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12–1 DNA
Photo credit: Jacob Halaska/Index Stock Imagery, Inc.
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2. Griffith and Transformation
In 1928, British scientist Fredrick Griffith was trying to learn how certain types of bacteria caused pneumonia.
He isolated two different strains of pneumonia bacteria from mice and grew them in his lab.
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3. Griffith and Transformation
Griffith finds a ‘transforming principle.’
Griffith experimented with the bacteria that cause pneumonia.
He used two forms: the S form (deadly) and the R form (not deadly).
A transforming material passed from dead S bacteria to live R bacteria, making them deadly.
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4. Griffith and Transformation
Griffith called this process transformation because one strain of bacteria (the harmless strain) had changed permanently into another (the disease-causing strain).
Conclusion: Griffith hypothesized that a factor must contain information that could change harmless bacteria into disease-causing ones.
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Avery and DNA
Avery and DNA
Oswald Avery repeated Griffith’s work to determine which molecule was most important for transformation.
Avery and his colleagues made an extract from the heat-killed bacteria that they treated with enzymes.
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Avery and DNA
Avery isolated and purified Griffith’s transforming principle.
Avery performed three tests on the transforming principle.
Qualitative tests showed DNA was present.
Chemical tests showed the chemical makeup matched that of DNA.
Enzyme tests showed only DNA-degrading enzymes stopped transformation.
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Avery and DNA
Conclusion: Avery identified DNA as the transforming principle.
Avery and other scientists discovered that the nucleic acid DNA stores and transmits the genetic information from one generation of an organism to the next.
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8. The Hershey-Chase Experiment
Alfred Hershey and Martha Chase studied viruses—nonliving particles smaller than a cell that can infect living organisms.
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9. The Hershey-Chase Experiment
Hershey and Chase studied viruses that infect bacteria, or bacteriophages. They performed 2 experiments.
Experiment 1: They tagged viral DNA with radioactive sulfur.
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10. The Hershey-Chase Experiment
Hershey and Chase studied viruses that infect bacteria, or bacteriophages. They performed 2 experiments.
Experiment 2: They tagged viral proteins with radioactive phosphorus.
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11. The Hershey-Chase Experiment
Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein.
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12. The Components and Structure of DNA
DNA is made up of nucleotides.
The parts of a DNA molecule is nitrogen bases and the backbone (deoxyribose – 5-carbon sugar & phosphate group)
phosphate group
deoxyribose (sugar)
nitrogen-containing
base
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13. The Components and Structure of DNA
There are four kinds of nitrogenous bases in in DNA:
adenine
guanine
cytosine
thymine
DNA is made up of nucleotides. Each nucleotide has three parts: a deoxyribose molecule, a phosphate group, and a nitrogenous base. There are four different bases in DNA: adenine, guanine, cytosine, and thymine.
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14. Rosalind Franklin used X-ray diffraction to get information about the structure of DNA.
Franklin’s x-ray images suggested that DNA was a double helix of even width.

15. The Components and Structure of DNA
Chargaff's Rules
Erwin Chargaff discovered that the percentages of guanine [G] and cytosine [C] and the percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA.
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16. Overall shape
Watson and Crick determined the three-dimensional structure of DNA by building models. Watson and Crick’s discovery built on the work of Rosalind Franklin and Erwin Chargaff.
They realized that DNA is a double helix that is made up of a sugar-phosphate backbone on the outside with bases on the inside.

17. Bonding
The backbone consists of the phospate group and deoxyribose sugar and is connected by covalent bonds.
The nitrogen containing bases (adenine, guanine, cytosine and thymine) are connected by hydrogen bonds.
hydrogen bond
covalent bond

18. The Components and Structure of DNA
DNA Double Helix
DNA is a double helix in which two strands are wound around each other. 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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12–1
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Avery and other scientists discovered that
DNA is found in a protein coat.
DNA stores and transmits genetic information from one generation to the next.
transformation does not affect bacteria.
proteins transmit genetic information from one generation to the next.
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The Hershey-Chase experiment was based on the fact that
DNA has both sulfur and phosphorus in its structure.
protein has both sulfur and phosphorus in its structure.
both DNA and protein have no phosphorus or sulfur in their structure.
DNA has only phosphorus, while protein has only sulfur in its structure.
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DNA is a long molecule made of monomers called
nucleotides.
purines.
pyrimidines.
sugars.
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Chargaff's rules state that the number of guanine nucleotides must equal the number of
cytosine nucleotides.
adenine nucleotides.
thymine nucleotides.
thymine plus adenine nucleotides.
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12–1
In DNA, the following base pairs occur:
A with C, and G with T.
A with T, and C with G.
A with G, and C with T.
A with T, and C with T.
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25. END OF SECTION