DNA Structure and Function (Chapter 13)

Cardboard Atoms and Bent-Wire Bonds

       A.Johann Friedrich Miescher discovered nucleic acids in 1868. 
       B.Linus Pauling discovered the helical structure of proteins in
          1951. 
       C.In 1953, Watson and Crick discovered the structure of the
          master molecule of life-DNA.
   I.Discovery of DNA Function 
       A.Early and Puzzling Clues 
             1.In 1928, Fred Griffith was working with S (pathogenic)
               and R (nonpathogenic) strains of a pneumonia-causing
               bacterium. 
             2.He performed four experiments summarized here: 
                  a.Inject mice with R cells; mice lived. 
                  b.Inject mice with S cells; mice died; blood samples
                    contained many live S cells. 
                  c.S cells were heat-killed then injected into mice;
                    mice lived. 
                  d.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. 
                  a.Both proteins and nucleic acids were candidates. 
                  b.In 1944, Oswald Avery showed that the substance
                    was DNA.
       B.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, these viruses were used in experiments by
               Hershey and Chase to prove which of these was the
               hereditary material (It was the nucleic acid). 
               a.35S-labeled proteins in the bacteriophage coat did
                    not enter the bacteria and thus were not
                    participating in providing directions for new virus
                    assembly. 
                  b.32P-labeled DNA in the viral core did enter the
                    bacteria and direct new virus assembly.
  II.DNA Structure 
       A.Components of DNA 
             1.DNA is composed of four kinds of nucleotides, each of
               which consists of: 
                  a.a five-carbon sugar-deoxyribose; 
                  b.a phosphate group; 
                  c.one of four bases-adenine (A), guanine (G),
                    thymine (T), cytosine (C).
             2.The nucleotides are similar, but T and C are single-ring
               pyrimidines; A and G are double-ring purines. 
             3.Edwin Chargaff, in 1949, noted two critical bits of data. 
                  a.The four kinds of nucleotide bases making up a
                    DNA molecule differ in relative amounts from
                    species to species. 
                  b.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. 
                  a.DNA exists as a long, thin molecule of uniform
                    diameter. 
                  b.The structure is highly repetitive. 
                  c.DNA is helical.
       B.Patterns of Base Pairing 
             1.Watson and Crick used numerous sources of data to
               build models of DNA. 
             2.The following features were incorporated into their
               models. 
                  a.Single-ringed thymine was hydrogen bonded with
                    double-ringed adenine, and single-ringed cytosine
                    with double-ringed guanine, along the entire length
                    of the molecule. 
                  b.The backbone was made of chains of
                    sugar-phosphate linkages. 
                  c.The molecule was double stranded and looked like
                    a ladder with a twist to form a double helix.
             3.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.
  III.DNA Replication and Repair 
       A.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."
       B.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.
       C.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.
       D.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.".
  IV.Focus on Health: When DNA Can't Be Fixed 

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