Meiosis

1
Meiosis

• Cell Division For Reproductive Cells

2
What is Meiosis?

• Meiosis is cell division for gametes (eggs in
  females and sperm in males)
• It occurs in two major phases, each with 4
  subphases
• Cells in meiotic division are not genetic clones
  like those in mitotic division
• DNA replicates before the beginning of meiosis

3
Meiosis I

• In meiosis I, chromosomes in a diploid cell
  resegregate, producing four haploid daughter
  cells. It is this step in meiosis that generates
  genetic diversity.

4
Stages of Meiosis I

• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• Cytokinesis
• 4 Haploid daughter cells are produced

5
Prophase I

• Homologous chromosomes pair and form synapses
• Bivalents paired chromosomes
• Chromosomes condense
• Bivalent has 2 chromosomes (1 coming from each
  parent cell) and 4 chromatids

6
Metaphase I

• Nuclear membrane disappears
• One kinetichore forms per chromosome
• Chromosomes attached to spindle fibers move
• Bivalents align in the center
• Either parental homologue can be on side,
  leaving a 50-50 chance that a daughter cell will
  get either the mothers or fathers chromosome

7
Anaphase I

• Chiasmata separate
• Chromosomes, each with 2 chromatids, move to
  separate poles
• Each of the chromosomes is haploid (23
  chromosomes), but each chromosome has 2 chromatids

8
Telophase I

• Nuclear envelopes MAY re-form, OR
• The cell may quickly enter Meiosis II

9
Cytokinesis

• Same as in mitosis two complete daughter cells
  form
• These daughter cells are genetically different
  than the parent cells

10
Meiosis II

• Meiosis II is similar to mitosis. However, there
  is no "S" phase. The chromatids of each
  chromosome are no longer identical because of
  recombination. Meiosis II separates the
  chromatids producing two daughter cells each with
  23 chromosomes (haploid), and each chromosome has
  only one chromatid.

11
Prophase II

• Chromatin in the nucleus condense and become
  visible as chromosomes
• Nucleolus disappears
• Centrioles begin migrating toward opposite ends
  of the cell
• Spindle fibers extend from the centromeres and
  may cross the cell to form the mitotic spindle

12
Metaphase II

• Nuclear membrane dissolves
• Proteins attach to the centromeres creating the
  kinetichores
• Microtubules attach at the kinetichores and the
  chromosomes begin moving
• Spindle fibers align the chromosomes along the
  middle of the cell nucleus (metaphase plate)
• This ensures that each new nucleus will receive 1
  copy of each chromosome.

13
Anaphase II

• Paired chromosomes separate at the kinetochores
  and move to opposite sides of the cell
• Unpaired chromosomes are called chromatids

14
Telophase II

• Chromatids arrive at opposite ends of the cell
• Nuclear membranes re-form around daughter nuclei
• Chromosomes disperse and are no longer visible
• Spindle fibers disappear

15
Cytokinesis

• Same as in mitosis two haploid daughter cells
  form
• These daughter cells are genetically different
  than the parent cells

16
Animations

• Meiosis I
• http//www.biology.arizona.edu/cell_bio/tutorials/
  meiosis/meiosis1_movie.html

• Meiosis II
• http//www.biology.arizona.edu/cell_bio/tutorials/
  meiosis/meiosis2_movie.html

17
Comparing Mitosis Meiosis

• Mitosis

• Meiosis

• Chromosome Behavior
• Homologous chromosomes independent
• Chromosome Number
• Identical daughter cells
• Genetic identity of progeny
• Identical daughter cells

• Chromosome Behavior
• Homologous chromosomes pair forming bivalents
  until anaphase I
• Chromosome Number
• Reduction in meiosis daughter cells haploid
• Genetic identity of progeny
• daughter cells have new assortment of parental
  chromosomes
• chromatids not identical, crossing over

18
Meiotic Errors

• Nondisjunction- homologues don't separate in
  meiosis 1
• Results in aneuploidy
• Usually embryo lethal
• Trisomy 21, exception leading to Downs syndrome
• Sex chromosomes
• Turner syndrome monosomy X
• Klinefelter syndrome XXY
• Translocation and deletion transfer of a piece
  of one chromosome to another or loss of fragment
  of a chromosome.

19
Mitosis, Meiosis, and Ploidy

• Mitosis can proceed independent of ploidy of
  cell, homologous chromosomes behave independently
  
• Meiosis can only proceed if the nucleus contains
  an even number of chromosomes (diploid,
  tetraploid).
• Trisomy 21 does not prevent meiosis

20
Review/Self-Test

• The biology Staff at the University of Arizona
  developed this self-test. See how well you
  understand!
• http//www.biology.arizona.edu/cell_bio/tutorials/
  meiosis/problems.html