How do cells divide and how does meiosis create genetic variation?
Compare mitosis and meiosis and explain how meiosis generates genetic variation.
The cell cycle, the stages and roles of mitosis and meiosis, and how meiosis produces genetically varied haploid gametes, for TCE Biology Unit 3.
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The cell cycle
A dividing cell passes through a repeating sequence called the cell cycle. Most of the cycle is interphase, made of three phases: G1 (cell growth), S (DNA replication, when each chromosome is copied into two identical sister chromatids), and G2 (preparation for division). Division itself is the mitotic (M) phase, followed by cytokinesis, when the cytoplasm splits. Checkpoints control progression through the cycle, and loss of this control can lead to uncontrolled division and cancer.
Mitosis
Mitosis produces two daughter nuclei that are genetically identical to the parent cell and to each other. It is used for growth, tissue repair, and asexual reproduction. The stages are:
- Prophase: chromosomes condense and become visible as two sister chromatids; the nuclear membrane breaks down; spindle fibres form.
- Metaphase: chromosomes line up single file along the cell equator.
- Anaphase: sister chromatids are pulled apart to opposite poles.
- Telophase: chromosomes reach the poles, nuclear membranes reform, and chromosomes decondense.
Cytokinesis then divides the cytoplasm, giving two diploid cells. Because there is no pairing of homologous chromosomes and no crossing over, the products are clones of the parent.
Meiosis
Meiosis produces four haploid gametes from one diploid cell, through two successive divisions, meiosis I and meiosis II, with only one round of DNA replication beforehand.
Meiosis I separates homologous chromosomes (the matching pairs, one from each parent):
- Prophase I: chromosomes condense, homologous chromosomes pair up to form bivalents, and crossing over occurs.
- Metaphase I: homologous pairs line up at the equator; their orientation is random (independent assortment).
- Anaphase I: whole homologous chromosomes are pulled to opposite poles, halving the chromosome number.
- Telophase I and cytokinesis: two haploid cells form, each chromosome still as two chromatids.
Meiosis II resembles mitosis and separates the sister chromatids of each chromosome, producing four haploid cells in total.
How meiosis generates variation
Meiosis is a major source of genetic variation, in three ways:
- Crossing over: during prophase I, homologous chromosomes exchange segments at points called chiasmata, producing recombinant chromosomes with new allele combinations.
- Independent assortment: in metaphase I, each homologous pair lines up independently, so maternal and paternal chromosomes are mixed randomly between the gametes.
- Random fertilisation: although this happens after meiosis, it multiplies the variation by combining any two gametes.
Together these processes mean that, apart from identical twins, no two offspring are genetically the same.
Comparing mitosis and meiosis
Mitosis gives two identical diploid cells in one division, with no crossing over and no pairing of homologues. Meiosis gives four varied haploid cells in two divisions, with crossing over and independent assortment. Mitosis supports growth and repair; meiosis supports sexual reproduction. Errors in meiosis, such as non-disjunction (chromosomes failing to separate), can lead to gametes with the wrong chromosome number and conditions such as Down syndrome.