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Meiosis. Homologous Chromosomes. Homologous chromosomes : pairs of chromosomes with genes for the same traits, but they can have different information about those traits. ½ the chromosomes in a cell come from Mom, ½ the chromosomes in a cell come from Dad. The Homolog’s. Humans.

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Homologous chromosomes
Homologous Chromosomes

  • Homologous chromosomes: pairs of chromosomes with genes for the same traits, but they can have different information about those traits

  • ½ the chromosomes in a cell come from Mom, ½ the chromosomes in a cell come from Dad


  • Humans have 46 chromosomes or 23 pairs of homologous chromosomes.

  • Somatic(body) cells contain 46 chromosomes.

  • 46 is the diploid (2N) number for humans.

    - Diploid means that a cell has both homologs for each chromosome

  • Somatic cells reproduce by mitosis

Reproductive cells
Reproductive Cells

  • Gametes(eggs and sperm) are called haploid (N)because they contain one set of chromosomes.

  • Eggs (Ovum)and sperm contain 23 chromosomes.

  • Eggs and sperm are produced by meiosis- a process that reduces chromosomes by half

Phases of meiosis
Phases of Meiosis

  • Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosome in a diploid cell.

  • Two steps

    • Meiosis I and Meiosis II

  • One diploid cell gives rise to 4 haploid cells

Who does it
Who does it?

  • Plants

  • Fungi

  • Animals (including you)

    • Males start meiosis at puberty.

    • Females start meiosis in utero.

Where does it occur
Where does it occur?

  • In your gonads

    • Females- the ovary

    • Males-the testicles

Why does it occur
Why does it occur?

  • Genetic Variation

    • Mixes genes from Mom and Dad to create a unique baby

  • Reduce the chromosome number by half

  • Ensures that each cell produced has one copy from each homologous pair of chromosomes

  • 23 in sperm + 23 in egg unite (fertilization) leads to a 46 zygote(baby)

Meiosis i
Meiosis I

  • Prior to this phase, each chromosome is replicated

    • Then its looks a lot like Mitosis with 4 phases

      • Prophase I

      • Metaphase I

      • Anaphase I

      • Telophase I and Cytokinesis

  • The major difference is what happens in Prophase I

Prophase i
Prophase I

  • Homologous chromosomes join to form a tetrad

  • Chromosomes in the tetrad exchange portions of themselves in process known as crossing-over

Crossing over

  • Results in new genetic combinations

    - It is another way to increase

    genetic diversity


Meiosis ii
Meiosis II

  • This time no chromosome replication occurs before entering prophase II.

    • Again it looks similar to Mitosis but now we have a separation of the chromosomes

      • Resulting in four new cells with half number of chromosomes (haploid cells)



2N = 4

Metaphase II

Telophase I

Anaphase II

Anaphase I

Interphase I

Metaphase I


Prophase I

N = Number of Chromosomes

Comparing mitosis and meiosis
Comparing Mitosis and Meiosis

  • Mitosis results in the production of two genetically identical diploid cells.

    • Allows an organism to grow, reproduce, and replace cells.

  • Meiosis results in the production of four genetically different haploid cells.

    • Allows organisms to reproduce sexually