Meiosis and chromosome number explained

Inheritance, variation and evolution • Reproduction

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Typical use of the cells produced by mitosis

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Growth, tissue repair and asexual reproduction, producing identical daughter cells.

Key concepts

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Chromosome number: diploid and haploid

Diploid cells contain two matching sets (pairs) of chromosomes: one set inherited from each parent. Human diploid cells contain 23 pairs (46 chromosomes) as an example. Gametes are haploid and contain a single set of chromosomes (23 in humans). Halving chromosome number in gametes prevents chromosome doubling at each generation and maintains a stable chromosome number across sexual reproduction. 0fileciteturn0file2

How meiosis halves chromosome number

Meiosis begins with a diploid cell that copies all of its genetic material so each chromosome consists of two sister chromatids. The cell then undergoes two rounds of division. The first division separates homologous chromosome pairs into two cells; the second division separates sister chromatids into four cells. Each final cell contains one copy of each chromosome and therefore a single set (haploid). Fertilisation fuses two haploid gametes and restores the diploid chromosome number in the zygote.

Stepwise process of meiosis

Step 1: DNA replication produces duplicated chromosomes (sister chromatids) and the nuclear membrane breaks down. Step 2 (meiosis I): Homologous chromosomes pair and may exchange sections by crossover; homologues then segregate to opposite poles, producing two daughter cells. Step 3 (meiosis II): Chromosomes line up again and sister chromatids separate, producing four haploid daughter cells. The two sequential divisions are the critical reason meiosis halves the chromosome number.

Source of genetic variation in meiosis

Pairing of homologous chromosomes enables DNA crossover, which exchanges chromosome segments between chromatids and produces new allele combinations. Independent assortment during the segregation of chromosome pairs produces many possible combinations of maternal and paternal chromosomes in gametes. The combination of crossover and independent segregation makes each gamete genetically distinct.

Comparison: meiosis versus mitosis

Mitosis produces two genetically identical diploid daughter cells from one diploid parent cell and serves growth and repair. Meiosis produces four non-identical haploid gametes from one diploid parent cell and serves sexual reproduction. The number of nuclear divisions and the genetic outcome (identical versus non-identical cells) distinguishes the two processes.

Biological limits and where meiosis occurs

Meiosis occurs only in specialised sex organs (ovaries and testes in mammals). Timing varies by sex: meiosis in females begins during embryonic development and produces a limited supply of ova; meiosis in males begins at sexual maturity and continues throughout life. The reduced chromosome number in gametes is required for correct development after fertilisation; errors in segregation cause aneuploidy (incorrect chromosome number).

Key notes

Important points to keep in mind

Meiosis includes one DNA replication followed by two cell divisions; that double division causes halving of chromosome number.

Haploid gametes restore diploid number on fusion during fertilisation.

Crossover and independent assortment generate genetic variation among gametes.

Mitosis produces two identical diploid cells; meiosis produces four non-identical haploid cells.

Meiosis occurs only in sex organs and has different timing in males and females.

Chromosome number differs between species; humans have 23 pairs in diploid cells.

Incorrect segregation of chromosomes causes abnormal chromosome numbers and developmental problems.

Remember the sequence: replicate → pair and segregate homologues → separate sister chromatids.