Chapter 16 The Molecular Basis of Inheritance

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Chapter 16 The Molecular Basis of
Inheritance
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Question?

• Traits are inherited on chromosomes, but what in
  the chromosomes is the genetic material?
• Two possibilities
• Protein
• DNA

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Qualifications

• Protein
• very complex.
• high specificity of function.
• DNA
• simple.
• not much known about it (early 1900s).

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For testing

• Name(s) of experimenters
• Outline of the experiment
• Result of the experiment and the importance of
  the result

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Griffith - 1928

• Pneumonia in mice.
• Two strains
• S - pathogenic
• R - harmless

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Griffiths Experiment
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Result

• Something turned the R cells into S cells.
• Transformation - the assimilation of external
  genetic material by a cell.

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Problem

• Griffith used heat.
• Heat denatures proteins.
• So could proteins be the genetic material?
• DNA - heat stable.
• Griffiths results contrary to accepted views.

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Avery, McCarty and MacLeod - 1944

• Repeated Griffiths experiments, but added
  specific fractions of S cells.
• Result - only DNA transformed R cells into S
  cells.
• Result - not believed.

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Hershey- Chase 1952

• Genetic information of a virus or phage.
• Phage - virus that attacks bacteria and
  reprograms host to produce more viruses.

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Bacteria with Phages
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Phage Components

• Two main chemicals
• Protein
• DNA
• Which material is transferred to the host?

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Used Tracers

• Protein - CHONS, can trace with 35S.
• DNA - CHONP, can trace with 32P.

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Experiment

• Used phages labeled with one tracer or the other
  and looked to see which tracer entered the
  bacteria cells.

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Result

• DNA enters the host cell, but the protein did
  not.
• Therefore
• DNA is the genetic material.

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Picture Proof
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Chargaff - 1947

• Studied the chemical composition of DNA.
• Found that the nucleotides were in certain
  ratios.

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Chargaffs Rule

• A T
• G C
• Example in humans,
• A 30.9
• T 29.4
• G 19.9
• C 19.8

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Why?

• Not known until Watson and Crick worked out the
  structure of DNA.

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Watson and Crick - 1953

• Used X-ray crystallography data (from Rosalind
  Franklin)
• Used model building.
• Result - Double Helix Model of DNA structure.
  (One page paper, 1953).

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Rosalind Franklin
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Book Movies

• The Double Helix by James Watson- His account
  of the discovery of the shape of DNA
• Movie The Double Helix

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DNA Composition

• Deoxyribose Sugar (5-C)
• Phosphate
• Nitrogen Bases
• Purines
• Pyrimidines

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DNA Backbone

• Polymer of sugar-phosphate.
• 2 backbones present.

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Nitrogen Bases

• Bridge the backbones together.
• Purine Pyrimidine 3 rings.
• Constant distance between the 2 backbones.
• Held together by H-bonds.

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Chargaffs Rule

• Explained by double helix model.
• A T, 3 ring distance.
• G C, 3 ring distance.

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Watson and Crick

• Published a second paper (1954) that speculated
  on the way DNA replicates.
• Proof of replication given by others.

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Replication

• The process of making more DNA from DNA.
• Problem when cells replicate, the genome must be
  copied exactly.
• How is this done?

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Models for DNA Replication

• Conservative - one old strand, one new strand.
• Semiconservative - each strand is 1/2 old, 1/2
  new.
• Dispersive - strands are mixtures of old and new.

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Replication Models
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Meselson - Stahl late 1950s

• Grew bacteria on two isotopes of N.
• Started on 15N, switched to 14N.
• Looked at weight of DNA after one, then 2 rounds
  of replication.

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Results

• Confirmed the Semiconservative Model of DNA
  replication.

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Replication - Preview

• DNA splits by breaking the H-bonds between the
  backbones.
• Then DNA builds the missing backbone using the
  bases on the old backbone as a template.

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Origins of Replication

• Specific sites on the DNA molecule that starts
  replication.
• Recognized by a specific DNA base sequence.

