Genetics Mini Exam I: The Discovery of the Double Helix

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  1. To solve the structure of the double helix, Watson and Crick used __

    __ helped with the structure determination
    model building—they built a scale model of what they thought a DNA molecule must look like

    Chargaff’s base ratios
  2. How did Chargaff come up with his base ratios?
    He used paper chromatography to measure the amounts of each of the four nucleotides, discovering that A=T and C=G
  3. What was the second piece of evidence that helped?
    The second piece of evidence was the X-ray diffraction pattern obtained when a DNA fiber is bombarded with X-rays. Some were diffracted, while others passed straight through. There were a series of spots and smears, which showed that DNA is a helix Though Rosalind Franklin developed the picture, Watson and Crick determined that the double helix was the only logical structure
  4. Eventually, it was determined that the __ had to be on the outside of the molecule and the two strands had to be __and __

    The combination of A pairing with T and C pairing with G would result in __
    • sugar-phosphate backbone
    • antiparallel
    • right-handed
    • identical sizes, which would fit perfectly in the center
  5. At first, people thought genes were made of __

    Cytochemistry, in which cells are __, showed that chromosomes are made of __ and __, in roughly equal amounts
    • protein
    • examined under the microscope after staining with dyes that bind specifically to just one type of biochemical
    • DNA and protein
  6. To determine whether it was DNA or protein that made genes, they looked at the __.

    The most fundamental requirement of the genetic material is that it be able to__
    • properties of genes
    • exist in an almost infinite variety of forms
  7. Each cell has a large number of genes, and each specifies a __, and each has a different structure. Therefore, the genetic material must have a __
    • different biological characteristic
    • great deal of chemical variability
  8. Why did the idea of variability initially ruled out DNA ?
    since DNA is uniform and proteins have variability with its amino acids and the massive amount of proteins
  9. The idea that genetic material was made of proteins remained strong until it was eventually overturned by two experiments. 

    Explain the first
    Avery, MacLeod, and McCarthy: transforming principle They prepared an extract from dead cells of Streptococcus pneumonia, a bacterium that causes pneumonia. Something in the extract transformed the harmless strain into one capable of disease. Eventually, they proved it was DNA.
  10. The idea that genetic material was made of proteins remained strong until it was eventually overturned by two experiments. 

    Explain the second.
    Hershey and Chase: when a bacterium is infected with the virus, the DNA enters but the protein stays out; and, the new viruses that are produced contain DNA, but only small amounts of protein
  11. Despite these experiments, why were many people not convinced that DNA was the genetic material?
    People worried about the specificity of the deoxyribonuclease enzyme that Avery used to inactivate the transforming principle. This result, a central part of the evidence that the transforming principle is DNA, would be invalid if the enzyme contained trace amounts of a contaminating protease and so was also able to degrade protein. When Watson and Crick determined the structure, it was much more accepted that genes are made of DNA
  12. Though this applied to bacteria, in 1979, an experiment was carried out that showed that human genes are made of DNA. Explain the experiment.
    Goeddel used DNA cloning to transfer the gene for somatotrophin into E. coli, allowing the bacteria to make somatotrophin.
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Genetics Mini Exam I: The Discovery of the Double Helix
2015-09-06 23:30:44
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