The Structure of DNA

1
The Structure of DNA

• Learning Targets
• What did scientists discover about the
  relationship between genes and DNA?
• What is the overall structure of the DNA
  molecule?

2
Interest Grabber
Section 12-1

• Genes are made of DNA, a large, complex molecule.
  DNA is composed of individual units called
  nucleotides. Three of these units form a code.
  The order, or sequence, of a code and the type of
  code determine the meaning of the message.

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3
Interest Grabber

• 1. On a sheet of paper, write the word cats. List
  
• the letters or units that make up the word
  cats.
• 2. Try rearranging the units to form other
  words.
• Remember that each new word can have only three
  units. Write each word on your paper, and then
  add a definition for each word.
• 3. Did any of the codes you formed have the same
  
• meaning?
• 4. How do you think changing the order of the
• nucleotides in the DNA codon changes the
  codons message?

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Section Outline
Section 12-1

• DNA
• A. Griffith and Transformation
• 1. Griffiths Experiments
• 2. Transformation
• B. Avery and DNA
• C. The Hershey-Chase Experiment
• 1. Bacteriophages
• 2. Radioactive Markers
• D. The Structure of DNA
• 1. Chargaffs Rules
• 2. X-Ray Evidence
• 3. The Double Helix

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5
Griffiths Experiment

• In 1928, British scientist Frederick Griffith was
  trying to figure out how certain types of
  bacteria produce a serious lung disease known as
  pneumonia.
• Griffith had isolated two slightly different
  strains, or types, of pneumonia bacteria from
  mice. Both strains grew very well in culture
  plates in his lab, but only one of the strains
  caused pneumonia.
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Figure 122 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)
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7
Griffiths Conclusions

• Griffith hypothesized that when the live,
  harmless bacteria and the heat-killed bacteria
  were mixed together, some factor was transferred
  from the heat-killed cells into the live cells.
  That factor might contain a gene with the
  information that could change harmless bacteria
  into disease-causing ones.
• He called this process as transformation.

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Averys Conclusion

• Avery and his colleagues made an extract, or
  juice, from the heat-killed bacteria. They then
  carefully treated the extract with enzymes that
  destroyed proteins, lipids, carbohydrates, and
  other molecules, including the nucleic acid RNA.
  Transformation still occurred.
• Avery and other scientists discovered that DNA is
  the nucleic acid that stores and transmits the
  genetic information from one generation of an
  organism to the next.
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Hershey Chases Experiment

• Hershey and Chase reasoned that if they could
  determine which part of the virusthe protein
  coat or the DNA coreentered the infected cell,
  they would learn whether genes were made of
  protein or DNA.
• The two scientists mixed the marked viruses with
  bacteria. Then, they waited a few minutes for the
  viruses to inject their genetic material. Next,
  they separated the viruses from the bacteria and
  tested the bacteria for radioactivity. Nearly all
  the radioactivity in the bacteria was from
  phosphorus (32P), the marker found in DNA.  

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 Hershey-Chase Experiment
Bacteriophage with phosphorus-32 in DNA
Phage infectsbacterium
Radioactivity inside bacterium
Bacteriophage with sulfur-35 in protein coat
Phage infectsbacterium
No radioactivity inside bacterium
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11
Hershey Chase Conclusion

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• Hershey and Chase concluded that the genetic
  material of the bacteriophage was DNA, not
  protein.

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Structure of DNA

• DNA is a long molecule made up of units called
  nucleotides.
• Each nucleotide is made up of three basic parts
  a 5-carbon sugar called deoxyribose, a phosphate
  group, and a nitrogenous base.
• There are four kinds of nitrogenous bases in DNA.
  Two of the nitrogenous bases, adenine and
  guanine, belong to a group of compounds known as
  purines. The remaining two bases, cytosine and
  thymine, are known as pyrimidines.

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Structure of DNA

• The backbone of a DNA chain is formed by sugar
  and phosphate groups of each nucleotide. The
  nitrogenous bases stick out sideways from the
  chain.

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 DNA Nucleotides
Purines
Pyrimidines
Adenine
Guanine
Cytosine
Thymine
Phosphate group
Deoxyribose
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15
Chargaffs Rules

• Chargaff discovered that the percentages of
  guanine G and cytosine C bases are almost
  equal in any sample of DNA. The same thing is
  true for the other two nucleotides, adenine A
  and thymine T.
• The observation that A T and G C
  became known as Chargaffs rules.
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Percentage of Bases in Four Organisms
Source of DNA A T G C
Streptococcus 29.8 31.6 20.5 18.0 Yeast 31.3 32.9
18.7 17.1 Herring 27.8 27.5 22.2 22.6 Human 30.9 2
9.4 19.9 19.8
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17
Franklins Conclusion

• Using X-ray diffraction technique, Franklin
  discovered that the strands in DNA are twisted
  around each other like the coils of a spring, a
  shape known as a helix. The angle of the X
  suggests that there are two strands in the
  structure. Other clues suggest that the
  nitrogenous bases are near the center of the
  molecule.
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Watson Crick

• DNA is a double helix in which two strands are
  wound around each other. A double helix looks
  like a twisted ladder or a spiral staircase.
• Each strand is made up of a chain of nucleotides.
  The two strands are held together by hydrogen
  bonds between adenine and thymine and between
  guanine and cytosine.
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 Structure of DNA
Nucleotide
Hydrogen bonds
Sugar-phosphate backbone
Key Adenine (A) Thymine (T) Cytosine (C) Guanine
(G)
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20
Checkpoint

• What are the complementary nucleotides on the DNA
  strand
• Ans A-T, G-C
• Identify the complementary nucleotides sequence
  on the other strand of DNA, if one strand has the
  following sequence ACTGCCA
• Ans TGACGGT