11 DNA: THE GENETIC MATERIAL
CHAPTER OUTLINE
Genes Are Made of DNA (p. 218)
11.1
11.2
11.3
The Discovery of Transformation (p. 218; Fig. 11.1)
A. In 1928, Griffith made a series of unexpected observations while experimenting with
Streptococcus pneumoniae.
B. He found that the virulent strain’s polysaccharide coat was necessary for infection.
C. He experimented further and found that the information specifying the polysaccharide coat
could be passed from dead, virulent bacteria to coatless, nonvirulent strains.
D. Hereditary information could thus be passed from dead cells to live ones, transforming them.
Experiments Identifying DNA as the Genetic Material (p. 219; Fig. 11.2)
A. The Avery Experiments
1. Avery’s experiments with the transforming principle from Griffith’s experiments
demonstrated conclusively that DNA is the hereditary material.
2. What Avery found was that the purified transforming principle had the same chemistry
as DNA, it behaved similarly to DNA, it was not affected by lipid or protein extraction,
it was not destroyed by protein- or RNA-digesting enzymes, but it was destroyed by
DNA-digesting enzymes.
B. The Hershey-Chase Experiment
1. In 1952, Hershey and Chase used radioactive labels to mark the DNA and the protein of
viruses.
2. They labeled the DNA of the viruses with radioactive phosphorus, while they labeled
the protein coat with radioactive sulfur.
3. They infected bacteria with these radioactive viruses, and found that the bacteria
contained the radioactive phosphorus, but not the sulfur.
4. This was additional evidence that DNA was the genetic material.
Discovering the Structure of DNA (p. 220; Figs. 11.3, 11.4)
A. As it became clear that the genetic material was DNA, researchers began to study its
structure.
B. We now know that DNA consists of subunits called nucleotides; in each nucleotide, a
nitrogen-containing base (purine or pyrimidine) and a phosphate group are bound to a central
sugar molecule.
C. Chargaff found that DNA always had equal amounts of purines (adenine and guanine) and
pyrimidines (thymine and cytosine).
D. More specifically, he found that the amount of adenine equaled the amount of thymine and
that the amount of cytosine was the same as the amount of guanine, a phenomenon now
called “Chargaff’s rule.”
E. Chargaff’s findings suggested base-pairing that was later found to occur inside the DNA
molecule.
F. Franklin suggested that the DNA molecule was in the form of a helix.
G. Watson and Crick then connected the ideas of a helix with base-pairing to further elucidate
the structure of DNA.
H. The DNA molecule has a sugar-phosphate backbone with base-pairing on its interior, and is
twisted into a double helix.
DNA Replication (p. 222)
11.4
How the DNA Molecule Copies Itself (p. 222; Figs. 11.5-11.8)
A. Hydrogen bonds between base pairs hold the two chains of a DNA molecule together, and
each chain of a DNA molecule is complementary to its pair.
1. If one chain has the bases ATTGCAT, its partner will have the complementary sequence
of TAACGTA.
B. This complementarity makes it possible for DNA to replicate itself.
C. There are three possible ways that DNA could replicate itself: conservative replication,
semiconservative replication, and dispersive replication.
D. The Meselson-Stahl Experiment
1. The alternative hypotheses of DNA replication were tested in 1958 by Meselson and
Stahl.
2. They used the isotopes 14N and 15N to label DNA at various times during replication.
3. They found that DNA replication was semiconservative; the process is called
semiconservative replication because in each new DNA molecule, one strand is “new”
DNA, and the complementary strand is the parent DNA molecule.
E. How DNA Copies Itself
1. An enzyme called DNA polymerase oversees DNA replication.
2. At a place called the replication fork, an enzyme called helicase unwinds the DNA,
primers are added to begin each new strand, and then DNA polymerase builds the new
strands by reading along each template strand and adding the correct complementary
nucleotide.
3. DNA polymerase can only add bases to the 3´ end of a strand.
4. Because the two strands in a DNA molecule run in opposite directions (5´ to 3´ for one,
and 3´ to 5´ for the other), the two new strands are built in different ways.
5. One strand has a free 3´ end and is built continuously, towards the replication fork; this
newly synthesized strand is called the leading strand.
6. The other strand had a free 5´ end and so must be built in segments away from the
replication fork; DNA ligase joins the segments, and this strand is called the lagging
strand.
7. DNA repair mechanisms proofread the DNA and repair damaged DNA. But sometimes,
there are mistakes.
Altering the Genetic Message (p. 226)
11.5
Mutation (p. 226; Fig. 11.9, 11.10; Table 11.1)
A. In the very large amount of DNA in each cell, mistakes during DNA proofreading do happen;
this generates genetic variation.
B. A mutation is a change in the DNA sequence of one or more genes; recombination is a change
in position of part of the genetic message.
C. Mistakes Happen
1. Mutations are rare but are the raw material for evolution.
D. Kinds of Mutation
1. Most mutations are detrimental and their effects may be minor or catastrophic.
E. Mutations in Germ-line Tissues
1. Only when a mutation occurs within a germ-line cell is it passed to subsequent
generations.
F. Mutations in Somatic Tissues
1. Changes in somatic cells are not passed on from generation to generation but are passed
on to cells that are descended from the original mutant cell.
2. A somatic mutation may have drastic effects on the individual in which it occurs.
G. Altering the Sequence of DNA
1. Point mutations are changes in the DNA sequence of an organism that involve only one
or a few base pairs of the coding sequence.
2. Sometimes the changes involve a base substitution, while other times either one or a few
bases are added (insertion) or lost (deletion); in a frame-shift mutation, the insertion or
deletion causes the genetic message to be out of register.
3. Some mutations may arise spontaneously, while others are the result of exposure to
mutagens.
H. Changes in Gene Position
1. Individual genes may move from one place to another by transposition or there may be
chromosomal rearrangements.
I. The Importance of Genetic Change
1. Evolution begins with alterations in the genetic message; some alterations enable an
organism to leave more offspring, and others reduce the ability to leave offspring.
2. Evolution can be viewed as the selection of particular combinations of alleles from a
pool of alternatives; the rate of evolution is limited by the rate that new alternatives are
created.
3. Genetic change through mutation and recombination provides the raw material for
evolution.