Chromosomes and Chromosome Number

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Chromosomes and Chromosome Number

• Human body cells have 46 chromosomes
• Each parent contributes 23 chromosomes
• Homologous chromosomes -one of two paired
  chromosomes, one from each parent

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Homologous Chromosomes

• Same length
• Same centromere position
• Carry genes that control the same inherited
  traits
• One came from Dad,
• one came from Mom!

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• An organism produces gametes to maintain the same
  number of chromosomes from generation to
  generation.

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Compare diploid and haploid number

• Diploid cell that contains two of each kind of
  chromosome (2N)
• Body cells(somatic cells) are diploid
• Haploid cell with only one kind of chromosome
  (N)
• Gametes (sex cells) are haploid

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Meiosis I

• The sexual life cycle in animals involves
  meiosis.
• Meiosis produces
• gametes.
• When gametes
• combine in fertilization,
• the number of
• chromosomes is restored.

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Meiosis I

• Prophase I- each chromosome pairs with its
  corresponding homologous chromosome to form a
  tetrad(4 chromatids)
• The nuclear envelope breaks down.
• Spindles form.
• Crossing-over- chromosomes will switch some
  genes. This gives us genetic variation.

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Crossing Over
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• Metaphase I
• Homologous pairs of chromosomes line up together
  at the middle of the cell
• Chromosome centromeres attach to spindle fibers.

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• Anaphase I
• homologous chromosomes separate move to
  opposite ends. (centromeres do not split)

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• Telophase I Cytokinesis
• spindle disappears and the cell divides

Meiosis I results in 2 haploid(1N) daughter
cells, each with half the number of chromosomes
as the original cell.
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Meiosis II
During the second meiotic division, nothing is
replicated, everything is going to be just
divided up
Prophase II as the spindle apparatus forms and
the chromosomes condense.
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Prophase II
as the spindle apparatus forms, the chromosomes
condense
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• Metaphase II
• a haploid number of chromosomes line up at the
  equator (middle)

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• Anaphase II
• sister chromatids are pulled apart at the
  centromere move to opposite poles

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• Telophase II (includes cytokinesis)
• when the 2 daughter cells divide into 4 new
  daughter cells, each new daughter cell has 23
  chromosomes. This is half the number of the
  original parent cell

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Difference in meiosis

• In males
• 4 mature sperm
• Males begin to produce sperm after puberty,
  produced constantly until death meiosis II
  occurs immediately after meiosis I
• Much smaller than egg
• May have X or Y chromosomes
• Have flagella to move

• In females
• 1 mature egg, 3 polar bodies which break down
• Women born with all eggs they will have, meiosis
  I occurs before birth, meiosis II occurs once a
  month
• Much larger
• Have all X chromosomes
• Has no method of movement

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Meiosis/gamete production

• Females have XX for last pair of chromosomes
• Males have XY for last pair
• Since all eggs are X, father determines the sex
  of the child since the sperm may be X or Y

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Meiosis

• There are 2 divisions of meiosis- Meiosis I and
  Meiosis II. It must be this way in order to end
  up with the sex cells (sperm egg) only having
  23 chromosome.
• Meiosis occurs in the testes of the male and is
  called spermatogenesis
• And in the ovaries of the female it is called
  oogenesis.
• 23 23 46 chromosomes

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Mitosis/Meiosis

• Mitosis
• Cell division producing body cells
• Results in 2 daughter cells
• One division
• Daughter cells have same number of chromosomes
  (2N)

• Meiosis
• Cell division producing sex cells
• Results in 4 daughter cells
• Two divisions
• Daughter cells have half the number of
  chromosomes (N)

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Cell division cell type of
daughter of cells
chromosomes Mitosis somatic(body)
2 46(diploid) Meiosis
gametes(sex) 4
23(haploid)
www.pbs.org How cells divideMitosis vs. meiosis