11 DNA: THE GENETIC MATERIAL
CHAPTER OUTLINE
Genes Are Made of DNA (p. 222)
11.1
11.2
11.3
The Discovery of Transformation (p. 222; 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. 223; 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. 226)
11.4
How the DNA Molecule Copies Itself (p. 226; 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. 230)
11.5
Mutation (p. 230; 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.
KEY TERMS
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transformation (p. 218) The Griffith experiment showed that information can pass from a dead strain
of a pathogenic bacterium to a non-virulent strain and transform that nonvirulent strain into one that is
pathogenic; this provided evidence that DNA is the genetic material.
nucleotides (p. 224) The building blocks of the nucleic acids.
double helix (p. 224)
base pairs (p. 224) Hydrogen bonds form between the base pairs of the DNA molecule, keeping the
molecule at a constant thickness.
complementarity (p. 226) Each strand of DNA is a complementary, mirror image of the other.
semiconservative replication (p. 226) When DNA replicates, each existing strand serves as a template
for a new complementary strand.
DNA replication (p. 228)
mutation (p. 230)
recombination (p. 230)
DNA fingerprinting (p. 231)
point mutation (p. 233)
frameshift mutation (p. 233)
transposition (p. 233)
chromosomal rearrangement (p. 233)
mutagens (p. 233) Agents (usually radiation or chemicals) that cause damage to DNA.
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
1.
The Excitement of Scientific Discovery. Since this chapter opens with a discussion of several major
experiments, it is a great opportunity to discuss how science is conducted. The process of scientific
discovery is an exciting one to scientists—it is what drives and motivates them. The Double Helix, by
James D. Watson (1968, A Mentor Book by New American Library, New York) is a brief yet enticing
look into the process of science, the sleuthing, the deduction, and the sheer joy of discovery by the man
who helped deduce the structure of DNA. Assign students to read this book, or portions of it, and get
their impressions.
2.
How Often Does Your DNA Make a Mistake? Pose the following problem to your students:
If DNA replication errors occur at a frequency of 10 -8 to 10-5 per cell division, how frequently do
mutations actually occur in each of us? Different types of cells in our bodies divide at very different
rates. Nerve cells do not divide again past a certain point in a person's development. Skin cells divide
continuously, and the cells lining your small intestine are replaced every few days. A million new red
blood cells are made every minute, which gives us a place to begin our calculations. If we have 1
million new red blood cells every minute, how many are made in 24 hours?
(1 million x 60 minutes per hour x 24 hours = 1.44 x 10 9 new red blood cells every day)
So how many errors occur in the new red blood cells produced each day?
(1.44 x 109 red blood cells x 10-8 errors per cell division = 1.44 x 101 or 14 per day)
These calculations can be carried out further. For a year, multiply by 365. For a lifetime, multiply this
last number by 75 or 80 years. Note: Red blood cells are somewhat atypical of body cells because in
the last stages of differentiation, the portion of the red blood cell containing the nucleus is pinched off,
presumably to make more room for hemoglobin. Red blood cells, therefore, do not divide again after
they are manufactured, so any mutation is not going to be passed along directly. This exercise simply
serves to illustrate the number of DNA mutations that are likely to occur in a large, multicellular
organism.
CRITICAL THINKING QUESTION
1.
If mutagens cause damage, what might researchers try to do to reverse damage to DNA?