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Mitosis Reviewed
Meiosis
PROPHASE
Starr et al.
Chapter 10
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Chromosomes, Chromatids, and Ploidy
METAPHASE
ANAPHASE
TELOPHASE
DNA must replicate: the chromosomes are duplicated.
Chromosomes must condense.
Chromosomes must line up so they can move to what will become daughter cells.
Chromosomes must actually move.
Cell cytoplasm has to be divided - each daughter cell needs a complete
membrane.
Chromosomes need to uncoil so they can be active in transcription again.
Sexual reproduction: Think about chromosome numbers…
centromere
one chromatid
its sister chromatid
One chromosome in the duplicated state
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Diploid or 2n
–  Chromosome number in organism (animals)
•  46 chromosomes in humans
•  8 in fruit flies
•  This is the chromosome number in all somatic cells.
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Haploid or n
–  Chromosome number in gametes
–  23 in humans
–  4 in fruit flies
–  eggs, sperm
Meiosis reduces chromosomes to haploid number.
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Meiosis: Prophase 1
primary
spermatocyte or
oogonium
4n
Meiosis I
Before chromosomes become
visible, DNA has already
replicated
DNA does not
replicate again
2 x 2n
Meiosis II
newly forming
microtubules
•  Prologue -- Interphase 1:
–  DNA replicates –  Sister chromatids remain joined at
centromeres
•  Prophase 1:
–  Chromatin condenses
–  Homologous chromosomes align
themselves
•  Synapsis
–  2 pairs of sister chromatids
–  Crossing over occurs •  Between homologous chromatids
–  Physical exchange
–  Identical portions of chromatids
–  Point of crossover = chiasma (plural:
chiasmata) Prophase I
4 x n
Crossing Over
•  At chiasmata •
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Exchanges sections of sister chromatids from 2 parental homologs
DNA is literally broken and re-ligated Bits of maternal chromatid are joined to bits of paternal chromatid.
Meiosis 1 concluded
spindle
equator
one pair of homologous
chromosomes
Metaphase I
Anaphase I
Telophase I
•  Homologous chromosomes are in synapsis at metaphase
–  Pulled apart by spindles
–  Cross-overs are resolved
•  Sister chromatids stay together
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Independent
Assortment
1
2
Meiosis 2
3
combinations possible
•  No DNA synthesis between meiosis 1 telophase and meiosis 2 prophase
–  Often there is no interphase
•  Meiosis 2 separates sister chromatids
–  Prophase 2
–  Metaphase 2
–  Anaphase 2 –  Telophase 2
•  Result
–  4 cells (gametes, for sperm)
–  Each with haploid number of chromosomes
–  With some recombination between paternal and maternal chromosomes
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MEIOSIS II begins with 2 cells produced in meiosis I
primary
spermatocyte or
oogonium
Meiosis I
4n
Before chromosomes become
visible, DNA has already
replicated
DNA does not
replicate again
2 x 2n
Meiosis II
PROPHASE II
4 x n
METAPHASE II
ANAPHASE II
TELOPHASE II
Fig. 10.4b p. 165
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Spermatogenesis
Human sperm production:
150 million/day --- 1700/second
stem cells undergo ~ 400 cell divisions by age 40
Evolutionary basis: sperm competition
spermatogonium
(diploid )
primary
spermatocyte
(diploid)
secondary
spermatocytes
(haploid)
spermatids
(haploid)
sperm (mature,
haploid male
gametes)
stem cell
Growth
Meiosis I,
Cytoplasmic Division
Meiosis II,
Cytoplasmic Division
Strategy:
The egg must provide for the embryo
Larger cells contain cytoplasm, mitochondria
Stem cells divide ~ 30 times Oogenesis
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three
polar
bodies
(haploid)
primary
oocyte
(diploid)
Meiosis I,
Cytoplasmic Division
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–  150,000,000 sperm/day
–  Primordial germ cells constantly
divide
Sperm from 30 year-old male
–  ~ 400 cell divisions
–  >10x female divisions
Sperm from 60 year old male
–  ~ 1,000 cell divisions
–  > 30x female divisions
•  Female
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~2,000,000 1o oocytes ---> ~300,000 ~500 ova released
1st meiotic division begins pre-puberty
Completed 1 ovum at a time
Any ovum, at any adult age
•  ~ 30 cell divisions
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Assume 1 error/ 1010 base pairs copied
Genome = 3 X 109 base pairs –  30 divisions ---> 10 errors
–  5% of DNA codes for proteins
•  < 1 changed amino acid in 30,000
proteins
secondary
oocyte
(haploid)
Growth
Origins of Mutations
•  Male
first polar
body
(haploid)
stem cell
oogonium
(diploid)
Alternative Strategies
Drosophila bifurca Each male produces ~ 100 sperm, 6 cm long
Take 17 days to mature sperm
Adult flies ~ 2 mm long.
Drosophila melanogaster sperm - 2 mm long, 1 day to maturity]
Homo sapiens sperm: ~ 2 µm
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ovum
(haploid)
Meiosis II,
Cytoplasmic Division
Recent molecular studies confirm
–  5x to 20x as many mutations
derive from father as from
mother
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Chromosomal changes
–  Long dormancy -->chromosome
damage
•  E.g.: Triploidy 21 more common in
older women
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Definitions
•  Gene
–  Stretch of DNA that –  Encodes a specific protein
•  (plus controller regions)
•  CF gene encodes ….
–  Encodes structural or other RNA
•  tRNA
•  rRNA
–  Includes associated introns and promoter regions
(Y)
•  Locus
(X)
–  Location of gene on chromosome
–  Visualized by fluorescent in situ hybridization (FISH)
•  Allele
–  Variants of a particular gene
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normal and mutant variants of CF gene
Blood type variants (A,B, O)
blue/brown eye color
etc ….
Basic Principles of Genetics
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Derive from meiosis
–  Segregation
•  One and only one copy of each gene is found in a gamete
Having only 2 of each
chromosome, one person can
one pair of
have only 2 alleles at any given duplicated
chromosomes
locus, but in a population, there
can be many different alleles.
Random fusion of
gametes 2n
germ cell
germ cell
each chromosome
duplicated during
interphase
Almost unlimited
assortment –  Independent assortment
•  Chromosomes sort into gametes independently of parental origin
–  #1 from mother, #2 from Dad …
•  Results in new combinations of genes in gametes –  Recombination
•  Results from crossing over
•  Means each chromosome carries a mix of paternal and maternal genes
Only identical twins
are genetically
identical
n
Meiosis I
separation of
homologues
Identical twins are
true clones of each
other
gametes
Meiosis II
separation of
sister
chromatids
gametes
2n
diploid number
restored at fertilization
zygote
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Is Sex Necessary?
Not for
Prokaryotes
Many plants
Some insects
bees
aphids
Some lizards
•  Parthenogenesis
–  Female produces offspring without male gametes
–  Occurs in some insects, lizards
–  Works well in stable environment
Absolutely for
Mammals
WHY?
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