Sexual Reproduction

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Sexual Reproduction

• Heredity - passing down of characteristics
  (genes)
• Alleles- different forms of the same gene

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Sexual Reproduction

• Why sexual reproduction?
• shuffles alleles new combinations
• provides genetic variation in species

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Meiosis

• Type of cell division
• One germ cell makes 4 gametes (egg/sperm) with
  ½ the of chromosomes.
• Occurs only in germ cells of gonads
• testes /ovaries
• Occurs in flowers
• ovary and anther
• Germ here doesnt mean bacteria. Its a term
  used for the type cells that gives rise to
  gametes in organisms.

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• While females are born with all the eggs they
  will ever have (around 400,000), only about 400
  will ever mature and ovulate between puberty and
  menopause.

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• Matrue males produce about 100 million sperm
  each day.

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• Sperm
• surrounding an egg

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• This shows how only one single sperm gets to
  penetrate the egg, releasing its nucleus of 23
  chromosomes to merge with the nucleus of the egg
  and its 23 chromosomes.

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Homologous Chromosomes

• Pair of chrom. similar in shape , size, and types
  of genes.
• Each locus (location of the gene) in same
  position on chrom.
• Humans have 23 pairs of homologues
• Housefly 6 prs
• Chicken 39 prs
• Apple 17 prs
• Dog 39 prs
• Cat 19 prs
• This is a karyotype
• (an image of an organisms
• chromosomes)
• This is a karyotype of a
• normal human male

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Chromosome numbers

• However many types of chrom. an organism has,
  that number is the n number of chrom. it has.

Body cells Egg/Sperm cells
Called Somatic cells Gametes
Term for chrom. Diploid (2 sets of chrom.) Haploid(1 set of chrom.)
n number 2n 1n
For Humans 46 chrom. 23 chrom.
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Homologous Chromosomes

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Animation

• Meiosis A animation
• Meiosis B animation
• Meiosis C animation

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Prophase I

• Longest and most complex phase (90 of meiosis).
• DNA condenses into chromosomes.
• Synapsis - a process when homologous chrom.
  come together, pair up, form a tetrad.

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Prophase I - Synapsis
Nonsister chromatids

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Prophase I - Crossing Over

• Crossing over may occur in the tetradbetween
  nonsister chromatids, ends break and reattach

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Crossing Over - Provides Variation

variation
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Metaphase I

• Shortest phase paired homologues align.
• INDEPENDENT ASSORTMENT occurs
• pairs of homologues line up independently of
  other pairs orientation toward the poles --
  random. Adds variation.

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Anaphase I

• Homologous chromosomes separate towards the poles
  (Tetrads separate)
• Sister chromatids remain attached

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Telophase I

• Each pole now has haploid set of chromosomes
  (however still doubled).
• Cytokinesis occurs two haploid daughter cells
  formed.

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Meiosis II

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

Prophase II / Metaphase II
Anaphase II
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Telophase II

• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs (2nd time).
• Four haploid daughter cells produced (chromosomes
  now back to single condition).
• gametes sperm or egg ovule or pollen grain

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Gamete Formation in Animals

• Diff. bet. male and female gametes.
• Male spermatogenesis
• all 4 develop into sperm cells.
• Female oogenesis
• cytokinesis is uneven.
• most cytoplasm goes to 1 of the 4 eggs(forms
  1 large egg cell)
• 3 other cells are small polar bodies which die

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Spermatogenesis
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Oogenesis
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human germ cell in ovary
n23 Still doubled
Polar Bodies
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2n 46
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n23 Still doubled
Ovum n23
diploid (2n)
meiosis I
meiosis II
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Mitosis vs Meiosis

• Mitosis
• Body (somatic cells)
• 2 daughter cells made(identical)
• Each w/ same kind of chrom. as parent cell
• 1 division process
• 1 cytokinesis
• No synapsis or crossing over
• Are diploid (2n)

• Meiosis
• Germ cells of gonads
• 4 gamete cells made(all different)
• Each w/ ½ chrom. as parent cell
• 2 divisions
• 2 cytokineses events
• Synapsis crossing over occurs in Prophase 1
• Are haploid (n)

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Nondisjunction

• When the tetrad (in Anaphase I) or the sister
  chromatids (in Anaphase II) do not separate,
  creating an abnormal of chrom. to occur in the
  gametes.
• Lethal most of the time

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Karyotype normal male
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Karyotype normal female
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• Downs Syndrome
• Trisomy 21
• 47, XY, 21
• The only trisomysurvivable toadulthood