2.6.3(Meiosis)

Define the term haploid

• Having only 1 set of chromosomes, represented by symbol ”n”

Define the term homologous chromosomes

• When maternal and paternal chromosomes are matching in haploid cells , containing the same genes at the same places, however hey may contain different alleles(versions) of some genes.

Describe the significance of variation in meiosis

• Meosis increases the genetic variation in species which increases chances of survival when the enviornment changes, as some individuals will have characteristics that enable them to be beter adapted to the change

State the name of the diploid cells that undergo meiosis in ovaries and testies and what phase of the cell cycle they are in before entering meiosis

• Gonads

• They are in the interphase

State the stages of meiosis

• There are 2 divisions in mitosis and in each division there are 2 stages:

  • Division one:

    1. Prophase 1

    2. Metaphase 1

    3. Anaphase 1

    4. Telophase 1

• There is then a short interphase

  • Division 2:

    1. Prophase 2

    2. Metaphase 2

    3. Anaphase 2

    4. Telophase 2

Describe what happens during Prophase 1

1. Prophase 1:

   • Chromosomes condense

   • Nuclear envolope breaks down

   • Centrioles produce spindle and move to opposite poles of the cell

   • The chromosomes come together in their homologous pairs

   • Crossing over occurs when non-sister chromatids wrap around each other and may swap sections so that alleles are shuffled

Describe what happens during Metaphase 1

2. Metaphase 1:

   • Homologous chromosomes line up at the equator of the cell and spindle fibres attach to the chromosomes by their centromeres

     The homologous pairs are arranged randomly, with the members of each pair facing opposite poles of the cell. the arrangement is independent assortment

Describe what happens during Anaphase 1

3. Anaphase 1:

   • The members of each pair of homologous chromosomes are pulled apart by motor proteins that drag them along the tubulin thread of the spindle

   • The centromeres do not divide, and each chromosome consists of 2 chromatids

   • The crossed over areas separate from each other, resulting in swapped areas of chromosome and allele shuffling

Describe what happens during Telophase 1

4. Telophase 1:

   • Animal cells: In most animal cells, 2 new nuclear envelopes form around each set of chromosomes ad the cell divides by cytokinesis. Then there is a short interphase when the chromosomes uncoil

   • Each new nucleus contains half the original number of chromosomes, but each chromosome consists of 2 chromatids

   • Plant cells: In most animal cells, the cell goes from anaphase1 straight to Prophase 2

Describe what happens during Prophase 2

• Prophase 2:

  • If the nuclear envelope has reformed, then they now brown down.

  • The chromosomes coil and condense, each one consists if 2 chromatids

  • the chromatids of each chromsome are no long identical due to the crossing over in prophase 1

  • Spindles form

Describe what happens during Metaphase 2

• Metaphase 2:

  • The chromosomes attach, by their centromere, to the equator of the spindle

  • The chromatids of each chromosome are randomly arranged

  • The way that they are arranged will determine how the chromatids separate during anaphase

Describe what happens during Anaphase 2

• Anaphase 2:

  • The centromeres divide

  • The chromatids of each chromosome are pulled apart by motor proteins that drag them along tubulin threads of the spindle, towards opposite poles

  • The chromatids are therefore randomly segregated

Describe what happens during Telophase 2

• Telophase 2:

  • Nuclear envelopes form around each of the 2 haploid nuclei

  • In animals, the 2 cells now divide to give 4 haploid cells

  • In plants, a tetrad of 4 haploid cells is formed

Describe how meiosis produces genetic variation

1. Crossing over during prophase 1 shuffles alleles

2. Independent assortment of chromosomes in anaphase 1 leads to random distribution of maternal and paternal chromosomes of each pair

3. Independent assortment of chromatids in anaphase 2 leads to further random distribution of genetic material

4. Haploid gametes are produced, which can undergo random fusion with gametes derived from another organism of the same species