Biology Webpage

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Biology Webpage

• Meiosis Lesson
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Meiosis
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How are human and animal chromosomes categorized?

• Sex chromosomes- the 23rd pair of chromosomes
  determines the gender of an organism and they
  also carry genes for other traits.
• Female XX
• Male XY
• Autosomes- All other chromosome pairs
• (1-22) which code for all the other traits.

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How many chromosomes are there in human cells?

• Humans have 23 pairs or a total of 46
  chromosomes.

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What are homologous chromosomes?

• Every organism produced by sexual reproduction
  has two copies of each autosome
• (one from each parent)
• Homologous chromosomes or homologues Two copies
  of each autosome
• - Same size
• - Same shape
• - nCarry genes for the same trait

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Journal

• What is the difference between
• sister chromatids and homologues?

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Diploid number of chromosomes

• Cells having two sets of chromosomes are said to
  be diploid they have both chromosomes for each
  homologous pair.
• All body cells have a diploid number of
  chromosomes
• Also referred to as 2n, 2(23)46

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Haploid number of chromosomes

• Cells contain only one set of chromosomes they
  have half the normal or diploid number of
  chromosomes.
• Only the reproductive cells called gametes (sperm
  and egg cells in humans) have haploid number.
• Also referred to as 1n, 1(23)23

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Haploid (n7)
Diploid (2n 14)
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Haploid Gametes 1n (Reproductive Cells)
Diploid zygote 2n (Offspring- also called a
zygote)
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Journal

• What is the difference between haploid and
  diploid?

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Meiosis

• Nuclear division that reduces the number of
  chromosomes in the new cells produced
  half_(haploid) the number in the original cell.
• In males this takes place in the sperm. It is
  called spermatogenesis.
• In females this takes place in the egg and it is
  called oogenesis.

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Meiosis has 2 parts

• Meiosis I- Homologous chromosomes are separated
• Meiosis II-
• _Sister chromatids are separated

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• Meiosis I
• Prophase I- chromosomes are paired with
  homologues to form tetrads
• M etaphase I- Homologues meet in the middle
• A naphase I- Homologues are pulled apart
• T elophase I- Two nuclei are formed
• Cytokinesis- Splits into two haploid cells

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• Meiosis II
• Prophase II- Nuclear membrane disappears
• M etaphase II- Chromosomes meet in the middle
• A naphase II- Sister Chromatids are pulled apart
• Telophase II- Two nuclei form
• Cytokinesis- Four haploid cells are formed

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Meiosis Provides Genetic Variation

• Recombination Prophase I homologues chromsomes
  _cross over and genetic material may be
  exchanged.

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Meiosis Provides Genetic Variation

• Law of Independent Assortment Homologues line
  up independently of each other!

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Journal

• What are the 2 ways that Meiosis
• provides for genetic variation?

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Nondisjunction

• When homologous chromosomes _fail_ to separate
  properly during meiosis.
• Results
• A gamete with an extra
• chromosome (trisomy)
• A gamete missing a
• chromosome (monosomy)

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Trisomy 21
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Polyploidy

• Organisms with more than the usual number of
  chromsomes
• Occurs in plants
• Caused by self fertilization

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• THE END!