Outline for todays lecture Ch. 13

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Outline for todays lecture (Ch. 13)

• Sexual and asexual life cycles
• Meiosis
• Origins of Genetic Variation
• Independent assortment
• Crossing over (recombination)

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Heredity

• Transmission of traits between generations
• Molecular basis of heredity is DNA replication
• Gene is a specific segment of DNA
• Physical location on the chromosome is called a
  genetic LOCUS (plural loci)
• e.g., the eye-color locus, Adh locus

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Asexual Life Cycles

• Single (diploid) individual is the parent
• Parent passes copies of ALL its genes to its
  offspring (reproduces clonally)
• Various mechanisms
• Mitotic cell division in unicellular Eukaryotes
• Vegetative reproduction, e.g., plant cuttings,
  hydra budding
• Parthenogenesis

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Sexual Life Cycles

• Two (diploid) parents give rise to offspring
• Offspring differ genetically from their parents
  and their siblings
• GAMETES are haploid reproductive cells that
  transmit genes across generations

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Sexual Life Cycles

• Key Point Sexual reproduction ? Genetic
  variation
• MOST eukaryotes reproduce sexually at least
  sometimes
• Most prokaryotes (e.g., bacteria) exchange genes
  at least occasionally

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Sexual Life Cycles Human Example

• 46 Chromosomes
• 22 Homologous pairs, called autosomes
• Same length
• Same centromere position
• Same sequence (/-)
• SAME GENES!!

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Sexual Life Cycles Human Example

• One pair of sex chromosomes
• i.e., sex-determining gene(s) reside on these
  chromosomes
• Females are XX
• Males are XY
• Only small region of homology ( same genes)
  between X, Y

X
Y
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Schematic drawing of a chromosome
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Diploid cell, n3 BEFORE DNA replication

• 3 Homologous Pairs
• 2 autosomes
• 1 sex chromosome (XX)
• One homologous chromosome from each parent
• DNA content 2C
• Ploidy 2n

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Diploid cell, n3, AFTER DNA replication

• 3 Homologous Pairs
• One homologous chromosome from each parent TWO
  SISTER CHROMATIDS
• DNA content 4C
• Ploidy 2n

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Sexual Life Cycles - animals

• Free-living stage is diploid
• Gametes formed by meiosis
• Haploid gametes merge genomes to form diploid
  zygote (syngamy)

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Sexual Life Cycles - Plants

• Diploid sporophyte forms haploid spores by
  meiosis
• Spores form gametophyte by mitosis
• Gametophyte forms gametes by mitosis
• Gametes merge to form diploid zygote

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Sexual Life Cycles - Fungi

• Free-living, multicellular organism is haploid
• Gametes formed by mitosis
• Gametes merge to form diploid zygote
• Zygote undergoes meiosis to form haploid cells

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Meiosis

• RECALL Function of MITOSIS is to faithfully
  replicate the parental genome in each daughter
  cell with no change in information content
• Function of MEIOSIS is to produce haploid cells
  from diploid cells
• Necessary for the formation of gametes
• Necessary for sexual reproduction

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Meiosis an overview

• Interphase 1
• Begin with two homologous chromosomes,
• DNA content 2C
• Ploidy 2n (diploid)

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Meiosis an overview

• Interphase 1
• Chromosomes replicate
• DNA content 4C
• Ploidy 2n

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Meiosis an overview

• Meiosis I
• Homologous chromosomes separate
• Cell Division 1
• Result is TWO haploid (ploidy n) cells with TWO
  SISTER CHROMATIDS of one of the two homologs

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Meiosis an overview

• Meiosis II
• Sister chromatids separate
• Cell Division 2
• Result is FOUR haploid daughter cells, each with
  an unreplicated chromosome ( 1C)
• Half as many chromosomes as the parent cell

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Meiosis I early Prophase I

• Homologous chromosomes pair
• Synaptonemal complex (proteins) attaches homologs
• synapsis
• Homologs form tetrad

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Meiosis I late Prophase I
Chiasmata
Spindle fiber

• Chromosomes cross over, form chiasmata
• Exchange of DNA between homologs occurs at
  chiasma
• Spindles form and attach to kinetochores as in
  mitosis

Tetrad
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Meiosis I Metaphase I

• Chromosomes lined up on metaphase plate in
  homologous pairs
• Spindles from one pole attach to one chromosome
  of each pair
• Spindles from the other pole attach to the other
  chromosome of the pair

Kinetochore
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Meiosis I Anaphase I

• Homologous chromosomes separate and move along
  spindle fibers toward pole
• Sister chromatids remain attached at centromeres
• Note that recombination has occurred!

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Meiosis I Telophase and cytokinesis

• Homologous chromosomes reach (opposite) poles
• Each pole has complete haploid complement of
  chromosomes
• Each chromosome consists of two sister chromatids

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Meiosis II Prophase II

• Spindle forms
• Chromosomes move toward metaphase plate

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Meiosis II Metaphase II

• Chromosomes reach metaphase plate, as in mitosis
• Kinetochores of sister chromatids attach to
  spindle fibers from opposite poles

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Meiosis II Anaphase II

• Centromeres of sister chromatids separate
• Sister chromatids move toward opposite poles

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Meiosis II Telophase and cytokinesis

• Mechanism as before
• Note that now FOUR HAPLOID DAUGHTER CELLS formed
  from each parent cell
• Note that some chromosomes are recombinant, some
  are not

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Meiosis I - Summary
Chiasma (site of crossing-over)
Tetrad formed by synapsis of homologs
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Meiosis I - Summary
Tetrads align at metaphase plate
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Meiosis I - Summary
Homologous chromosomes separate Sister
chromatids remain paired
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Meiosis II - Summary
Sister chromatids separate Haploid daughter
cells result
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Origins of Genetic Variation

• Independent Assortment of Chromosomes
• Recombination among chromosomes
• Crossing over
• Recombination within chromosomes
• Random fertilization

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Independent Assortment of Chromosomes
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Independent Assortment of Chromosomes

• Number of possible combinations of chromosomes
  within a gamete
• Two homologs A, B Mom A1B1, Dad A2B2
• Four combinations A1B1, A1B2, A2B1, A2B2
• Three homologs Mom A1B1C1, Dad A2B2C2
• Eight combinations
• A1B1C1, A1B1C2, A1B2C1, A1B2C2, A2B1C1, A2B2C1,
  A2B1C2, A2B2C2
• n homologs 2n combinations

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Crossing-over Recombination within chromosomes

• Averages 2 per chromosome per meiosis in
  humans, flies
• If no crossing-over, genes on same chromosomes
  would always be inherited together

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Crossing-over Recombination within chromosomes
Human genome has 20K genes. Suppose each gene
assorts independently. How many combinations?
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Review Mitosis vs. Meiosis

• Event Mitosis Meiosis
• DNA Replication Interphase Interphase I
• Cell Divisions 1 2
• Daughter cells 2
  4
• Ploidy of daughters 2n (diploid) n
  (haploid)
• Synapsis of homologs? No Yes
• Crossing-over No Yes
• (recombination)
• Biological Purpose Duplicate cells Generate
• faithfully gametes

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Meiosis, Genetic variation, and Evolution

• Role of segregation
• Role of crossing-over
• What about LIMITS to evolution?
• E.g., body size

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For Thursday Introduction to Mendelian Genetics

• Read Chapter 14 through p. 260