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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Meiosis Process
Meiosis is a special type of cell division that produces gametes (sex cells) in sexually reproducing organisms. Unlike mitosis, which creates genetically identical cells, meiosis creates cells with half the number of chromosomes, which is crucial for sexual reproduction.
Key Definitions:
Meiosis is essential for sexual reproduction because it:
Humans have 46 chromosomes (23 pairs) in most cells. After meiosis, gametes contain 23 chromosomes (one from each pair). When fertilisation occurs, the zygote receives 23 chromosomes from each parent, restoring the diploid number of 46.
Meiosis consists of two consecutive cell divisions, called meiosis I and meiosis II, with distinct phases in each division. The entire process transforms one diploid cell into four haploid cells.
The first division is where the chromosome number is reduced from diploid to haploid. It includes several phases:
The longest and most complex phase:
Chromosomes line up:
Separation begins:
The second division resembles mitosis but occurs without DNA replication. Each haploid cell from meiosis I divides again:
Preparation for second division:
Alignment for separation:
Final separation:
In humans, meiosis occurs differently in males and females:
This difference contributes to the fact that women have a limited number of eggs, while men continuously produce sperm.
Understanding the differences between these two types of cell division is crucial for your IGCSE exam:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Number of daughter cells | Two | Four |
| Chromosome number | Maintained (diploid to diploid) | Halved (diploid to haploid) |
| Genetic variation | None (identical cells) | High (due to crossing over and random assortment) |
| Purpose | Growth, repair, asexual reproduction | Gamete production for sexual reproduction |
| Crossing over | Does not occur | Occurs during prophase I |
Meiosis creates genetic diversity through three main mechanisms:
During prophase I, homologous chromosomes exchange genetic material. This creates new combinations of genes on each chromosome.
The random alignment of chromosome pairs during metaphase I means maternal and paternal chromosomes can be distributed in many different combinations.
When gametes combine during fertilisation, any sperm can fertilise any egg, adding another layer of genetic variation.
The independent assortment of chromosomes alone can produce 2^23 (over 8 million) possible combinations of chromosomes in human gametes. When you add crossing over and random fertilisation, the potential genetic diversity is practically limitless!
This explains why siblings (except identical twins) can look so different from each other, even though they have the same parents.
Different organisms use meiosis at different points in their life cycles:
In animals (including humans), most cells are diploid. Meiosis occurs only in specialised cells in reproductive organs to produce haploid gametes. After fertilisation, the zygote is diploid again.
Many plants have a more complex life cycle with alternation of generations. They have both diploid (sporophyte) and haploid (gametophyte) stages. Meiosis produces spores, which grow into the haploid generation.
Sometimes meiosis doesn't proceed correctly, leading to chromosomal abnormalities:
This occurs when chromosomes fail to separate properly during meiosis. It can result in gametes with too many or too few chromosomes. When these gametes participate in fertilisation, the resulting zygote may have chromosomal disorders like Down syndrome (trisomy 21).
Meiosis is a fundamental process that:
Understanding meiosis helps explain inheritance patterns, genetic disorders and the incredible diversity of life on Earth.
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