9.5 Meiosis Function in Sexual Reproduction

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9.5 Meiosis Function in Sexual Reproduction
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• Objectives
• Describe how homologous chromosomes are alike and
  how they differ.
• Contrast haploid and diploid cells.
• Summarize the process of meiosis.

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• Key Terms
• meiosis
• karyotype
• homologous chromosome
• sex chromosome
• diploid
• gamete
• haploid
• fertilization
• zygote
• tetrad

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• Each offspring in a sexually reproducing species
  inherits a unique combination of genes from its
  two parents. This combined set of genes
  contributes to a unique combination of traits in
  each individual

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• Homologous ChromosomesSexual reproduction
  depends in part on meiosisa type of cell
  division that produces four cells, each with half
  the number of chromosomes as the parent cell.
• In animals, meiosis occurs in the sex organsthe
  testes in males and the ovaries in females.

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• A typical human body cell has 46 chromosomes. A
  display of the 46 chromosomes of an individual is
  called a karyotype. 23 pairs
• Each chromosome has a twin that resembles it in
  size and shape. You inherit one chromosome of
  each pair from your mother and the other from
  your father. The two chromosomes of each matching
  pair are called homologous chromosomes

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• Each homologous chromosome in a pair carries the
  same sequence of genes controlling the same
  inherited characteristics.
• For example, if a gene influencing eye color is
  located at a particular place on one chromosome,
  then the homologous chromosome also has a gene
  for eye color in the same place.

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• However, the two genes may be slightly different
  versions. One chromosome might have the form of
  the gene for brown eye color, while the
  homologous chromosome might have another form for
  blue eye color

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• Altogether, humans have 23 homologous pairs of
  chromosomes.
• For the karyotype of a human female, the 46
  chromosomes fall neatly into 23 homologous pairs.
  
• But in males, the two chromosomes of one pair do
  not look alike.

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• This 23rd pair of chromosomes, called the sex
  chromosomes, determines the person's sex.
• The sex chromosomes occur in two forms, called X
  and Y.
• Like all mammals, human males have one X
  chromosome and one Y chromosome, while females
  have two X chromosomes. Only small parts of the X
  and Y chromosomes are homologous

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• Diploid and Haploid CellsHaving two sets of
  chromosomes, one inherited from each parent, is a
  key factor in the life cycles of all sexually
  reproducing organisms.

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• Almost all human cells are diploid that is, they
  contain two homologous sets of chromosomes.
• The total number of chromosomes, 46 in humans, is
  referred to as the diploid number (abbreviated
  2n, as in 2n 46).

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• The exceptions are egg and sperm cells, known as
  sex cells, or gametes. Each gamete has a single
  set of chromosomes, one from each homologous
  pair. A cell with a single set of chromosomes is
  called a haploid cell. For humans, the haploid
  number (abbreviated n) is 23.
• These haploid cells are produced through the
  process of meiosis.

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• In the human life cycle, the nucleus of a haploid
  sperm cell from the father fuses with the nucleus
  of a haploid egg cell from the mother in the
  process of fertilization.
• The resulting fertilized egg, called a zygote, is
  diploid. It has two homologous sets of
  chromosomes, one set from each parent

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• The Process of MeiosisAs a result of the
  alternation of fertilization and meiosis, life
  cycles of all sexually reproducing organisms
  involve alternating diploid and haploid stages.
• Producing haploid gametes by meiosis keeps the
  chromosome number from doubling in every
  generation.

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• Meiosis Versus Mitosis Meiosis produces haploid
  daughter cells from specialized cells in diploid
  organisms.
• Meiosis is different from mitosis in two major
  ways.
• The first major difference is that meiosis
  produces four new offspring cells, each with one
  set of chromosomesthus half the number of
  chromosomes as the parent cell

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• In contrast, mitosis produces two offspring
  cells, each with the same number of chromosomes
  as the parent cell.
• The second major difference is that meiosis
  involves the exchange of genetic material between
  homologous chromosomes.

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• The Two Meiotic Divisions
• Meiosis consists of two distinct partsmeiosis I
  and meiosis II.
• In meiosis I, homologous chromosomes, each
  composed of two sister chromatids, are separated
  from one another.
• In meiosis II, sister chromatids are separated
  much as they are in mitosis. However, the
  resulting cells are haploid rather than diploid.

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• Prophase I.
• Proteins cause the homologous chromosomes to
  actually stick together along their length. The
  paired chromosomes, now consisting of four
  chromatids, are referred to as tetrads. The
  tetrads attach to the spindle.
• The second new step is that the sister chromatids
  in the tetrads exchange some genetic material in
  the process known as crossing over.

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• Metaphase I During metaphase I, the tetrads move
  to the middle of the cell and line up across the
  spindle

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• Anaphase I In this stage, homologous chromosomes
  separate as they migrate to opposite poles of the
  spindle. Notice that the sister chromatids
  migrate togethereach chromosome is made up of
  two copies. Although this cell started with four
  chromosomes, there are only two chromosomes (each
  with two copies) moving to each pole.

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• Telophase I and Cytokinesis In telophase I, the
  chromosomes arrive at the poles. Each pole now
  has a haploid daughter nucleus because it has
  only one set of chromosomes, even though each
  chromosome consists of two sister chromatids.
• Cytokinesis usually occurs along with telophase
  I, forming two haploid daughter cells. The
  chromosomes in each daughter cell are still
  duplicated.

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• Prophase II In each haploid daughter cell, a
  spindle forms, attaches to the centromeres, and
  moves the individual chromosomes to the middle of
  the cell.

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• Metaphase II The chromosomes line up in the
  middle of the cell with spindle microtubules
  attached to each sister chromatid.

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• Anaphase II The sister chromatids separate and
  move to opposite poles

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• Telophase II and Cytokinesis The chromatids, now
  considered individual chromosomes, arrive at the
  poles. Cytokinesis splits the cells one more
  time. The process of meiosis is completed,
  producing four haploid daughter cells as a final
  result.