Meiosis

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Meiosis

• Chapter 8
• Part II

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Agenda

• Textbook Pages 136-143 Meiosis and crossing
  over, Pages 181-193 The structure of genetic
  material.
• Powerpoint

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Quiz dates

• 12/7 Germ cell vs Somatic cell, Meiosis, crossing
  over, Homologous chromosomes.
• 12/9 TBA

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Octopus Sex

• Male reaches under his mantle with tentacle,
  removes packet of sperm, and insert it into
  females egg chamber
• Eggs are fertilized and give rise to new octopuses

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Limpet Sex

• Larva can become adult of either sex
• Depends on whether other limpets are present and
  what their sex is
• Adults can change sex in response to new arrivals

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Slipper snails

• Hermaphroditic
• Stacked on top of each other
• Young are always male
• Can stay male or change to female
• Cannot go back to male

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Sexual Reproduction

• Chromosomes are duplicated in germ cells
• Germ cells undergo meiosis and cytoplasmic
  division
• Cellular descendents of germ cells become gametes
• Gametes meet at fertilization

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Meiosis
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Asexual Reproduction

• Single parent produces offspring
• All offspring are genetically identical to one
  another and to parent

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Sexual Reproduction

• Involves
• Meiosis
• Gamete production
• Fertilization
• Produces genetic variation among offspring

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Homologous Chromosomes Carry Different Alleles

• Cell has two of each chromosome
• One chromosome in each pair from mother, other
  from father
• Paternal and maternal chromosomes carry different
  alleles

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Alleles- Different forms of a gene
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What is a gene?

• Chemical factors that determine traits

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Sexual Reproduction Shuffles Alleles

• Through sexual reproduction, offspring inherit
  new combinations of alleles, which leads to
  variations in traits
• This variation in traits is the basis for
  evolutionary change

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Gamete Formation

• Gametes are sex cells (sperm, eggs)
• Arise from germ cells

ovaries
anther
ovary
testes
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Chromosome Number

• Sum total of chromosomes in a cell
• Germ cells are diploid (2n)
• Gametes are haploid (n)
• Meiosis halves chromosome number

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Meiosis Two Divisions

• Two consecutive nuclear divisions
• Meiosis I
• Meiosis II
• DNA is NOT duplicated between divisions
• Four haploid nuclei are formed

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Meiosis I
Each homologue in the cell pairs with its
partner,
then the partners separate
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Meiosis II

• The two sister chromatids of each duplicated
  chromosome are separated from each other

two chromosomes (unduplicated)
one chromosome (duplicated)
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Stages of Meiosis

• Meiosis I
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I

• Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

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Meiosis I - Stages
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92
46x2
46x2
46x2
23x4
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Prophase I

• Each duplicated, condensed chromosome pairs with
  its homologue
• Homologues swap segments !!!!!!! Take note !!!!
• Each chromosome becomes attached to microtubules
  of newly forming spindle

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Crossing-Over
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Crossing-Over
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Crossing-Over
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Crossing Over

• Each chromosome becomes zippered to its homologue
• All four chromatids are closely aligned
• Non-sister chromosomes exchange segments

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Effect of Crossing Over

• After crossing over, each chromosome contains
  both maternal and parental segments
• Creates new allele combinations in offspring

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Metaphase I

• Chromosomes are pushed and pulled into the middle
  of cell
• Sister chromatids of one homologue orient toward
  one pole, and those of other homologue toward
  opposite pole
• The spindle is now fully formed

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Anaphase I

• Homologous chromosomes segregate from each other
• The sister chromatids of each chromosome remain
  attached

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Telophase I

• The chromosomes arrive at opposite poles
• The cytoplasm divides
• There are now two haploid cells
• This completes Meiosis I

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Meiosis II - Stages
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Prophase II

• Microtubules attach to the kinetochores of the
  duplicated chromosomes
• Motor proteins drive the movement of chromosomes
  toward the spindles equator

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Metaphase II

• All of the duplicated chromosomes are lined up at
  the spindle equator, midway between the poles

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Anaphase II

• Sister chromatids separate to become independent
  chromosomes
• Motor proteins interact with microtubules to
  move the separated chromosomes to opposite poles

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Telophase II

• The chromosomes arrive at opposite ends of the
  cell
• A nuclear envelope forms around each set of
  chromosomes
• The cytoplasm divides
• There are now four haploid cells

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Random Alignment

• Either the maternal or paternal member of a
  homologous pair can end up at either pole
• The chromosomes in a gamete are a mix of
  chromosomes from the two parents

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Possible Chromosome Combinations

• As a result of random alignment, the number of
  possible combinations of chromosomes in a gamete
  is
• 2n
• (n is number of chromosome types)

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Possible ChromosomeCombinations
1
2
3
or
or
or
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Fertilization

• Male and female gametes unite and nuclei fuse
• Fusion of two haploid nuclei produces diploid
  nucleus in the zygote
• Which two gametes unite is random
• Adds to variation among offspring

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Factors Contributing to Variation Among Offspring

• Crossing over during prophase I
• Random alignment of chromosomes at metaphase I
• Random combination of gametes at fertilization

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Mitosis Meiosis Compared

• Mitosis
• Functions
• Asexual reproduction
• Growth, repair
• Occurs in somatic cells
• Produces clones

• Meiosis
• Function
• Sexual reproduction
• Occurs in germ cells
• Produces variable offspring

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Prophase vs. Prophase I

• Prophase (Mitosis)
• Homologous pairs do not interact with each other
• Prophase I (Meiosis)
• Homologous pairs become zippered together and
  crossing over occurs

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Results of Mitosis and Meiosis

• Mitosis
• Two diploid cells produced
• Each identical to parent
• Meiosis
• Four haploid cells produced
• Differ from parent and one another

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Pre-Natal Tests - help to determine if an at risk
baby is healthy
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Human Karyotype
Sex chromosomes
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Sex Chromosomes
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Down Syndrome (trisomy 21), 47,XY,21. 47 the
total number of chromosomes (46 is normal). XY
the sex chromosomes (male). 21 designates the
extra chromosome as a 21.
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Klinefelters Syndrome 47, XXY
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Turners Syndrome

• Only one complete X
• The second could also be incomplete
• In almost all cases, unable to repro. Or
  menstruate

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Hermaphrodite

• XX/XY or XO
• 350-400 Known cases

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Gene Therapy - an absent or faulty gene is
replaced by a working gene Has not been very
successful