natural selection. In a varied population some individuals may survive a new disease, drought or pest, so the population can adapt.

Where meiosis happens depends on the life cycle. In animals it happens in the testes and ovaries and produces the gametes directly. In flowering plants it happens in the anthers and the ovules and produces spores, which then divide by mitosis to form the pollen grains and the embryo sac that contain the gametes. In many fungi and algae the zygote itself divides by meiosis.

Worked example 1: how much variation?

(a) How many genetically different gametes can a human ( $n = 23$ ) make by independent assortment alone? (b) How many different zygotes can one couple produce in this way? (c) Repeat (a) for cassava ( $2 n = 36$ ).

(a) $2^{23} = 8\,388\,608$ , about 8.4 million.
(b) Each zygote combines one of $2^{23}$ eggs with one of $2^{23}$ sperm: $(2^{23})^2 = 2^{46} \approx 7.0 \times 10^{13}$ .
(c) $n = 18$ : $2^{18} = 262\,144$ .

These are underestimates: crossing over in every meiosis creates further new combinations, so in practice no two gametes are likely to be identical.

Definition

Non-disjunction: the failure of homologous chromosomes (in meiosis I) or of sister chromatids (in meiosis II) to separate, so that gametes receive one too many or one too few chromosomes.

Aneuploidy: having one or more chromosomes more or fewer than the normal number, e.g. trisomy (2n + 1) or monosomy (2n − 1). Polyploidy: having extra complete sets of chromosomes.

When meiosis goes wrong

Down syndrome (trisomy 21): three copies of chromosome 21, 47 chromosomes in all. It causes learning difficulties of varying degree and a higher risk of heart defects, and people with Down syndrome lead full lives with good support. Most cases arise from non-disjunction in the mother's eggs. The risk rises with the mother's age, partly because every egg cell has been held paused in prophase I since before the mother herself was born, sometimes for over 40 years.
Turner syndrome (45, X): a female with one X chromosome and no second sex chromosome.
Klinefelter syndrome (47, XXY): a male with an extra X chromosome.

Polyploidy and sterility. In a triploid banana, each kind of chromosome has three homologues. At metaphase I they cannot all pair, and they are shared between the two poles at random, two to one pole and one to the other. With 11 kinds of chromosome, the chance that a gamete gets a balanced set is tiny, so the plant is almost sterile and its fruit develops without seeds. Bread wheat is hexaploid but fertile, because its six sets are three different pairs of genomes, and each chromosome still has exactly one partner.

Worked example 2: counting after non-disjunction

In a human testis, the chromosome 21 pair fails to separate in meiosis I in one cell; meiosis II is normal. (a) How many chromosomes are in each of the four sperm? (b) What happens if each fertilises a normal egg?

(a) After meiosis I, one cell has 24 chromosomes (both 21s) and the other 22 (no 21). Meiosis II gives two sperm with 24 and two with 22.
(b) A normal egg has 23. $24 + 23 = 47$ : trisomy 21, Down syndrome. $22 + 23 = 45$ : monosomy 21, which is not viable, so the embryo is lost early in pregnancy.

Compare non-disjunction in meiosis II (Figure 1, right): only two of the four gametes are abnormal.

Mitosis and meiosis compared

Feature	Mitosis	Meiosis
Number of divisions	One	Two (meiosis I and II)
Cells produced	Two	Four
Chromosome number	Unchanged: 2n → 2n (or n → n)	Halved: 2n → n
Genetic make-up of the products	Identical to each other and the parent	Different from each other and the parent
Synapsis and bivalents	Never	In prophase I
Crossing over and chiasmata	No	Yes, in prophase I
What lines up at metaphase	Individual chromosomes	Bivalents (metaphase I); individual chromosomes (metaphase II)
What separates at anaphase	Sister chromatids	Homologous chromosomes (anaphase I); sister chromatids (anaphase II)
Where in animals	Body cells everywhere	Testes and ovaries only
Function	Growth, repair, asexual reproduction	Gametes or spores for sexual reproduction; variation

LabXchange: "Nondisjunction in Meiosis". Watch for the difference between non-disjunction in meiosis I and in meiosis II, and how each leads to trisomy or monosomy after fertilisation.

Warning

Common mistakes

"Meiosis only happens in animals." Plants and fungi use meiosis too; in plants it makes spores.
Writing that non-disjunction in meiosis II makes all four gametes abnormal. Only the cell in which the chromatids failed to separate is affected: two abnormal, two normal.
Calling Down syndrome a "disease" that can be caught. It is a chromosome difference present from conception.

Tip

Comparison questions: write paired statements, one for each division, on the same feature: "In mitosis, sister chromatids separate at anaphase; in meiosis I, homologous chromosomes separate." Unpaired facts score less.

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Practice: the role of meiosis and comparison

Explain why meiosis is necessary in organisms that reproduce sexually.
Name three sources of genetic variation in sexual reproduction, and say when each happens.
How many combinations of chromosomes can a bean plant ( $2 n = 22$ ) produce in its gametes by independent assortment alone?
Define non-disjunction and aneuploidy.
A gamete with 22 chromosomes fertilises a normal human gamete. How many chromosomes will the zygote have, and what is this condition called?
Explain why triploid bananas are seedless and must be grown from suckers.
Give four differences between mitosis and meiosis.
Suggest a disadvantage to farmers of growing only bananas propagated from suckers.

Show answersHide answers

It halves the chromosome number so that fertilisation restores the diploid number, keeping it constant from generation to generation; it also creates genetic variation.
Crossing over (prophase I), independent assortment (metaphase I), and random fertilisation (when gametes fuse).
$n = 11$ : $2^{11} = 2\,048$ .
Non-disjunction: failure of homologues or sister chromatids to separate in meiosis. Aneuploidy: a chromosome number that is one or more above or below the normal number.
22 + 23 = 45 chromosomes: monosomy (2n − 1).
With three homologues of each chromosome, pairing and separation in meiosis are irregular and almost all gametes get unbalanced sets, so fertilisation and seed formation fail; the plants must be propagated vegetatively.
Any four from the table: one versus two divisions; two versus four cells; diploid versus haploid products; identical versus varied products; no synapsis versus bivalents; no crossing over versus chiasmata; sister chromatids versus homologues separate at anaphase (I).
All plants of a variety are genetically identical clones, so a disease or pest that attacks one can attack them all, and there is no variation for selection or breeding.

Key points

Meiosis halves the chromosome number and creates variation by crossing over, independent assortment ( $2^{n}$ ) and random fertilisation.
Non-disjunction gives gametes with n + 1 or n − 1 chromosomes, causing trisomy (e.g. Down syndrome) or monosomy.
Triploid bananas are sterile because three homologues cannot separate evenly.
Mitosis: one division, identical diploid cells. Meiosis: two divisions, four different haploid cells.

1A fruit without seeds

2The role of meiosis

3When meiosis goes wrong

4Mitosis and meiosis compared

A fruit without seeds

The role of meiosis

When meiosis goes wrong

Mitosis and meiosis compared