Chapter 11 Notes

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Chapter 11 Notes

• Chromosomes, Sexual Reproduction and Meiosis

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• CHROMOSOME
• Supercoiled strand of DNA

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• CHROMOSOME STRUCTURE
• 1. Centromere narrowed area on the chromosome
  that divides the chromosome into two arms.
• 2. Telomeres the tips of the chromosome
• 3. Genes sections of DNA on the chromosome that
  code for proteins.

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Genes
Centromere
Telomeres
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• DIPLOID CELLS
• 2N
• Contain 2 non-identical copies of each chromosome
  (called homologous chromosome pairs)
• Include all somatic (body cells)
• Ex skin cell, brain cell

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• HOMOLOGOUS CHROMOSOMES
• Chromosomes that have a corresponding chromosome
  from the opposite sex parent.

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• FEATURES OF HOMOLOGOUS CHROMOSOMES
• 1. Identical in size
• 2. Centromeres in same position.
• Carry same complement of genes (same at the
  same location).
• Not identical because the DNA sequences between
  corresponding genes vary.
• Alleles corresponding genes on homologous
  chromosomes that may have different DNA
  sequences.

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Genes
Homologous Chromosomes
allele
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• HAPLOID CELLS
• N
• do not have homologous chromosome pairs. (Only
  contain one member of each homologous chromosome
  pair.)
• Include gametes (egg and sperm)

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• COMPARISON CHROMOSOME S IN DIPLOID AND HAPLOID
  CELLS
• Haploid cells have half the number of chromosomes
  of diploid cells.

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• SEXUAL REPRODUCTION
• Gametes (sperm and egg) from different parents
  unite to produce a new organism.

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• ADVANTAGES OF SEXUAL REPRODUCTION
• Produces new genetic combinations (asexual
  reproduction produces clones of parent)

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• WHY ARE GAMETES HAPLOID?
• Sperm (haploid) egg (haploid) zygote (diploid)

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• If sperm and egg were diploid, zygote would have
  double the correct number of chromosomes

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• FORMATION OF HAPLOID CELLS
• Occurs by meiosis

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• MEIOSIS
• Cell division that cuts the chromosome number in
  half by separating the homologous chromosomes of
  a dipoid cell into four haploid cells.

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• PURPOSES OF MEIOSIS
• Produce haploid gamete cells
• Keep chromosome number constant from generation
  to generation
• Increases genetic variation

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• CELL CYCLE
• Interphase
• Meiosis I and Cytokinesis I
• Meiosis II and Cytokinesis II

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• INTERPHASE
• DNA is replicated
• All chromosomes are doubled (2 identical sisters
  attached at the centromere)

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• MEIOSIS I AND CYTOKINESIS I
• Separates the pairs of homologous chromosomes
  into two daughter cells.

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• 1. Prophase I
• homologous chromosomes pair together to form a
  tetrad (composed of four sister chromatids)
• Homologous sister chromatids swap genes in a
  process called crossing over. This increases
  genetic variation.

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Tetrad
Crossing Over
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• 2. Metaphase I
• Tetrads line up in middle of cell
• Anaphase I
• Tetrads split and homologous chromosomes move to
  opposite poles
• Telophase and Cytokinesis I
• 2 daughter nuclei form
• cytoplasm is divided between 2 cells

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• RESULT OF MEIOSIS I
• Homologous chromosomes have been separated into
  two daughter cells

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THERE IS NO INTERPHASE BETWEEN MEIOSIS I AND II
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• MEIOSIS II
• Separates sister chromatids producing two more
  daughter cells
• Very similar to mitosis

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• Prophase II often skipped.
• Metaphase II Doubled chromosomes line in the
  middle of both cells
• Anaphase II sister chromatids split and move to
  opposite poles.
• Telophase/Cytokinesis II
• 4 daughter nuclei form
• cytoplasm is divided between four daughter cells

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• RESULTS OF MEIOSIS
• 1. 4 haploid daughter cells
• 2. each daughter cells has half the number of
  chromosomes as the parent cell.
• 3. All daughter cells are genetically different
  from each other and from the parent.

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Secondary Spermatocytes
Spermatids
Sperm
N
N
Primary Spermatocyte
N
Diploid 2N
N
N
N
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• SPERMATOGENESIS
• Meiotic cell division that produces four haploid
  sperm cells in males
• Location testes
• Entire process requires two months

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• PART III MEIOSIS IN HUMANS

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• HOW DO SPERMATIDS SPECIALIZE TO BECOME SPERM?
• 1. Formation of Flagellum (for propulsion)
• Formation of Acrosome (sac of enzymes that will
  digest outer layers of egg)
• Formation of Mid-piece (lined with mitochondria
  for production of ATP)
• Loss of excess cytoplasm (increase efficiency)

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• OOGENESIS
• Meiotic cell division in females that produces
  one haploid egg and three polar bodies
• Location ovaries
• Entire process requires 15-30 years

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• POLAR BODY
• Discarded set of chromosomes formed by uneven
  division of the cytoplasm

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Ovum (Egg)
Secondary Oocyte
N
N
Primary Oocyte
Diploid 2N
Polar Body
Polar Bodies
N
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• KARYOTYPE
• Picture of the chromosomes from one cell allowing
  the chromosome content of that cell to be
  studied.

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• KARYOTYPE OF A HUMAN DIPLOID CELL
• 46 chromosomes (23 homologous pairs)
• 44 autosomes (non-sex chromosomes)
• 2 sex chromosomes (Male XY, female XX)
• (male sex chromosomes are not truly homologous,
  but act as a homologous pair anyway)