Bio 102 Practice Problems
The Double Helix
Multiple choice: unless otherwise directed, circle the one best answer.
1. Experiments by Avery, McCarty and MacLeod were consistent with the hypothesis that DNA is the genetic
material. However, at the time many scientists still didn't believe that DNA was the genetic material for a
variety of logical reasons. Which one of the following was NOT cited as a reason to doubt that DNA was the
genetic material?
A. DNA is made up of only four nucleotides and is too simple to encode complex traits.
B. The experiments were done with bacteria, and bacteria may be too simple to have genes.
C. Proteins are more complex and variable in size and structure than DNA. Thus it seems more likely that
proteins are the genetic material.
D. Avery, McCarty and MacLeod couldn't conclusively prove that their DNA was not contaiminated with a
tiny amount of protein.
E. Frederick Griffith had earlier shown conclusively that the genetic material in Streptococcus pneumoniae
was a complex carbohydrate capsule.
2. Which of the following investigators demonstrated that DNA could direct the replication of a virus, thus
providing evidence that DNA is the genetic material?
A.
B.
C.
D.
E.
Watson and Crick
Avery, McCarty and MacLeod
Beadle and Tatum
Meselson and Stahl
Hershey and Chase
3. Which one of the following statements is true about the structure of DNA?
A.
B.
C.
D.
E.
A-T base pairs are held together more strongly than G-C pairs.
The 5´ end of one strand is always adjacent to the 3´ end of the opposite strand.
A base pair always consists of two purines or two pyrimidines.
There are three phosphates between the 3´ carbon of one nucleotide and the 5´ carbon of the next.
An A-T base pair is narrower than a G-C base pair.
4. Many scientists contributed to our understanding that DNA is the genetic material, but only Watson and Crick
won the Nobel Prize. What was it about their discovery that was especially important?
A. For the first time, we understood how the genetic material was connected to a specific sequence of amino
acids.
B. The work of Watson and Crick provided a clear explanation for Mendel’s Law of Independent
Assortment.
C. They were the first scientists to clearly show that a diploid human cell contains 46 chromosomes, each of
which contains many genes.
D. The structure of DNA provided an explanation for how information in DNA could be copied.
E. The structure of DNA was published in a highly prestigious journal.
5. The Hershey-Chase experiment demonstrated that:
A.
B.
C.
D.
E.
DNA was unquestionably the genetic material
DNA was the transforming principle
DNA was necessary for host cells to produce bacteriophages
DNA was composed of base-paired nucleotides
DNA was a double-helix structure
6. Which of the following correctly describes base-pairing in DNA:
A. A purine always pairs with another purine
B. Bases are joined by phosphodiester bonds
C. A−T base pairs are joined by three hydrogen bonds
D. A and G are complementary bases
E. A purine always pairs with a pyrimidine
7. A scientist who provided evidence that DNA is the genetic material of bacterial viruses was:
A. Martha Chase
B. Frederick Griffith
C. Oswald Avery
D. Jim Watson
E. Rosalind Franklin
8. Watson and Crick’s structure for DNA provided convincing evidence that DNA was the genetic material
because:
A.
B.
C.
D.
E.
it showed that three-base codons must represent amino acids
it showed that radioactive phosphorus but not sulfur would be incorporated into the molecule
it showed that proteins could access the bases by binding in the major groove
it showed that DNA could coil around histones and condense to form visible chromosomes
it showed how an exact copy of the DNA molecule could be produced
Short answer (show your work or thinking to get partial credit):
1. You are analyzing a double-stranded DNA sample, and you find that 23% of the sample is guanine (G)
nucleotides. What are the percentages of the other three bases?
In double-stranded DNA, G pairs with C, so the amounts of these two bases should be equal: C=23%
Now, G+C = 46%, so the remaining 54% must be A and T
Since A pairs with T, these two should be equal; therefore, A=27% and T=27%
2. Why couldn't Hershey and Chase have used radioactive nitrogen in their experiment?
Hershey and Chase wanted to distinguish between DNA and protein. Both DNA and protein molecules
contain nitrogen, so both DNA and protein would be labeled by radioactive nitrogen.
