Meiosis

1
Meiosis
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Objectives

• 4.2.1 State that meiosis is a reduction
  division of a diploid nucleus to form haploid
  nuclei.
• 4.2.2 Define homologous chromosomes.
• 4.2.3 Outline the process of meiosis, including
  pairing of homo- logous chromosomes and
  crossing over, followed by two divisions,
  which results in four haploid cells.
• 4.2.4 Explain that non-disjunction can lead to
  changes in chromo- some number, illustrated by
  reference to Down syndrome (trisomy 21).
• 4.2.5 State that, in karyotyping, chromosomes
  are arranged in pairs according to their size
  and structure.
• 4.2.6 State that karyotyping is performed using
  cells collected by chorionic villus sampling
  or amniocentesis, for pre- natal diagnosis of
  chromosome abnormalities.
• 4.2.7 Analyze a human karyotype to determine
  gender and whether non-disjunction has
  occurred.

3
Remember mitosis stages in the cell cycle

• Interphase 3 sub-phases
• G1 phase (1st gap) centers on growth.
• S phase (synthesis) when chromosomes are
  copied.
• G2 phase (2nd gap) the cell completes
  preparations for cell division.
• Organelles multiply.
• Chomosomes form
  from chromatin.
• Centrosomes are
  duplicated.
• Microtubles form.
• Then the cell divides (M).

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Stages in the cell cycle

• Mitosis is broken into 4 sub-phases
• Interphase Prophase Metaphase
  Anaphase Telophase

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Genetically identical nuclei

• By this method, each
• daughter cell ends up
• with one copy of
• every chromosome.
• An abnormal number
• of chromosomes in
• a cell is usually fatal.

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Definition of meiosis

• Meiosis is a reduction division of a diploid (2n)
  nucleus to form haploid (1n) nuclei.
• Eukaryotic species have a characteristic number
  of chromosomes in the nucleus.
• Human somatic (body) cells have 46 chromosomes.
• Human gametes (sperm eggs) have 23 chromosomes.

Blue red represent chromosomes from mother
father.
Sperm has an uneven mix of red blue, and
some that are combinations.
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Homologous chromosomes

• Homologous chromosomes are chromosome pairs of
  the same length, centromere position, and
  staining pattern that possess genes for the same
  characters at corre-sponding loci. One
  homologous chromosome is inherited from the
  father and one from the mother.

A set of homologous chromosomes pro- duces a
karyotype.
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Homologous chromosomes

• Homologous chromosomes are chromosome pairs of
  the same length, centromere position, and
  staining pattern that possess genes for the same
  characters at corre-sponding loci.

Homologous pairs of chromosomes
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Mitosis vs. meiosis

• Meiosis reduces chromosome number from 2n to 1n.
• Chromosomes replicate before either mitosis or
  meiosis.
• In mitosis 1 cell division produces 2 diploid
  daughter cells.
• In meiosis 2 cell divisions produce 4 haploid
  daughter cells.

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Process of meiosis

• Meiosis reduces chromosome number from 2n to 1n.
• In interphase chromosomes replicate and form
  genetically identical sister chromatids, joined
  at the centromere.

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Process of meiosis

• Meiosis reduces chromosome number from 2n to 1n.
• In prophase I homologous chromosomes pair up and
  exchange segments called crossing over a
  major event.

Note tetrads
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Process of meiosis

• Crossing over non-sister chromatids of
  homologous chromosomes exchange genetic
  information the genomes of the mother father
  of this parent mix.

From parents Crossing
Haploid (1n) Mother Father
over gametes
No sperm or eggs are identical.
Tetrad the 4 homologous chromosomes
One parents DNA
The crossing points within the tetrads are called
chiasmata.
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Process of meiosis

• Crossing over independent assortment introduce
  variation, as does the random joining of a unique
  sperm and egg at fertilization.
• There are 23 chromo- some
  pairs in human
  germ cells, therefore
  they can line up in
  223 8,388,608 dif-
  ferent ways. Sperm
  and egg can produce
  gt 64 trillion unique
  individuals.

This is the advantage of sexual reproduction over
asexual.
mix in 223 different ways
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Process of meiosis

• Meiosis reduces chromosome number from 2n to 1n.
• In metaphase 1 homologs line up at metaphase
  plate.
• In anaphase 1 homologous chromosomes separate.

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Process of meiosis

• Meiosis reduces chromosome number from 2n to 1n.
• After telophase 1, a 2nd cell division separates
  the sister chromatids. Each gamete has only 1
  copy of DNA.

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Non-disjunction

• Non-disjunction an accident of meiosis or
  mitosis in which the members of a pair of
  homologous chromosomes or sister chromatids fail
  to move apart properly.
• Some gametes get 2 of one chromosome others get
  none.
• Ex Down syndrome (trisomy 21)

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Non-disjunction

• Down syndrome (trisomy 21)
• 3 copies of chromosome 21

The karyotype
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Karyotyping

• In karyotyping, chromosomes are arranged in pairs
  according to their size and structure.

A picture of ones chromosomes
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Karyotyping

• Karyotyping is performed using cells collected by
  chorionic villus sampling or amniocentesis, for
  pre-natal diagnosis of chromosome abnormalities.

The chorion is a membrane forming the placenta.
20
Karyotyping

• Social and ethical issues of karyotyping unborn
  fetuses
• Abortion of fetuses with abnormalities
• Abortion based on gender or superficial
  characteristics
• Eye color?

21
Karyotyping

• Analyze a human karyotype to determine gender and
  whether non-disjunction has occurred.
• What is the sex?

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Karyotyping

• Analyze a human karyotype to determine gender and
  whether non-disjunction has occurred.
• XYY syndrome - Affected individuals are usually
  very tall and thin. Many experience severe acne
  during adolescence. Additional symptoms may
  include antisocial or behavioral problems and
  learning disabilities. Intelligence is usually
  normal, although
  IQ, on
  average, is 10 to 15
  points lower than
  siblings.

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Karyotyping

• Analyze a human karyotype to determine gender and
  whether non-disjunction has occurred.
• Trisomy 18 is a disorder that shows symptoms as
  soon as the child is born. About 25 of Edward's
  syndrome victims die before they are one month
  old, and only 10 live for one year.