DNA the Genetic Material

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DNA the Genetic Material

• Chapter 9

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Questions

• What are genes made of? Is it protein or DNA?
• How do chromosomes function in heredity?
• What is the history behind the discovery of DNA
  as the genetic material?

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Friedrich (Fritz) Miescher 1869

• Isolated the first crude preparation of DNA. He
  named it nuclein.
• His task was to study white blood cells.
• These cells were difficult to extract from the
  lymph glands, but they were found in great
  quantities in the pus from infections.
• Miescher collected bandages from a nearby clinic
  and washed off the pus.
• He used an alkaline solution to lyse the cells
  and extract the material in the nucleus

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THOMAS HUNT MORGAN (1866-1945)

• About 1910 Thomas Hunt Morgan established that
  genes were carried on chromosomes.
• He used Drosophila melanogaster, fruit flies for
  his research
• Determined the existence of sex linked inheritance

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Phoebus Levene 1929

• Each nucleotide building block of DNA is make up
  of a phosphate group linked to a deoxyribose
  sugar which is linked to a nitrogenous base.

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Experiments by Frederick Griffith 1928

• Disease strain smooth colonies, S, encapsulated
  in a coating
• Disease causing bacteria killed mice
• Harmless strain rough colonies, R, not
  encapsulated
• Harmless bacteria did not kill mice
• Heat killed disease S bacteria did not kill mice
• Heat killed disease S bacteria mixed with
  harmless R bacteria killed mice

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Results of Griffiths Experiments

• His experiments showed that genetic information
  could be transformed from one bacterium to
  another
• Transformation is a change in phenotype caused
  when bacterial cells take up foreign genetic
  material.

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Griffiths Experiment
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Oswald Averys Experiments 1944

• Wanted to find out what had caused the results of
  Griffiths experiments.
• He destroyed all cell parts but DNA, that is
  proteins, lipids, carbohydrates transformation
  still occurred.
• When DNA was destroyed no transformation occurred.

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DNA is responsible for transformation

• Avery determined that DNA is the hereditary
  material involved in transformation in
  pneumococcus bacteria.
• DNA is the nucleic acid that stores and transmits
  the genetic information from one generation to
  the next
• A gene is made of DNA

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Beadle and Tatum 1941

• One gene one enzyme hypothesis
• Worked with Neurospora, a bread mold,
• haploid, no dominant, masking alleles
• Each gene translates to an enzyme needed by the
  organism. They examined mold mutated by X-rays
  where one gene was prevented from making certain
  enzymes that the Neurospora needed to grow. Only
  when a certain vitamin, needed to produce the
  enzyme, was added did the mold grow
• One gene makes one protein

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Barbara McClintock, 1944

• Discovers that genes can be transposed from one
  position to another on a chromosome.
• Jumping gene hypothesis

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Erwin Chargaff 1950- Chargaffs Rules

• in DNA the amounts of adenine and thymine are
  about the same, as are the amounts of guanine and
  cytosine
• AT
• CG

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Linus Pauling 1951

• Discovered that the molecules of some proteins
  have helical shapesarrived at through the use of
  atomic models
• Proposed a three stranded shape for DNA

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Rosalind Franklin 1952

• Through technique of X-ray crystallography
  discovered that DNA was shaped in a double
  stranded helix not three as had been proposed by
  Linus Pauling

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Watson and Crick 1953

• James Watson and Francis Crick proposed a model
  for DNA - a double-stranded, helix with
  complementary strands aligned anti-parallel to
  each other
• Phosphate groups located on the outside of the
  chain alternate with deoxyribose sugar groups.
  The two chains are held together by hydrogen
  bonds between pairs of nitrogen basesadenine (A)
  with thymine (T) and guanine (G) with cytosine
  (C).
• With Maurice Wilkins they win Nobel Prize 1962

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Hershey -Chase Experiments 1952

• Their experiments proved that DNA was the genetic
  material and not proteins.
• Bacteriophage- virus that infects a bacteria
  composed of DNA or RNA and a protein coat.
• Proteins contain almost no phosphorus and DNA
  contains no sulfur.
• They used radioactive markers. When P-32 found
  inside bacteria they proved that DNA was the
  genetic material
• Bacteria and viruses have DNA too
• The Hershey-Chase Experiment

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Building blocks of DNA

• The DNA molecule is shaped like a twisted ladder

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DNA is semi conservative

• Half the DNA ladder is a template for copying the
  whole thing