The chromosomal basis of inheritance

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Introduction to Genetic Analysis
Chapter 3 The chromosomal basis of inheritance
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The chromosome theory of inheritance

• The chromosome theory correlates the segregation
  of Mendels hereditary particles with the
  behavior of cellular structures (chromosomes)
• Sutton and Boveri recognized that the behavior of
  Mendels particles during the production of
  gametes precisely parallels the behavior of
  chromosomes at meiosis.

3
Definitions

• Chromatin
• The material that chromosomes are made of.
  Contains DNA as genetic material and proteins to
  organize the structure of the chromosome.
• nondisjunction
• The failure to separate and distribute equally a
  pair of homologous chromosomes.
• Nucleosome
• The repeating unit within chromatin. Composed of
  DNA wrapped around an octamer of histone
  proteins.

4
Key questions revisited

• How do we know that genes are parts of
  chromosomes?
• This is shown by the exact match between the
  patterns of gene inheritance and chromosomes
  behavior during meiosis. DNA sequencing has
  proven that genes are part of chromosomes.

5
Key questions revisited

• How are genes arranged on chromosomes?
• In a linear array from one end of the single DNA
  molecule to the other

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Key questions revisited

• What are chromosomes made of?
• Chromosome are made of chromatin DNA proteins.
  DNA is the genetic material, proteins organize
  the structure. The DNA is made of genes and
  intergenic regions between genes.

7
Key questions revisited

• How is chromosome number maintained?
• DNA replicates before cell division and a copy
  passes to each daughter cell. Genes control the
  accuracy of this process (e.g. genes encoding
  kinetochore or microtubules proteins)

8
Key questions revisited

• What are the chromosomal basis for Mendels laws?
• When a heterozygous A/a undergoes meiosis, the
  pairing of homologous chromosomes carrying the
  two alleles followed by their pulling apart
  ensures that half of the gametes possesses the
  chromosome with the A allele and the other half
  possesses the chromosome with the a allele.

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Key questions revisited

• What are the chromosomal basis for Mendels laws?
• In a dihybrid A/a B/b, spindle fiber attachment
  and pulling action are separate and independent
  for both chromosome pairs. Hence A can go to the
  same pole as B in some meiosis, or the same pole
  as b in others.

10
Key questions revisited

• How does all the DNA fit into a tiny nucleus?
• DNA is embedded in several levels of compaction
  through the action of many proteins. DNA is
  coiled and supercoiled, and the supercoils are
  looped upon a central scaffold.

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Chromosomes are the result of progressive levels
of packing
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Chromosomes are the result of progressive levels
of packing
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Experiment 10B Nucleosome Structure Revealed

• Markus Noll decided to test Kornbergs model
• He did this via the following procedure
• Digest DNA with the enzyme DNase I
• Accurately measure the molecular weight of the
  resulting DNA fragments
• The rationale is that the linker DNA is more
  accessible than the core DNA to the DNase I
• Thus, the cuts made by DNase I should occur in
  the linker DNA

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The Hypothesis

• The experiment tests the beads-on-a-string model
  of chromatin structure
• It the model is correct, DNase I should cut in
  the linker region
• Thereby producing DNA pieces that are about 200
  bp long

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(No Transcript)
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The Data
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Interpreting the Data
These longer pieces were all in multiples of 200
bp
At low concentrations, DNase I did not cut at all
the linker DNA
And this, three
All chromosomal DNA digested into fragments that
are 200 bp in length
This fragment contains two nucleosomes