3. Suppose you are studying a recently discovered Martian organism. Its cells contain nucleic acids, but to your
surprise, you find six nucleotides in a Martian DNA-like molecule: A, T, G, C, and two unusual nucleotides,
xenine (X) and martiacine (M). You determine the amounts of each nucleotide in a DNA sample, but
unfortunately you spill some coffee on your lab notebook (we told you not to bring food into the lab!) and can
only read part of the data:
a. What are the missing percentages?
A and X are equal, and so are T and M. That means G and C probably pair with each other and are equal
to each other. A, T, X and M add up to 40%, so G+C=60% and therefore G and C are 30% each.
b. In this alien DNA, what are the base-pairing rules?
Must be A with X, T with M and G with C
4. Sketch a purine nucleotide below. Label its three major parts, and indicate its 3´ and 5´ ends:
5' end
Phosphate
Base
(2 rings for purine)
P
O
Sugar
(deoxyribose)
OH
3' end
5. You have isolated the first bacterium from mars. Its DNA is a little different in structure from that of earthly
organisms. Instead of our familiar A, T, C and G, you find that the organism’s DNA contains 21% selestine
(S), 29% xenosine (X), and 21% rhodocine (R). You have not yet developed an assay to measure the
concentration of the fourth base, but you have identified it as nucleosine (N). What are the base-pairing rules
for this organism?
S pairs with R
X pairs with N
6. At right is the structure of a nucleotide.
base
a. Label the three major components of the nucleotide.
b. Label the 5’ and 3’ ends
c. Would this nucleotide be used in making DNA or RNA? How do you
know?
DNA: it has a deoxyribose with only one –OH group (RNA would be
made with ribose, with both a 2’ and 3’ OH)
d. Is this nucleotide a purine or a pyrimidine? How do you know?
Purine: the two-ring structure on the base shows this
5’
phosphate
3’
sugar
7. Because a cell’s DNA is so crucial, the cell has many mechanisms that can repair damage to DNA or changes
that might otherwise be transmitted to the offspring as a mutation. One key repair system recognizes mispaired DNA bases that might have been inserted incorrectly during replication. This repair system, however,
is much better at recognizing and removing an improper G-A base pair than it is at recognizing and removing
an improper G-T base pair. Given what you know about nucleotide and DNA structure, why do you think this
is true?
A G-A base-pair is a purine-purine base-pair, so it would have a total of four rings on the two bases. Normal
base-pairs have a total of three rings (1 purine + 1 pyrimidine). The extra ring would create a “bulge” that
could be detected by an enzyme. The G-T pair is wrong but is still a purine-pyrimidine pair, so it would
be less obvious.
8. Two base pairs are shown below. Label each base as either “G”, “A”, “T” or “C”.
_G___
__C__
_A__
__T__
True or False? Read carefully: a question is false unless it is completely true!
T
□ ■
T
□ ■
T
■ □
F
F
F
1. Although Griffith’s discovery of transformation did not convince scientists that DNA is the
genetic material, additional experiments by Avery, McCarty and MacLeod provided definitive
proof that it is.
2. Hershey and Chase demonstrated that bacteriophage DNA labeled with radioactive sulfur
phosphorous entered bacterial cells. They concluded that the virus DNA, not protein, is the
component that programs the host cell to make more viruses.
3. Pairing between two nucleic acid strands (DNA-DNA, DNA-RNA or RNA-RNA) is always
antiparallel.
T
□ ■
T
□ ■
T
T
F
F
■ □
□ ■
4. Avery, McCarty and MacLeod showed that treatment with a protease destroyed the transforming
ability of dead, smooth Streptococcus pneumoniae cells.
5. A DNA molecule that contains 18% of the purine guanine (G), would also contain 18% of the
other purine, adenine (A).
F
6. It would require more heat to separate a C-G base-pair than an A-T base pair.
F
7. Hershey and Chase provided evidence that DNA is the genetic material by finding that the
“transforming principle” was inactivated by DNase but not proteinase.