CELL CYCLE

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CELL CYCLE

• Meiosis

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Chromosome Pairing

• Humans have 46 chromosomes arranged in 23 pairs
• 22 of the 23 pairs are homologous, meaning that
  the 2 chromosomes in each pair are similar with
  regard to shape, size, banding pattern, etc.
• The 23rd pair is homologous in females but not in
  males

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Haploid vs. Diploid

• The body cells of every species have a
  characteristic number of chromosomes
• Diploid any cell that contains two complete sets
  of chromosomes (2n)
• Haploid a cell with only one complete set of
  chromosomes these are also known as sex cells or
  gametes (n)

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Haploid vs. Diploid (cont.)

• Gametes must be haploid
• If they werent, the chromosome number would
  double 46 46 92 92 92 184
• Even one extra chromosome can cause serious
  defects

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Meiosis

• To keep the number of chromosomes stable from
  generation to generation, haploid cells are
  produced by meiosis
• Meiosis is a type of cellular reproduction in
  which the number of chromosomes is reduced by
  half, so that the daughter cells are haploid
• Haploid gametes combine to create a diploid
  offspring which grows by mitosis into an adult
• As an adult, the offspring will produce haploid
  gametes

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Phases of Meiosis

• Meiosis is divided into 2 parts
• Meiosis I
• Meiosis II
• These 2 parts are divided by interkinesis

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Meiosis I

• Prophase I
• Chromosomes condense
• Each homologous pair becomes attached to one
  another forming a TETRAD recall that tetrads do
  not form in mitosis

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Meiosis I (cont.)

• Metaphase I
• same as mitosis
• Anaphase I
• same as mitosis
• Telophase I
• same as mitosis

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Interkinesis

• Cytokinesis occurs forming 2 new cells
• Each chromosome in the haploid cells consists of
  2 sister chromatids

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Meiosis II

• Prophase II
• Spindle fibers form again
• Chromosomes condense
• Metaphase II
• Spindle fibers line up the chromosomes in the
  center of the cell

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Meiosis II (cont.)

• Anaphase II
• Sister chromatids are separated and pulled to
  opposite poles of the cell
• Telophase II
• Nuclei form
• Spindle fibers break down

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Cytokinesis

• Just as in mitosis, meiosis is followed by
  cytokinesis
• Cytokinesis produces 4 haploid cells
• Each cell contains one copy of one complete set
  of chromosomes (n)

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Human Cytokinesis

• In the human male, cytokinesis divides the
  cytoplasm evenly creating 4 sperm cells
• In the human female, cytokinesis divides the
  cytoplasm unevenly so that one cell receives
  almost all of the cytoplasm

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