Biology 1 - Lesson 12: Meiosis and Genetic Variation
Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing haploid gametes or spores from diploid cells. This process underlies sexual reproduction in eukaryotes, ensuring offspring inherit genetic material from both parents. Meiosis promotes genetic variation through independent assortment and crossing over, driving evolutionary potential in populations.
Comparison of Mitosis and Meiosis
Before examining meiosis in detail, it is helpful to contrast it with mitosis. While mitosis maintains the diploid chromosome set, meiosis creates haploid cells:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of Divisions | 1 | 2 (Meiosis I and Meiosis II) |
| Number of Daughter Cells | 2, genetically identical | 4, genetically distinct |
| Chromosome Number | Maintained (diploid → diploid) | Halved (diploid → haploid) |
| Role | Growth, tissue repair, asexual reproduction | Sexual reproduction, genetic diversity |
Phases of Meiosis
Meiosis consists of two successive divisions, Meiosis I and Meiosis II, each with its own subphases (prophase, metaphase, anaphase, telophase). Importantly, DNA replication occurs only once—before Meiosis I.
DNA Replication)] --> B[Meiosis I
Homologous chromosomes separate
Diploid -> Haploid] B --> C[Meiosis II
Sister chromatids separate
Four haploid cells]
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Meiosis I (Reductional Division):
- Prophase I: Homologous chromosomes pair (synapsis) forming tetrads. Crossing over (exchange of genetic material) occurs at chiasmata, creating recombined chromosomes.
- Metaphase I: Homologous pairs line up at the metaphase plate. Independent assortment ensures random orientation of maternal and paternal homologs.
- Anaphase I: Homologous chromosomes (each with two sister chromatids) move to opposite poles. Sister chromatids remain attached.
- Telophase I & Cytokinesis: Two haploid cells form, although each chromosome still has sister chromatids.
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Meiosis II (Equational Division):
- Prophase II: Spindle reforms in each haploid cell. Chromosomes still consist of two sister chromatids.
- Metaphase II: Individual chromosomes (sister chromatids) align at the metaphase plate.
- Anaphase II: Sister chromatids separate, moving to opposite poles as individual chromosomes.
- Telophase II & Cytokinesis: Nuclei re-form around separated chromosomes, resulting in four haploid cells, each genetically distinct from the parent cell and one another.
Sources of Genetic Variation
Meiosis introduces diversity in several ways:
- Crossing Over: Occurs during Prophase I, when homologous chromosomes exchange segments at chiasmata. This shuffles alleles between maternal and paternal chromosomes.
- Independent Assortment: In Metaphase I, different combinations of maternal and paternal homologs randomly align at the metaphase plate, generating a vast array of possible chromosome distributions (2n combinations for n pairs).
- Random Fertilization: The fusion of gametes from two individuals adds another layer of variation, exponentially increasing the range of possible offspring genotypes.
Biological Significance
Sexual reproduction and meiosis underlie genetic variability in populations, fueling evolution by natural selection. Offspring inherit novel combinations of alleles, some of which may confer advantages (or disadvantages) in changing environments. Over generations, this genetic diversity shapes adaptation, speciation, and the dynamics of ecosystems.
Conclusion
Meiosis and the formation of haploid gametes enable sexually reproducing species to shuffle genetic information across generations. Through crossing over, independent assortment, and random fertilization, meiosis fosters biological variation, ultimately influencing the evolutionary trajectories of populations and the diversity of life on Earth.
