MEIOSIS

1
MEIOSIS
Modified with permission from Robert Goodman,
Biology Department,Citrus Community College
2
Meiosis

• The form of cell division by which gametes, with
  half the number of chromosomes, are produced.
• Diploid (2n) ? haploid (n)
• Meiosis is sexual reproduction.
• Two divisions (meiosis I meiosis II)

3
Meiosis

• Sex cells undergo meiosis to produce gametes
• Gametes have half the of chromosomes.
• Male sperm
• Female egg
• Meiosis only occurs in gonads
• Male Testes spermatogenesis
• Female Ovaries oogenesis
• Meiosis is similar to mitosis with some
  chromosomal differences.

4
Spermatogenesis
Head Mid piece
Tail
5
Interphase I

• Similar to mitosis interphase
• Chromosomes replicate (S phase)
• Each duplicated chromosome consists of two
  identical sister chromatids attached at their
  centromeres
• Centriole pairs also replicate

6
Interphase I

• Nucleus and nucleolus visible.

7
Meiosis I (four phases)

• Cell division reduces the chromosome number by
  one-half.
• Four phases
• a. prophase I
• b. metaphase I
• c. anaphase I
• d. telophase I

8
Prophase I

• Longest and most complex phase (90)
• Chromosomes condense
• Synapsis occurs
• Homologous chromosomes come together to form a
  Tetrad
• Two homologous chromosomes which is four
  chromatids (sister and nonsister chromatids)

9
Prophase I - Synapsis

10
Homologous Chromosomes

• Pair of chromosomes (maternal and paternal) that
  are similar in shape and size.
• Homologous pairs (tetrads) carry genes
  controlling the same inherited traits.
• Each locus (position of a gene) is in the same
  position on homologues.
• Humans have 23 pairs of homologous chromosomes.
• 22 pairs of autosomes
• 1 pair of sex chromosomes

11
Homologous Chromosomes

12
Sex Chromosomes
XX chromosome - female
XY chromosome - male
13
Crossing Over

• Crossing over (variation) may occur between
  nonsister chromatids at the certain areas.
• Segments of nonsister chromatids break and
  reattach to the other chromatid.

14
Crossing Over - Variation

VARIATION
15
Prophase I

spindle fiber
centrioles
aster fibers
16
Metaphase I

• Shortest phase
• Tetrads align on the metaphase plate
• Independent assortment occurs
• Orientation of homologous pair to poles is random
• Variation
• Formula 2n
• Example 2n 4
• Then n 2
• Thus 22 4 combinations

17
Metaphase I


OR
18
Question

• In terms of Independent Assortment
• How many different combinations of sperm could a
  human male produce?

19
Answer

• Formula 2n
• Human chromosomes 2n 46
• n 23
• 223
• 8 million combinations

20
Anaphase I

• Homologous chromosomes separate and move towards
  the poles
• Sister chromatids remain attached at their
  centromeres

21
Anaphase I
22
Telophase I

• Each pole now has haploid set of chromosomes.
• Cytokinesis occurs and two haploid daughter cells
  are formed.

23
Telophase I
24
Meiosis II

• No interphase II
• (or very short - no more DNA replication)
• Remember Meiosis II is similar to mitosis

25
Prophase II

• Same as prophase in mitosis

26
Metaphase II

• Same as metaphase in mitosis

27
Anaphase II

• Same as anaphase in mitosis
• Sister chromatids separate

28
Telophase II

• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember four haploid daughter cells
  produced.
• gametes sperm or egg

29
Telophase II
30
Meiosis
31
Variation

• Important to population as the raw material for
  Natural Selection.
• Question
• What are the three sexual sources of genetic
  variation?

32
Answer

• 1. Crossing over (prophase I)
• 2. Independent assortment (metaphase I)
• 3. Random fertilization
• Remember variation is good!

33
Question

• A cell containing 20 chromosomes (diploid) at the
  beginning of meiosis would, at its completion,
  produce cells containing how many chromosomes?

34
Answer

• 10 chromosomes (haploid)

35
Karyotype

• A method of organizing the chromosomes of a cell
  in relation to number, size, and type.

36
Fertilization

• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg

37
Question

• A cell containing 40 chromatids at the beginning
  of meiosis would, at its completion, produce
  cells containing how many chromosomes?

38
Answer

• 10 chromosomes