104 Notes to Instructors
Notes to Instructors
Chapter 16 The Molecular Basis of Inheritance
What is the focus of these activities?
Almost all introductory biology students know that DNA is the hereditary material in
What are the particular activities designed to do?
Activity 16.1 Is the hereditary material DNA or protein?
This activity is designed to help students organize and review the experiments and
Activity 16.2 How does DNA replicate?
This activity is designed to give students a better understanding of both the overall
What misconceptions or difficulties can these activities reveal?
Activity 16.1
This activity asks students to review and integrate the evidence from a series of
experiments that together demonstrated that DNA (not protein) is the hereditary material.
In addition, many students don’t understand the experiments of Meselson and Stahl. We
offer a couple of reasons:
1. Many don’t understand why growing bacteria in a medium that contains only 15N
means that the DNA the bacteria produce will ultimately contain 15N in its
nucleotides. It is useful to let them know that, like plants, many bacteria are capable
of manufacturing macromolecules (like nucleotides) from inorganic precursors.
2. Students may not understand why DNA that contains 15N will layer out in a
centrifuge tube lower than DNA that contains 14N. They generally don’t understand
Activity 16.2
Students tend to encounter a number of difficulties, misconceptions, and missing
conceptions as they model DNA replication. Here are several possible problems:
1. Many students don’t understand what 5⬘and 3⬘mean relative to the structure of
DNA. To explain this, it is useful to draw a deoxyribose molecule and show how the
2. Many students want to know why DNA polymerase can add nucleotides only to an
open hydroxyl group at the 3⬘end of a nucleotide. Remind them that enzymes
3. The majority of students have difficulty understanding both why and how Okasaki
4. Students may not understand how RNA primers on the lagging strand are removed
by a second type of DNA polymerase. Again, the model helps explain this. Knowing
that this removal is done by a second polymerase also helps students understand why
these fragments need to be ligated together by the action of ligase.
5. Most students are able to understand which strand of replicating DNA is the leading
strand and which is the lagging strand if they begin replication at one end of a DNA
Notes to Instructors 105
Answers
Activity 16.1 Is the hereditary material DNA or protein?
Accumulating and Analyzing the Evidence
Build a concept map to review the evidence used to determine that DNA was the genetic
material, the structure of DNA, and its mode of replicaton. Keep in mind that there are
many ways to construct a concept map.
• First, develop a separate concept map for each set of terms (A to D on the next
page). Begin by writing each term on a separate sticky note or sheet of paper.
Here is an example:
106 Activity 16.1
DNA
contains
made of
Chromosomes
Nucleus
Proteins
Map A
Griffith
mice
S strain of Streptococcus
R strain of Streptococcus
Map B
Hershey and Chase
bacteria
bacteriophage (phage) (only a protein and
DNA)
35S
32P
Map C
Watson and Crick
X-ray crystallography
Chargaff’s rule
purine structure
dispersive
semiconservative
nucleic acid bases
14N
15N
bacteria
density equilibrium centrifugation
Activity 16.1 107
1. In the early to mid-1900s, there was considerable debate about whether protein or
DNA was the hereditary material.
a. For what reasons did many researchers assume that protein was the genetic
material?
Biologists understood that chromosomes segregated to opposite poles in mitosis
Terms
b. What key sets of experiments led to the
understanding that, in fact, DNA and
not protein was the hereditary material?
c. What evidence did each experiment
provide?
Griffith experimented with a lethal S strain
of bacteria and a nonlethal R strain of the
same bacterial species. S strain injected
These experiments indicated that some
factor from the heat-killed S strain was
able to transform live R into S bacteria.
cause bacteria to produce more T2 phage.
Hershey and Chase grew two different
cultures of phage, one on cells that
contained amino acids labeled with
radioactive sulfur, and the other on cells
that contained nucleotides labeled with
radioactive phosphorus. They used these
centrifuge. If the researchers cultured
these cells further, they could show that
new viruses were produced. As a result,
they demonstrated that it was the DNA in
the viruses that was transferred to the
bacteria and caused the production of new
viruses. (There was arguably some protein
108 Activity 16.1
2. Watson and Crick were the first to correctly describe the structure of DNA. What
evidence did they use to do this? How did they use this evidence to put together or
propose the structure of DNA?
