Mitosis

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Mitosis Meiosis

• Cell Division

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Recall Observations

• Late 1800s cell division
• Threads
• Walther Flemming changes during cell division
• Threads appear paired prior to cell division
• Paired threads separate just prior to division
• Mitosis (thread)

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Observations on thread

• Number of threads differ between species
• Roundworms 4 Peas 14 Humans 46
• Same between individuals within a species
• All roundworms 4
• Same between cells within an individual

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Gamete fusion (fertilization)

• 1876 - demonstrated in sea urchins
• Sperm cell egg cell embryo
• 1883 - threads in gametes (sperm egg cells)
  number 1/2 of those in body cells
• All roundworm body (somatic) cells 4
• Roundworm gametes 2
• 1887 - proposal special type of division
  produces sperm egg cells Meiosis

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• So, two types of cell divisions?
• One creates daughter cells with same number of
  threads as parent cells
• The other creates daughter cells with 1/2 that
  number. Original is restored when gametes fuse.

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Thread number

• 1888 - Threads are named chromosomes (colored
  body)
• Ex 12 distinct types (size, shape), but 24
  total
• a-k autosomes
• X sex chromosome
• Paired threads homologous chromosomes
• One Maternal, one Paternal

Karyotype
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Thread number

• 12 unique types (size, shape), but 24 total
• n of unique types of chromosomes
• n 12
• 2n 24
• Ploidy of complete sets of homologous
  chromosomes
• 1 set of 12 haploid
• 2 sets 12 24 diploid 2 versions of each
  unique type
• Polyploids (more than 2n) also exist

Karyotype
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Clicker Q

• If n 15, how many chromosomes does a triploid
  organism have?
• 15
• 30
• 45
• 60

How many complete sets of chromosomes does it
have?
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Chromosome terminology

• Chromosome
• Sister chromatids (exact replicas)
• Centromere

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Do chromosomes always look like that?

• Nope. Usually exist as unwound, invisible,
  stringy, chromatin

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Chromatin

• Unwound, unreplicated DNA with genes on them

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When does Mitosis occur?

• Fraction a of cells life
• Chromosome replication (DNA synthesis) is
  separate from Mitosis
• Significant time spent in Gap phases (G1,
  G2)duplicating organelles, growing, etc.

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Major events of Mitosis

• Chromosomes have already replicated!!
• Sister chromatids separate --gt equal allocation
  of chromosomes to daughter cells
• Remember, the point is to make a cell that is an
  exact replica of the parent

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Stages of Mitosis

• Chromosomes replicate (S)
• Centrosomes replicate (M)
• Chromosomes condense (M)

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Prophase Metaphase
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Anaphase Telophase
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Cytokinesis
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Summary

• Homologs do not associate.
• All chromosome align linearly on metaphase plate.

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Major events of Meiosis

• 2 cell divisions Meiosis I Meiosis II
• Meiosis I
• Homologs form tetrads
• Non-sister chromatids cross over!
• Homologs associate on metaphase plate, then
  separate.
• Meiosis II
• Sister chromatids separate!
• Full compliment (diploid ) restored upon
  fertilization

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Homologs replicate
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• Homologous pairs separate
• Sister chromatids separate

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Meiosis I Stages
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Meiosis II Stages
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How is heritable variation produced?

• Homologous chromosomes align independently on the
  metaphase plate
• Varies the combination of chromosomes in daughter
  cells
• Crossing over (recombination) between non-sister
  chromatids at chiasmata shuffles alleles
• Varies combinations of alleles on chromosomes in
  daughter cells

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Independent assortment
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Overhead

• How many different chromosome arrangements (at
  the metaphase plate of Meiosis I) could a cell
  have that is 2n 6?
• 2
• 3
• 4
• 5

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Even selfing produces variation