THINKING PRACTICE QUESTIONS!
DNA: The Molecular Basis of Life!
1. A molecule of DNA was analyzed and found to contain 20% thymine. Caclulate the percentage of adenine, guanine, and
cytosine in this molecule.
2. Table 1 shows the distribution of nucleotides that originated from analysis of single stranded DNA and double stranded
DNA. Identify which of the samples A, B, an dC are most likely double stranded and which are single stranded, and
record your answers.
Table 1
Sample
Adenine
Guanine
Thymine
Cytosine
A
10%
40%
10%
40%
B
35%
10%
35%
20%
C
15%
25%
20%
40%
3.
4.
5.
6.
In a double helix, there is a complete turn every 3.4 nm, or 10 nucleotides. The human genome contains approximately 3
billion nucleotides. Calculate how long an average human cell’s DNA would be if its structure were unwound. Calculate
how many complete turns would exist in this molecule.
Meselson and Stahl’s experiment indicated that DNA replicates semiconservatively. Calculate the percentage of DNA
double helixes that would not contain any of the original parent strands after three generations of replication. Show all
your work.
Calculate how long a DNA molecule would be if it had 5 million bases on one of its strands. Calculate how many turns
would be found in its double-helical structure.
The distribution of nucleotides in the table below originated from analysis of single-stranded DNA and double-stranded
DNA. Identify which samples are double stranded and which samples are single stranded. Support your answer.
Adenine
Guanine
Thymine
Cytosine
Sample A
15%
45%
10%
30%
Sample B
40%
10%
40%
10%
Sample C
20%
25%
20%
35%
PROTEIN SYNTHESIS
1.
In a hypothetical situation, 85 amino acids exist and there are still only 4 nucleotides found in nucleic acid. Calculate
the minimum number of nucleotides required to code for this large number of amino acids. What is the maximum
number of amino acids your answer would code for?
2. The amino acid sequence for a certain peptide is Leu-Tyr-Arg-Trp-Ser. How many nucleotides are necessary in the
DNA to code for this peptide?
3. A molecular biologist isolated two DNA fragments of a genetic sequence that were believed to be the promoter region
for a certain gene from a bacterial cell. The biologist subjected the DNA fragments to heat and recorded the
temperature at which the double strands pulled apart. DNA helix A unwound at 84 degrees, whereas DNA helix B
unwound at 65 degrees. Which of the two strands is most likely to be the promoter? Explain.
4. A researcher was trying to determine whether two molecules (molecule A and molecule B) were corepressors or
inducers in their respective operon systems. Data were collected regarding the levels of protein and the amount of
gene transcription for the genes in their respective operons. The data is shown in the table below.
Level of protein
Transcription of gene 1
Transcription of gene 2
Molecule A
High
Low
Low
Low
High
High
Molecule B
High
High
High
Low
Low
Low
5.
6.
7.
a) Determine whether molecule A and molecule B are inducers or corepressors. Justify your response.
b) Identify which system resembles the lac operon system and which resembles the trp operon system.
c) State a generality about operon systems involving an inducer and about operon systems involving a
corepressor.
Determine whether or not the following mutations would be harmful to an organism. Translate the mRNA sequence
into protein to help you decide. The mutation is UNDERLINED.
a) AUG UUU UUG CCU UAU CAU CGU
AUG UUU UUG CCU UAC CAU CGU
b) AUG UUU UUG CCU UAU CAU CGU
AUG UUU UUG CCU UAA CAU CGU
c) AUG UUU UUG CCU UAU CAU CGU
AUG UUU CUU GCC UUA UCA UCG U
d) AUG UUU UUG CCU UAU CAU CGU
AUG UUU UUG CCU AUC AUC GU
e) AUG UUU UUG CCU UAU CAU CGU
UGC UAC UAU UCC GUU UUU GUA
Which of the following amino acid changes can result from a single base-pair substitution?
a) arg to leu
b) cys to glu
c) ser to thr
d) ile to ser
The following is a fragment of DNA that codes for part of a protein required in cellular respiration:
3’ – TACATAGCATGTATAAGCATAAATGTAGTACTAATT – 5’
a) Write the complementary strand for this fragment.
b) Which of the two strands will RNA polymerase use as the coding strand?
c) Transcribe the coding strand into mRNA.
d) Translate the mRNA into amino acids using the genetic code.
e) Generate a list of anticodons that would be found on tRNAs that would deliver the first two amino acids.
f) Change one base in the fourth codon that would result in a silent mutation.
g) If the seventh base is changed from G to A, what effect would this have on the protein synthesized? What
class of mutation does this belong to?
h) If the seventh base is completely eliminated, what effect would this have on the protein synthesized? What
class of mutation does this belong to?
i) Which of the mutations in questions (g) and (h) would be more deleterious to an organism? Why?