DNA Structure and Function

  1. Discovery of DNA Function
    1. Early and Puzzling Clues
      1. In 1928, Fred Griffith was working with S (pathogenic) and R (nonpathogenic) strains of streptococcus pnumoniae
      2. He performed four experiments summarized here:
        1. Inject mice with R cells; mice lived.
        2. Inject mice with S cells; mice died; blood samples contained many live S cells.
        3. S cells were heat-killed then injected into mice; mice lived.
        4. Live R cells plus heat-killed S cells were injected into mice; mice died; live S cells were found in the blood.

      3. Some substance from the S cells had transformed the R cells.
        1. Both proteins and nucleic acids were candidates.
        2. In 1944, Oswald Avery showed that the substance was DNA.
    2. Confirmation of DNA Function
      1. Viruses called bacteriophages use bacterial cells for reproduction.
      2. Because they consist of only a protein coat and a nucleic acid core, Hershey and Chase used these viruses in experiments to prove which of these was the hereditary material.

        1. S35 labeled proteins in the bacteriophage coat did not enter the bacteria and thus were not participating in providing directions for new virus assembly.
        2. P32 labeled DNA in the viral core did enter the bacteria and direct new virus assembly.

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  2. DNA Structure
    1. Components of DNA
      1. DNA is composed of four kinds of nucleotides, each of which consists of:
        1. a five-carbon sugar–deoxyribose;
        2. a phosphate group;
        3. a nitrogenous base–adenine (A), guanine (G), thymine (T), cytosine (C).
      2. The nucleotides are similar, but
        1. T and C are single-ring pyrimidines.
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        3. A and G are double-ring purines.

      3. Edwin Chargaff, in 1949, noted two critical bits of data.
        1. The four kinds of nucleotide bases making up a DNA molecule differ in relative amounts from species to species.
        2. The amount of A = T, and the amount of G = C.
      4. Rosalind Franklin used X-ray diffraction techniques to produce images of DNA molecules.
        1. DNA exists as a long, thin molecule of uniform diameter.
        2. The structure is highly repetitive (.34h m, 3.4h m)
        3. DNA is helical (lucky guess from Linus Pauling)
    2. Patterns of Base Pairing
      1. Watson and Crick built a model of DNA.
      2. The base pairing is constant for all species but the sequence of base pairs in a nucleotide strand is different from one species to the next.
  3. DNA Replication and Repair
    1. How a DNA Molecule Gets Duplicated
      1. First, the two strands of DNA unwind and expose their bases.
      2. Then unattached nucleotides pair with exposed bases.
      3. Thus, replication results in DNA molecules that consist of one "old" strand and one "new" strand; this is designated "semiconservative replication."

    2. Several enzymes participate in replication:
      1. One kind of enzyme unwinds the two nucleotide strands.
      2. DNA polymerases attach free nucleotides to the growing strand.
      3. DNA ligases seal new short stretches of nucleotides into one continuous strand.
    3. Monitoring and Fixing the DNA
      1. DNA polymerases, DNA ligases and other enzymes engage in DNA repair.
      2. DNA polymerases "proofread" the new bases for mismatched pairs, which are replaced with correct bases.
    4. Regarding the Chromosomal Proteins
      1. The DNA of humans and other eukaryotes is highly organized to prevent tangling.
      2. Some histones (a type of protein) act as spools to wind the DNA into units forming looped regions and "domains."