History of DNA structure and its importance

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History of DNA structure and its importance

• How did we learn that DNA is the key to coding
  for all characteristics of living things?

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A timeline

• 1928
• British scientist -- Frederick Griffith
  studies bacteria looking for cause of
  pneumonia
• found two specific strains or cultures of
  bacteria that looked different when growing
  on petri dishes
• -one grew in smooth-edged groups -other one
  produced colonies that were rough and ragged
  around the edges

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Why important?

• Visual differences made it easy to recognize and
  distinguish between the strains of bacteria
• Also, Griffith found that
• smooth-edged colonies of bacteria caused
  disease
• rough-edged colonies were harmless

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Griffiths Experiment
Section 12-1
Heat-killed, disease-causing bacteria (smooth
colonies)
Harmless bacteria (rough colonies)
Control(no growth)
Harmless bacteria (rough colonies)
Heat-killed, disease-causing bacteria (smooth
colonies)
Disease-causing bacteria (smooth colonies)
Dies of pneumonia
Dies of pneumonia
Lives
Lives
Live, disease-causingbacteria (smooth colonies)
Go to Section
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Results of Griffiths 1928 experiment Discovery
of process of Transformation

• Somehow the heat-killed bacteria had passed their
  disease-causing ability to the harmless strain
• The harmless strain had been transformed into a
  disease-causing strain
• Hypothesized that some factor was responsible
  for this change

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Timeline cont

• 1944
• -American, Oswald Avery, continued bacteria
  research of Griffith
• -Knew were 4 types of organic compounds that
  make up all life
• - used enzymes to destroy lipids,
  carbohydrates, proteins, and RNA in an
  extract from the disease causing bacteria.

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• Transformation still occurred, so obviously the
  molecules they had destroyed were not responsible
  for transformation.
• Only organic molecule left that had not been
  destroyed was DNA
• When repeated experiment with DNA-destroying
  enzymes, no transformation occurred.DNA was the
  key to heredity

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Timeline cont

• 1952
• -Americans Alfred Hershey and Martha Chase
• -worked with viruses called bacteriophages
• -viruses are simple DNA or RNA core and a
  protein coat around them
• -when infect, bacteriophages inject DNA or RNA
  into cell and protein coat is left outside
• -used radioactive markers to trace
• phosphorus-32 (32P) for DNA
• sulfur-35 (35S) for protein coat

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Figure 16.2a The Hershey-Chase experiment phages
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Hershey-Chase Experiment
Section 12-1
Bacteriophage with phosphorus-32 in DNA
Phage infectsbacterium
Radioactivity inside bacterium
Bacteriophage with sulfur-35 in protein coat
Phage infectsbacterium
No radioactivity inside bacterium
Go to Section
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Results of Hershey-Chase

• When viruses were separated from the bacteria and
  tested for radioactivity, all of the
  radioactivity from the bacteria was found to be
  32P
• Conclusion genetic material of the
  bacteriophage that was transferred was DNA

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Race for the structure of DNA

• 1940
• -Erwin Chargaff discovers that percentages
  of A and T are equal in any sample of DNA same
  is true for C and G
• 1944
• - Linus Pauling discovers that proteins can
  have a helical shape
• 1952
• - Rosalind Franklin takes pictures of DNA
  molecule using technique called X-ray
  diffraction, shows that DNA has helical shape

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Figure 16.4 Rosalind Franklin and her X-ray
diffraction photo of DNA
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• 1951-1952
• -Maurice Wilkins works with X- ray diffraction
  and sees same pattern as Franklin, shares info
  with James Watson
• April, 1953
• -James Watson and Francis Crick build first
  model of DNA
• (are awarded Nobel Prize in 1960s)

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Figure 5.x3 James Watson and Francis Crick
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Basic DNA structure

• Exists as a double helix
• Uprights made up of alternating deoxyribose
  (sugar) and phosphate groups
• Bases are attached to the sugars
• Bases in DNA are adenine, thymine, cytosine, and
  guanine
• A pairs with T, C pairs with G and vice-versa
• A and G are purines larger, double rings
• T and C are pyrimidines smaller, single rings

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What determines primary structure of a protein?

• Gene unit of inheritance that determines the
  sequence of amino acids
• made of DNA (polymer of
• nucleic acids)
• Building blocks of nucleic acids are nucleotides
  
• phosphate group, pentose sugar, nitrogenous
  base (A,T,C,G,U)

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Figure 16.6 Base pairing in DNA
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Chromosome Structure of Eukaryotes
Section 12-2
Nucleosome
Chromosome
DNA double helix
Coils
Supercoils
Histones
Go to Section
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2 Important Gene Functions

• Carry information from one generation to
  the next.
• 2. Have to be easily copied each time a cell
  divides.

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DNA Replication

• DNA opens up and makes a complete copy of itself
  necessary during mitosis and meiosis
• New nucleotides float in and pair in a
  complementary fashion A to T, C to G and vice
  versa

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Figure 16.7 A model for DNA replication the
basic concept (Layer 1)
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Figure 16.7 A model for DNA replication the
basic concept (Layer 2)
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Figure 16.7 A model for DNA replication the
basic concept (Layer 3)
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Figure 16.7 A model for DNA replication the
basic concept (Layer 4)
Semi-conservative process
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Replication

• Begin at origins of replication
• Occurs in a Replication bubble each end is
  called a Replication fork
• Catalyzed by DNA polymerase