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Prokaryotic

• Circular DNA.
• 1 origin site.
• Replication runs in both directions from the
  origin site.

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Eukaryotic Cells

• Many origin sites.
• Replication bubbles fuse to form new DNA strands.

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DNA Elongation

• By DNA Polymerases.
• Adds DNA triphosphate monomers to the growing
  replication strand.
• Matches A to T and G to C.

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Energy for Replication

• From the triphosphate monomers.
• Loses two phosphates as each monomer is added.

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Problem of Antiparallel DNA

• The two DNA strands run antiparallel to each
  other.
• DNA can only elongate in the 5--gt 3 direction.

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Leading Strand

• Continuous replication toward the replication
  fork in the 5--gt3 direction.

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Lagging Strand

• Discontinuous synthesis away from the replication
  fork.
• Replicated in short segments as more template
  becomes opened up.

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Homework

• Read Chapter 16
• Chapter 48 pt. 2 today
• Chapter 16 Mon. 1/14
• Newsweek article reaction paper Wed. 1/16

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Priming

• DNA Polymerase cannot initiate DNA synthesis.
• Nucleotides can be added only to an existing
  chain called a Primer.

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Primer

• Make of RNA.
• 10 nucleotides long.
• Added to DNA by an enzyme called Primase.
• DNA is then added to the RNA primer.

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Priming

• A primer is needed for each DNA elongation site.

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Okazaki Fragments

• Short segments (100-200 bases) that are made on
  the lagging strand.
• All Okazaki fragments must be primed.
• RNA primer is removed after DNA is added.

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Enzymes

• Replaces RNA primers with DNA nucleotides.
• DNA Ligase - joins all DNA fragments together.

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Other Proteins in Replication

• Helicase - unwinds the DNA double helix.
• Single-Strand Binding Proteins - help hold the
  DNA strands apart.

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Enzyme Summary
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DNA Replication Error Rate

• 1 in 1 billion base pairs.
• About 3 mistakes in our DNA each time its
  replicated.

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Reasons for Accuracy

• DNA Polymerase self-checks and corrects
  mismatches.
• DNA Repair Enzymes - a family of
  enzymes that checks and corrects DNA.

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DNA Repair

• 50 different DNA repair enzymes known.
• Failure to repair may lead to Cancer or other
  health problems.

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Example

• Xeroderma Pigmentosum -Genetic condition where a
  DNA repair enzyme doesnt work.
• UV light causes damage, which can lead to cancer.

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Xeroderma Pigmentosum
Cancer
Protected from UV
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Thymine Dimers

• T-T binding from side to side causing a bubble in
  DNA backbone.
• Often caused by UV light.

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Excision Repair

• Cuts out the damaged DNA.
• DNA Polymerase fills in the excised area with new
  bases.
• DNA Ligase seals the backbone.

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Problem - ends of DNA

• DNA Polymerase can only add nucleuotides in the
  5---gt3 direction.
• It cant complete the ends of the DNA strand.

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Result

• DNA gets shorter and shorter with each round of
  replication.

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Telomeres

• Repeating units of TTAGGG (100- 1000 X) at the
  end of the DNA strand (chromosome)
• Protects DNA from unwinding and sticking
  together.
• Telomeres shorten with each DNA replication.

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Telomeres
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Telomeres

• Serve as a clock to count how many times DNA
  has replicated.
• When the telomeres are too short, the cell dies
  by apoptosis.

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Implication

• Telomeres are involved with the aging process.
• Limits how many times a cell line can divide.

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Telomerase

• Enzyme that uses RNA to rebuild telomeres.
• Can make cells immortal.
• Found in cancer cells.
• Found in germ cells.
• Limited activity in active cells such as skin
  cells

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Comment

• Control of Telomerase may stop cancer, or extend
  the life span.

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NEWS FLASH

• The DNA of Telomers is actually used to build
  proteins.
• These proteins seem to impede telomerase.
• Feedback Loop??

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Summary

• Know the Scientists and their experiments.
• Why DNA is an excellent genetic material.
• How DNA replicates.
• Problems in replication.