Lecture 21: Meiosis

1
Lecture 21 Meiosis
2
Lecture outline 10/25/05

• Review mitosis
• Finish cell cycle
• Mutations in checkpoint genes can lead to cancer
• Sexual and asexual life cycles
• Meiosis
• Mechanics of meiosis
• Independent assortment and Crossing over produce
  genetic variation

3

• Mitosis produces an exact copy of the parent
  cell.
• Used for growth and asexual reproduction.
• Meiosis produces reduced (haploid) gametes, which
  are genetically unique.
• Necessary for sexual reproduction.

4
The human life cycle
Gametes are the only haploid cells
Meiosis occurs during gamete formation
5
Examples of Asexual Reproduction

• In asexual reproduction one parent produces
  genetically identical offspring by mitosis.
• (basically just branching)

Hydra
6
Aspens connected underground
www.robertturnerphoto.com
Dandelions make asexual seeds
7
Siblings share 1/2 of their genes, on average
Each get one chromosome from mother and one from
father, but they might not both get the same
chromosome from a parent.
8
Diploid cells have two copies of each chromosome

• Homologous chromosomes contain copies of the same
  genes
• One maternal, one paternal

9
The human life cycle
Gametes are the only haploid cells
Meiosis occurs during gamete formation
10

• Plants and some algae
• Exhibit an alternation of generations
• The life cycle includes both diploid and haploid
  multicellular stages

11

• In most fungi, and some protists
• Meiosis produces haploid cells that give rise to
  a haploid multicellular adult organism
• The haploid adult carries out mitosis, producing
  cells that will become gametes

12
Germline vs Soma
Germ means seed
Soma means body

• Meiosis only occurs in germ cells.
• Produces gametes
• Eggs and sperm
• Human oocytes start meiosis before birth and
  arrest in prophase 1
• then they mature one at a time at ovulation
• Human spermatocytes produce sperm continuously
• 28 d for meiosis
• another 35 d to become mature sperm

13
Mitosis vs Meiosis
Not paired
diploid
Sister chromatids separate in meiosis 2
Homologs pair in meiosis 1
haploid
14
Meiosis 1
INTERPHASE
MEIOSIS I Separates homologous chromosomes
PROPHASE I
METAPHASE I
ANAPHASE I
Sister chromatids remain attached
Chiasmata
Sister chromatids
Spindle
Homologous chromosomes separate
Tetrad
Pairs of homologous chromosomes split up
Chromosomes duplicate
Tertads line up
Homologous chromosomes (red and blue) pair and
exchange segments 2n 6 in this example
Figure 13.8
15
Meiosis 2
MEIOSIS II Separates sister chromatids
TELOPHASE II AND CYTOKINESIS
TELOPHASE I AND CYTOKINESIS
METAPHASE II
PROPHASE II
ANAPHASE II
Cleavage furrow
Haploid daughter cells forming
Sister chromatids separate
chromosomes are still double
Figure 13.8
16
Chromosome condensation and pairing

• The Synaptonemal Complex holds the homologs
  together

Segments of this cartoon show the same
Chromosome at different times in meiosis
17
What makes Meiosis different from Mitosis?

• Synapsis and crossing over
• Homologous chromosomes physically connect and
  exchange genes
• Paired homologous chromosomes at metaphase 1
• Separation of homologues, producing haploid
  gametes
• At anaphase I of meiosis, homologous pairs
  separate
• In anaphase II of meiosis, the sister chromatids
  separate

18
Meiosis produces genetic variation

• Random alignment of chromosomes in meiosis 1
• Independent assortment of maternal and paternal
  chromosomes into gametes
• Crossing over

223 8x106 possible combinations of chromssomes
in a gamete
Random fusion of gametes will produce about 223 x
223 64 trillion diploid combinations!
Futher shuffling of genes within chromosomes
19
(No Transcript)
20
Independent Assortment

• Each pair of chromosomes sorts its maternal and
  paternal homologues into daughter cells
  independently of the other pairs

Figure 13.10
21
Crossing Over

• Produces recombinant chromosomes that carry genes
  derived from two different parents

22
Crossovers are random

• This tetrad has 3 crossovers
• 23
• 13
• 24

23
Spindle fibers from one pole attach to BOTH
sister chromatids
Spindle fibers from the other pole attach to the
other homolog

• Crossovers are essential for correct alignment at
  metaphase 1

Chiasmata hold the pair together while Tug of
war Aligns tetrads