Chapter 13Meiosis and Sexual Life Cycles

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Chapter 13 Meiosis and Sexual Life Cycles
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Heredity

• Heredity the transmission of traits from one
  generation to the next
• Asexual reproduction clones
• Sexual reproduction variation
• Human life cycle 23
  pairs of homologous chromosomes (46)
  1 pair of sex and 22 pairs of
  autosomes karyotype gametes are
  haploid (1N)/ all other cells are diploid
  (2N) fertilization (syngamy) results
  in a zygote
• Meiosis cell division to produce haploid
  gametes

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Karyotype
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Alternative life cycles I

• Fungi/some algae meiosis produces 1N cells that
  divide by mitosis to produce 1N adults (gametes
  by mitosis)

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Alternative life cycles II

• Plants/somealgae
• Alternation of generations 2N sporophyte, by
  meiosis, produces 1N spores spore divides by
  mitosis to generate a 1N gametophyte gametes
  then made by mitosis which then fertilize into 2N
  sporophyte

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Meiosis

• Preceded by chromosome replication, but is
  followed by 2 cell divisions (Meiosis I Meiosis
  II)
• 4 daughter cells 1/2 chromosome number (1N)
  variation

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Meiosis I

• Division in meiosis I occurs in four phases
  prophase, metaphase, anaphase, and telophase.
• During the preceding interphase the chromosomes
  are replicated to form sister chromatids.

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Meiosis I

• In prophase I, the chromosomes condense and
  homologous chromosomes pair up to form tetrads.
• In a process called synapsis, special proteins
  attach homologous chromosomes tightly together.
• At several sites the chromatids of homologous
  chromosomes are crossed (chiasmata) and segments
  of the chromosomes are traded.

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Meiosis I

• At metaphase I, the tetrads are all arranged at
  the metaphase plate.
• In anaphase I, the homologous chromosomes
  separate and are pulled toward opposite poles.

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Meiosis I

• In telophase I, movement of homologous
  chromosomes continues until there is a haploid
  set at each pole.
• Cytokinesis by the same mechanisms as mitosis
  usually occurs simultaneously

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Meiosis II

• very similar to mitosis
• During prophase II a spindle apparatus forms,
  attaches to kinetochores of each sister
  chromatids, and moves them around.

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Meiosis II

• At metaphase II, the sister chromatids are
  arranged at the metaphase plate.
• At anaphase II, the centomeres of sister
  chromatids separate and the now separate
  sisters travel toward opposite poles.

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Meiosis II

• In telophase II, separated sister chromatids
  arrive at opposite poles
• Cytokinesis separates the cytoplasm.
• At the end of meiosis, there are four haploid
  daughter cells.

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Meiosis vs. mitosis

• Synapsis/tetrad/chiasmata (prophase I)
• Homologous vs. individual chromosomes (metaphase
  I)
• Sister chromatids do not separate (anaphase I)
• Meiosis I separates homologous pairs of
  chromosomes, not sister chromatids of individual
  chromosomes.

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Origins of Genetic Variation, I

• Independent assortment homologous pair of
  chromosomes position and orient randomly
  (metaphase I) and nonidentical sister chromatids
  during meiosis II
• Combinations possible 2 with n the
  haploid number of the organism

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Origins of Genetic Variation, II

• Crossing over (prophase I) the reciprocal
  exchange of genetic material between nonsister
  chromatids during synapsis of meiosis I
  (recombinant chromosomes)
• Random fertilization 1 sperm
  (1 of 8 million possible chromosome combinations)
  x 1 ovum (1 of 8 million different possibilities)
  64 trillion diploid combinations!