Watson and Crick collected the following evidence:
• Chargaff’s rule indicated that the amount of adenine in DNA was equal to the
amount of thymine and that the amount of guanine was equal to the amount of
cytosine.
3. How did the results of Meselson and Stahl’s experiments show that DNA replicates
semiconservatively? To answer this, answer the following questions.
a. Diagram the results that would be expected for each type of replication proposed.
Meselson and Stahl grew bacteria for many generations in a medium containing
heavy nitrogen (N15). The bacteria used the heavy nitrogen to make the
Activity 16.1 109
b. What evidence allowed Meselson and Stahl to eliminate the conservative model?
If the DNA replicated conservatively, after one replication cycle the two DNA
molecules produced should have been the original DNA molecule and an
c. What evidence allowed them to eliminate the dispersive model.
If the dispersive model were correct, after the first division cycle they would
expect to find one band of DNA at an intermediate density. At the end of the
An E. coli cell that contains a single circular chromosome made of double-stranded DNA
is allowed to replicate for many generations in 15N medium until all of the E. coli cells’
DNA is labeled with 15N. One E. coli cell is removed from the 15N medium and is placed
into medium in which all of the available nucleotides are 14N labeled. The E. coli cell is
allowed to replicate until eight E. coli are formed.
1. Given this situation, which of the following is true?
110 Activity 16.1
16.1 Test Your Understanding
Activity 16.2 How does DNA replicate?
Working in groups of three or four, construct a dynamic (working or active) model of
DNA replication. You may use the materials provided in class or devise your own.
Building the Model
• Develop a model of a short segment of double-stranded DNA.
• Include a key for your model that indicates what each component represents in
the DNA molecule—for example, adenine, phosphate group, deoxyribose.
Your model should describe the roles and relationships of all the following enzymes and
structures in replication:
Activity 16.2 111
parental DNA lagging strand Okazaki fragment
nucleotide excision repair telomerase DNA polymerase
daughter DNA 5⬘end helicase
2. Draw the sequence of events that occurred to explain your answer.
Use your model to answer the questions.
1. Explain how Meselson and Stahl’s experiments support the idea that DNA
replication is semiconservative.
Three alternative models for replication of DNA were possible. See Figure 16.10 on
page 312 for diagrams of the conservative, semiconservative, and dispersive models.
Meselson and Stahl grew bacteria in a culture medium that contained nucleotides
labeled with heavy nitrogen (one extra neutron added), or 15N. After many
generations, the DNA in the bacteria was completely labeled with 15N nucleotides.
2. A new form of DNA is discovered that appears to be able to replicate itself both in
the 3⬘→5⬘direction and in the 5⬘→3⬘direction. If this is true, how would this
newly discovered DNA replication differ from DNA replication as we know it?
3. Amazingly, an alien species of cellular organism is found alive in the remains of a
meteorite that landed in the Mojave Desert. As a scientist, you are trying to
determine whether this alien life-form uses DNA, protein, or some other type of
compound as its hereditary material.
a. What kinds of experiments would you propose to determine what the hereditary
material is?
112 Activity 16.2
There are a number of different ways of doing this. Here is one possibility: You
could try to grow some of the organism’s cells in culture and then observe the
b. Assuming that the hereditary material turns out to be similar to our DNA,
describe the simplest experiments you could run to try to determine if it is
double-stranded like our DNA, triple-stranded, or something else.
Here is one possible way of doing this: You could do X-ray crystallography of the
4. Some researchers estimate that the mutation rate for any given gene (or its DNA) in
certain strains of bacteria is about 108. This means that one error or mutation in a
given gene is introduced for every 100-million cell divisions.
a. What can cause mistakes in replication?
An incorrect or mismatched nucleotide can be added during replication.
b. How are such mistakes normally corrected?
DNA polymerase inserts new nucleotides during replication. It also checks or
Activity 16.2 113