4.3.7Genetics, Variation and Interdependence
Comparing Meiosis and Mitosis — AQA A Level Biology 7402 Revision Notes & Topical Questions
5 min read 9 parts Lesson 20 of 46 1 quiz
Lesson summary
4.3.7 Comparing Meiosis and Mitosis — AQA A Level Biology 7402 notes: Compare mitosis and meiosis in terms of purpose, process, the number and genetic nature of the products, and explain the significance of each.Mitosis is the division of the nucleus that produces two genetically…
- Interphase
- Telophase and cytokinesis
- Exam-style question 1
- Exam-style question 2
- Exam-style question 3
- Genetic variation
Section 1 of 9Purposes
Mitosis is the division of the nucleus that produces two genetically identical diploid daughter cells.
Meiosis
It is used for growth, repair of tissues, replacement of cells and asexual reproduction. Meiosis produces four genetically different haploid cells.
It is used to make gametes in sexual reproduction (or spores in plants), halving the chromosome number so that it is restored at fertilisation,
- and to generate variation.
Section 2 of 9Comparison table
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Daughter cells | Two | Four |
| Chromosome number of daughters | Same as parent (diploid) | Half the parent (haploid) |
| Genetic identity | Identical to the parent and to each other | Different from parent and from each other |
| Pairing of homologous chromosomes | No | Yes, forming bivalents |
| Crossing over | No (normally) | Yes, in prophase I |
| Independent assortment | No | Yes, in metaphase I |
| Where it occurs | All dividing body cells | Reproductive organs |
| Role | Growth, repair, asexual reproduction | Formation of gametes, variation |
Section 3 of 9Comparing the stages
1Interphase
Interphase in both cases DNA replicates once, before division, so each chromosome has two sister chromatids.
2Prophase
Prophase in mitosis chromosomes condense and the nuclear envelope breaks down; in meiosis I homologous chromosomes pair and cross over.
3Metaphase
Metaphase in mitosis single chromosomes line up on the equator; in meiosis I pairs line up, and in meiosis II single chromosomes line up.
4Anaphase
Anaphase in mitosis, centromeres divide and chromatids are separated; in meiosis I whole chromosomes separate and centromeres do not divide,
- and in meiosis II the centromeres divide and chromatids are separated.
5Telophase and cytokinesis
Telophase and cytokinesis in both, nuclei re-form and the cytoplasm divides.
Section 4 of 9Significance of the differences
The genetic identity of mitosis is essential for a multicellular organism, because every body cell needs the same set of genes, and for asexual reproduction, where the offspring are clones.
Significance of the differences
The chromosome halving in meiosis is essential in sexual reproduction, otherwise the number would double each generation.
The variation created by crossing over, independent assortment and random fertilisation provides the raw material for natural selection, helping a species to survive changing conditions, for example the evolution of resistance to disease.
Both processes can go wrongerrors in mitosis (and the loss of control over the cell cycle) lead to cancer, while non-disjunction in meiosis leads to chromosome abnormalities in offspring.
Section 5 of 9Calculations and links
Questions often ask for chromosome numbers or DNA amounts.
Calculations and links
Suppose a cell has 2n = 10. At the start of mitosis, after replication, there are 10 chromosomes and 20 chromatids; the daughter cells have 10 each.
For meiosis, the daughter cells after meiosis I have 5 chromosomes (each of two chromatids) and after meiosis II there are four cells with 5 chromosomes, each of one chromatid.
This topic links to the cell cycle, DNA replication, sexual and asexual reproduction, and the genetic diversity of populations.
Practise by writing a one-line summary for each row of the comparison table and checking that you could explain the reason for each difference.
Section 6 of 9Exam technique for comparison questions
A comparison question requires you to state a point about both, not just one.
1
Do not write that 'mitosis produces identical cells' and leave the meiosis part out.
2
Use comparative words such as 'whereas', 'but' and 'in contrast'.
3
A good approach is to prepare for the most common comparison points: number of divisions, number of cells, chromosome number, genetic identity, pairing and crossing over,
- and function.
4
Think about what is the same, too.
5
In both, DNA is replicated once during interphase, spindle fibres attach to centromeres, chromosomes condense,
- and the cell is under control of the cell cycle.
6
Both occur in eukaryotic cells and involve movement of chromosomes by the spindle, which needs ATP.
You might also be asked to explain why meiosis provides an advantage over asexual reproduction.
Exam technique for comparison questions
The answer is that the genetic variation among the offspring means that some are likely to survive if the environment changes, for instance if a new pathogen appears,
- whereas clones are all equally vulnerable.
Asexual reproduction, however, can produce many offspring quickly without needing a mate, which is an advantage in a stable environment.
Section 7 of 9Exam-style questions
Give three differences between mitosis and meiosis. [3]3 marks
Exam-style questionsMitosis has one division and meiosis has two. [1]Mitosis gives two genetically identical diploid cells whereas meiosis gives four genetically different haploid cells. [1]Homologous chromosomes pair and cross over in meiosis but not in mitosis. [1]Explain why it is important that gametes are produced by meiosis and not by mitosis. [3]3 marks
Exam-style questionsMeiosis halves the chromosome number so that the diploid number is restored at fertilisation. [1]With mitosis the number would double each generation. [1]Meiosis also produces genetic variation, through crossing over and independent assortment. [1]A cell with 2n = 12 divides. State the number of chromosomes in each daughter cell if the division is (a) mitosis (b) meiosis. [2]2 marks
(a) 12 chromosomes. [1]
(b) 6 chromosomes. [1]
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Mitosis | Nuclear division giving two genetically identical cells. |
| Meiosis | Division giving four genetically different haploid cells. |
| Diploid | Having two sets of chromosomes. |
| Haploid | Having one set of chromosomes. |
| Genetic variation | Differences in alleles between individuals. |
| Cytokinesis | Division of the cytoplasm after nuclear division. |
| Clone | A genetically identical organism or cell. |
Section 9 of 9Exam technique
1
Saying meiosis produces two cells, or that mitosis produces four.
2
Writing that chromosomes line up in pairs in mitosis.
3
Forgetting that in meiosis II the chromatids are not identical because of crossing over.
4
Saying that variation arises in mitosis. It is only through mutation.
5
Confusing a cell with 46 chromosomes after meiosis I with a diploid cell; it is haploid but each chromosome still has two chromatids.
Marked practiceQuizzes for this lesson
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