Gametogenesis

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Gametogenesis

• the formation of gametes in animals
• although Gametogenesis in males and females is
  generally the same there are some general
  differences

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p. 170, Fig 5.18
Spermatogenesis
Oogenesis
1 Spermatocyte (2n)
1 Oocyte (2n)
Meiosis I
2 Spermatocytes (n)
2 Oocyte (n)
Meiosis II
Spermatids (n)
Ootid (n)
Polar Bodies (n)
Spermatozoa (n)
Ovum (n)
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Males vs. FemalesBattling Sexes

• Males
• equal division of cytoplasm
• since sperm cells are streamlined for movement,
  they cannot carry excess weight
• make more sex cells than females

• Females
• cytoplasm does not divide equally after each
  division
• one daughter cell called the ootid receives most
  of the cytoplasm
• uses the extra cytoplasm and organelles for
  future cell divisions (if fertilization)
• the other cells called polar bodies die
• only one ovum (egg cell) is produced by meiosis

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Males vs. Femalesby the Numbers

• Males
• the diploid spermatocytes - the cells which give
  rise to sperm cells - are capable of many mitotic
  cell divisions before meiosis ever begins
• can produce one billion sperm cells everyday

• Females
• baby females have two million primary oocytes in
  their ovaries
• most are absorbed into the body
• of this number 400 to 500 will be released during
  the reproductive years
• primary oocytes have already entered meiosis I
• remained suspended in prophase I until the female
  reaches reproductive age or puberty

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Menstrual Cycling

• starting at the first menstrual cycle, meiosis
  will resume in one oocyte at a time, once a month
  until menopause
• menopause is the end of menstrual cycling in
  women
• usually occurs between the ages of 40 to 55
• the remaining oocytes left in the ovaries are
  nonfunctional or unresponsive and are no longer
  released

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Chromosomes

• All chromosomes in females are found as
  homologous pairs
• Not all chromosomes in males are found as
  homologous pairs

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• There is one different pair of chromosomes in
  males and females.
• In females the pair is always two rod shaped
  chromosomes (XX)
• In males there is one rod-shaped chromosome and
  one hook-shaped chromosome (XY)

These chromosomes are called Sex Chromosomes
Chromosomes that are not sex chromosomes are
called Autosomes
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Abnormal Meiosis

• Like most processes sometimes meiosis makes
  mistakes
• eg. Nondisjunction
• occurs when two homologous chromosomes move to
  the same pole during meiosis
• the result is one daughter will be missing a
  chromosome (22) and the other will have an extra
  chromosome (24)
• cells that lack or have too much genetic
  information will not function properly
• Aside

Nondisjunction can also happen during mitosis,
but the effects are less devastating than during
meiosis. WHY?
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Abnormal Meiosis

• Trisomy
• the condition where there are three homologous
  chromosomes in place of a homologous pair
• zygote with 47 chromosomes
• eg. Down Syndrome or Klinefelter Syndrome
• Monosomy
• the condition where there is a single chromosome
  in place of a homologous pair
• zygote with 45 chromosomes
• eg. Turner Syndrome

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Karyotyping

• Karyotype Chart
• a picture of chromosomes arranged in homologous
  pairs in descending order by size, with sex
  chromosomes placed last
• How?
• 1. mix a small amount of tissue with a solution
  that stimulates mitotic division
• 2. add another solution which stops the division
  at metaphase (the best phase to karyotype since
  the chromosomes are the most condensed)
• 3. the metaphase cells are placed on a slide and
  stained so that distinctive bands appear
• 4. a photograph is taken and blown up and then
  the individual chromosomes are cut out and
  matched with its homologous pair.

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Homework

• Read p. 167-174
• p. 174 4,5,9