Chp' 8: The Cellular Basis of Reproduction

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Chp. 8 The Cellular Basis of Reproduction
Inheritance
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Cell Division What is it For?

• Replacing lost or damaged cells
• Growth
• Genes passed on from cell to cell
• Some organisms reproduce this way

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Eukaryotic Chromosomes

• Not always visible
• Only seen when cell is dividing
• Chromatin
• Diffuse, extended form chromosomes are in when
  cell not dividing
• DNA proteins
• How many chromosomes do humans have???

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Eukaryotic Chromosomes

• Chromosomes duplicate before cell divides
• 2 sister chromatids with identical genes
• Held together by centromere
• When cell divides, one chromosome goes to each
  daughter cell

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The Cell Cycle

• Orderly sequence of events
• Interphase
• Most of a cells time
• Cell performs its various functions metabolizes
• Doubles almost everything in cytoplasm
• S phase chromosomes duplicate
• G1 G2 phases gap phases
• G1 chromatin
• G2 sister chromatids

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The Cell Cycle

• Mitotic phase (M phase)
• Cell actually dividing
• Mitosis nucleus contents dividing
• Cytokinesis cytoplasm divides

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The Cell Cycle
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Mitosis

• Unique to eukaryotes
• Very accurate error in chromosome distribution
  1 in 100,000 cell divisions
• Phases (dont forget Interphase precedes these)
• Prophase
• Metaphase
• Anaphase
• Telophase

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Prophase (Early)

• Chromosomes seen as 2 sister chromatids
• Nucleoli disappear
• Mitotic spindle begins to form
• Centrosomes moving towards poles

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Prophase (Late)

• Nuclear membrane breaks up
• Spindle microtubules attach to sister chromatids

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Metaphase

• Mitotic spindle fully formed
• Centromeres line up at imaginary equatorial plate
• One spindle microtubule attaches to each sister
  chromatid

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Anaphase

• Sister chromatids separate
• Each daughter chromosome is pulled towards a pole
• Cell elongates

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Telophase

• Chromosomes have reached poles
• Cleavage furrow forms
• Nuclear envelope forms
• Nucleoli appear
• Spindle disappears
• Cytokinesis occurs

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Cancer Cells Out Of Control

• What is cancer?
• Disease of the cell cycle
• Excessive division, bizarre behavior
• Malignant
• Metastasis spread of cancer cells

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Cancer Treatment

• Surgery
• Radiation therapy disrupts cell division
• Chemotherapy drugs disrupt cell division

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Meiosis The Basis of Sexual Reproduction

• Like begets like
• Siblings resemble each other

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Homologous Chromosomes

• Somatic cell
• Typical body cell
• Normal human how many chromosomes?
• Break one open during metaphase make a
  karyotype
• The matching pairs are called homologous
  chromosomes

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Karyotype
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Human Chromosomes

• Autosomes
• Non-sex chromosomes
• How many pairs?
• Sex chromosomes
• Determine gender
• Normal female ??
• Normal male ??
• You get 23 chromosomes from each parent

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Cells of a Sexual Organism

• Diploid (2n)
• Cells contain 2 homologous sets of chromosomes
• All somatic cells
• Haploid (n)
• Cells contain a single set of chromosomes
• Sex cells (gametes)

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Human Life Cycle

• Haploid sperm fertilizes haploid egg
• Resulting zygote is diploid
• Growth, development
• Human adult
• Some cells go through meiosis
• Gametes (sperm egg)

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Human Life Cycle
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Meiosis

• Produces haploid gametes in diploid organisms
• Meiosis I (preceded by Interphase)
• Meiosis II

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Prophase I

• Homologous chromosomes stick together in pairs
  ---gt tetrad
• Homologous chromosomes can now exchange
  corresponding segments
• Crossing over
• Rearranges genetic information
• Spindle begins to form
• Tetrads move towards
  center of cell

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Metaphase I

• Tetrads aligned in middle of cell
• Spindle microtubules attach
• One to each pair of sister
  chromatids

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Anaphase I

• Sister chromatids migrate to poles
• Homologous pairs now separated

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Telophase I

• Cell divides
• Cytokinesis
• Left with 2 haploid daughter cells
• Chromosomes still together as sister chromatids

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Meiosis II

• Essentially the same as mitosis
• Sister chromatids split apart
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
• Result 4 haploid daughter cells

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Mitosis vs. Meiosis
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The Origins of Genetic Variation

• Independent assortment
• Random fertilization
• Crossing over

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Independent Assortment

• Homologous chromosomes differ genetically
• Orientation of homologous pairs at Metaphase I is
  by chance
• Thus, all of your gametes are not exactly alike
• Total number of chromosome combinations in an
  individual 2n
• n haploid number
• How many combinations???

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Independent Assortment
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Random Fertilization

• Any one of 8,388,608 eggs can be fertilized by
  any one of 8,388,608 sperm
• How many combinations???

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Crossing Over

• Occurs between homologous pairs during Prophase I
• Chiasma site of crossing over
• Genetic recombination
• Production of gene combinations different from
  those carried by parental chromosomes
• Adds to variation

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Crossing Over
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Accidents in Meiosis

• Errors in chromosome number
• Nondisjunction
• When a member of a chromosome pair fails to
  separate at Anaphase
• Happens occasionally in either Meiosis I or
  Meiosis II Why???

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Nondisjunction
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Hypothesis Why Meiotic Accidents Happen

• Meiosis begins in a womans ovaries before she is
  born
• Not completed until time of ovulation
• One egg matures per cycle
• Cell is stuck in meiosis for decades
• Being stuck may lead to meiotic errors

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Errors in Chromosome Number

• Trisomy 21
• Down syndrome
• Most common chromosome
  number abnormality
• Most common serious birth defect in U.S.
• 1 in 700
• Incidence increases markedly with age of the
  mother

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Down Syndrome
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Maternal Age Down Syndrome
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Abnormal Numbers of Sex Chromosomes

• Also caused by nondisjunction
• Klinefelter Syndrome XXY
• Turner Syndrome XO
• Metafemale XXX
• XYY Normal male

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Abnormal Numbers of Sex Chromosomes