Chromosomes

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Unit 8

• Chromosomes
• Meiosis
• Genetics

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Review

• What is a chromosome?
• What is a gamete?
• When can chromosomes be seen in the nucleus of a
  cell?
• What is this process called?
• What are genes?
• Where are genes located?

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Chromosomes

• DNA wrapped around protein cores (histones) and
  coiled further into a rod-shaped form
• Only seen during cell division (mitosis)
• Since mitosis is occurring, how much DNA is
  present?

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Chromosomes

• Chromatid one half of the chromosome
• Centromere the constricted middle of the
  chromosome
• Chromatin the DNA and histones in the nucleus of
  a non-dividing cell

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Chromosome Number

• Each species has a characteristic number of
  chromosomes
• Adders tongue fern has 2524 chromosomes
• Does the complexity of the organism correspond to
  its chromosome number?

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Chromosomes are categorized as either sex
chromosomes or autosomes.

• Humans have two sex chromosomes
• XX female
• XY male
• All other chromosomes are autosomes.
• (22 of the 23 pairs)

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Homologous chromosomes

• Every cell of an organism (except gametes) has
  two copies of each autosomeone from each parent.
• Homologous chromosomes the two copies of each
  autosome also known as homologues.
• Are the same size and shape, and carry genes for
  the same traits

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Karyotype

• A karyotype is a photomicrograph of the
  chromosomes in a dividing cell
• The chromosomes are arranged by size, with the
  largest pair being designated pair 1, and with
  the sex chromosomes being designated the last
  pair.

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What is the sex of this individual?
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What about this one?
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Diploid vs. Haploid Cells

• Diploid cells having two sets of chromosomes
  (2n)
• diploid for humans 46
• Haploid cells having only one set of chromosomes
  (n)
• haploid for humans 23
• Which cells in our bodies would be haploid?
• Why is it necessary for these cells to be haploid?

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Meiosis

• Meiosis the process of making gametes
• Reduces the number of chromosomes in new cells to
  half the number in the original cell
• Mitosis cell division
• Meiosis nuclear division
• Meiosis Animation

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Interphase

• G1 (gap 1) phase cell growth
• S (synthesis) phase DNA is copied
• G2 (gap 2) phase growth and preparation for cell
  division

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Chromatids, Chromosomes, EiyiyiI cant keep it
all straight!

• G1 DNA has not been copied yet (46 chromosomes)
• G2 DNA has been copied (still 46
  chromosomesmade up of 92 sister chromatids)

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Meiosis I

• Prophase I DNA coils tightly into chromosomes
  spindle fibers appear nucleus dissolves each
  chromosome lines up next to its homologue
  (synapsis)
• Each pair of homologous chromosomes is called a
  tetrad.
• Genes on one chromosome are adjacent to the
  corresponding genes on the other chromosome

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Crossing-Over

• During synapsis, the chromatids within a
  homologous pair twist around one another.
• Portions of chromosomes may break off and attach
  to adjacent chromatids on the homologous
  chromosomecrossing-over
• This process permits the exchange of genetic
  material between maternal and paternal
  chromosomes.

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Meiosis vs. Mitosis
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Size of Genomes

• Does the of genes have anything to do with the
  complexity of the organism?

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Classical Genetics

• The Legacy of Gregor Mendel
• Or
• The Monk with the Missing Peas

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The Big Question

• Why do children look like their parents?

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Genetics

• The branch of biology which deals with heredity
• -or-
• Why do children look like their parents?

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The Father of Genetics

• Gregor Mendel was the 1st person to succeed in
  predicting how traits are transferred from one
  generation to the next

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Why is he so important?

• He studied one trait at a time
• He analyzed his data mathematically
• He looked at multiple traits
• He used multiple trials

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The First Generation

• Mendel chose true-breeding pea plants as his
  parental generation (when self-pollinated, always
  produced the same type of offspring)
• He crossed a true-breeding tall plant with a
  true-breeding short plant
• All of the offspring were tall!

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The Second Generation

• Next, he crossed two tall offspring plants with
  each other
• ¾ of the offspring in the second generation were
  tall ¼ were short

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How to self-pollinate a pea plant
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The Second Generation, cont.

• Mendel did similar monohybrid crosses with the
  other traits as well.
• In every case, he found that one trait seemed to
  disappear in the F1 generation and reappear in ¼
  of the F2 plants
• This is where dominant and recessive come from

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The Law of Segregation

• Based on the results of his second generation
  crosses
• The two alleles for each trait must separate when
  gametes are formed
• A parent, therefore, passes on at random only one
  allele for each trait to each offspring

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Mendels Dihybrid Crosses

• Performed another set of crosses where he used
  peas that differed from each other in two traits
  rather than just one

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The first generation

• Took true-breeding pea plants that had round
  yellow seeds (RRYY) and crossed them with
  true-breeding pea plants that had wrinkled green
  seeds (rryy).
• The F1 plants all had round yellow seeds

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The second generation

• F1 plants self-pollinated
• F2 plants
• 9 round yellow
• 3 round green
• 3 wrinkled yellow
• 1 wrinkled green

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The Law of Independent Assortment

• Genes for different traits are inherited
  independently of each other
• When a pea plant with the genotype RrYy produces
  gametes, the alleles R and r will separate from
  each other (the law of segregation) as well as
  from the alleles Y and y and vice versa
• These alleles can then recombine in four
  different ways
• We now know that this is only true if genes are
  located on different chromosomes or are far apart
  on the same chromosome

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Testcross

• A cross of an individual of unknown genotype with
  an individual of a known genotype (usually
  homozygous recessive)
• Unknown R_ x rr
• If any offspring show the recessive phenotype,
  then the unknown parent must have been
  heterozygous

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Pedigrees

• A graphic representation of an individuals
  family tree, which permits patterns of
  inheritance to be recognized.

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When are pedigrees used?

• When testcrosses cannot be made
• When number of offspring is too small
• Or if results of testcross would take too long