Chromosomes and Cell Division

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Chromosomes and Cell Division
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Chromosomes

• Your somatic cells (all excluding egg and sperm
  cells) contain 46 chromosomes
• Chromosomes contain the genetic instructions for
  the structure function of your body

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Chromosomes

• Autosomes these are the 44 chromosomes
  containing similar types of information in cells
  of both males females
• Sex chromosomes these are 2 of the 46
  chromosomes that look like an XX for females and
  XY for males

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Microtubules

• Microtubules form the Spindle Apparatus which is
  the machinery responsible for moving
  chromosomes during cell division

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Note the pink spindle apparatus moving the
chromosomes
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Chromosome numbers

• Diploid 2n
• 23 chromosomes that you received from your mother
  plus 23 chromosomes you received from your father
• 2 sets of chromosomes
• Seen in somatic cells (body cells other than egg
  and sperm)

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Homologous Chromosomes

• Similar chromosomes
• Contains the same TYPE of information
• E.g. chromosome number 1 you received from mom
  and chromosome number 1 you received from dad.

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This photo shows homologous chromosome pairs
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MITOSIS

• Cell division in cells that are NOT your sex
  cells
• Allows growth and repair
• 2n gtgtgtgt2n

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Interphase Chromosome replication
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Interphase

• Exact copies are made of the chromosomes
• Exact copies are termed sister chromatids
• Sister chromatids are joined in the middle at the
  centromere

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Mitosis - Prophase

• Nuclear envelope breaks down
• Spindle apparatus forms
• Chromosomes thicken

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Mitosis - Metaphase

• The microtubules of the spindle apparatus attach
  to the centromere of sister chromatids
• Sister chromatids line up along the cell equator

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Mitosis - Anaphase

• Sister chromatids move to opposite poles of the
  cell

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Mitosis - Telophase

• Sister chromatids have arrived at opposite poles
  of the cell
• Nuclear envelope reforms around each set of
  chromosomes
• Cytoplasm cleaves yielding 2 identical daughter
  cells

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Meiosis

• This cell division occurs in the gonads (ovaries
  and testes)
• Resulting cells are the egg and sperm each with
  23 chromosomes
• Chromosome numbers are cut in half from 46 to 23
• Diploid (2n) to Haploid (n)

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Interphase

• Interphase in meiosis is just as it was in
  mitosis
• Chromosomes are copied

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Meiosis Prophase I

• Nuclear envelope breaks down
• Chromosomes thicken
• Spindle apparatus forms
• Homologous chromosomes pair up (this does NOT
  happen in mitosis)
• Crossing over occurs randomly

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Meiosis Metaphase I

• Homologous pairs line up along the equator
• Lining up is random resulting in more variation

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Meiosis Anaphase I

• Homologous pairs split apart

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Meiosis Telophase I

• Chromosomes have arrive at opposite poles
• New nuclear envelope reforms
• 2 new cells are NOT identical and are still
  diploid

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Meiosis - Prophase II

• Now, onto Prophase II without an interphase stage
• Chromosomes thicken
• Nuclear envelope breaks down
• Spindle apparatus forms

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Meiosis Metaphase II

• Single chromosomes line up on the equator

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Meiosis Anaphase II

• Chromosomes split apart
• But, chromosomes are NOT identical

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Meiosis Telophase II

• Chromosomes have arrived at opposite poles
• Nuclear envelope reforms
• 4 resulting haploid cells
• Each of the four cells is genetically different

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Chromosomal inheritance

• Sex determination
• XX is female
• XY is male

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Downs Syndrome

• Mistake during meiosis
• Commonly due to an extra chromosome 21

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