SBI 4UW
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Name:
In a DNA molecule a purine pairs with a pyrimidine. Explain why A-C and G-T pairs do not form.
Differentiate between a purine and a pyrimidine.
A molecule of DNA was analyzed and found to contain 20% thymine. Calculate the percentage of adenine,
guanine and cytosine in this molecule.
a) The following table shows the distribution of nucleotides that originated from analysis of single-stranded
DNA and double-stranded DNA. Identify which of the samples A, B and C are most likely doublestranded and which are single-stranded.
Sample
A
B
C
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Nucleic Acids and Proteins Questions
Adenine
10%
35%
15%
Guanine
40%
10%
25%
Thymine
10%
35%
20%
Cytosine
40%
20%
40%
b) One strand of a DNA molecule contains the nucleotide proportions 15% A, 30% T, 20% G, 35% C.
Calculate the proportions of the four base pairs that would be expected I the double-stranded from of this
DNA.
In a double helix, there is a complete turn every 3.4 nm, or 10 nucleotides. The human genome contains
approximately 3 billion nucleotides.
a)
Calculate how long (in metres) an average human cell’s DNA would be if its structure were unwound.
b)
Calculate how many complete turns would exist in this molecule.
c)
Assume that the DNA molecule in a particular chromosome is 75 mm long. Calculate the number of
nucleotide base pairs this molecule has.
Outline the function for each of the following enzymes: DNA gyrase, DNA helicase, DNA polymerase I,
DNA polymerase III, DNA ligase and RNA primase.
Compare the mechanisms by which the leading and lagging strands are replicated. Explain why it is
necessary for two mechanisms to exist.
Distinguish between a replication fork and a replication bubble.
Explain why it is necessary for eukaryotic DNA to have multiple replication origin sites.
Meselson and Stahl’s experiments indicated that DNA replicates semiconservatively. Calculate the
percentage of DNA double helixes that would contain one of the original parent strand after four rounds of
replication.
Sketch a diagram of the DNA replication fork. Label the 3' ends of DNA segments versus the 5' ends.
One of the consequences of DNA polymerase III only being able to elongate and not initiate a
complementary strand is that RNA primers need to be added. Since DNA polymerase III only works in the 5'
to 3' direction, an end of one strand is consistently missing some of its base pairs after replication. To
compensate, eukaryotic cells have noncoding sequences of DNA at the ends called telomeres. Hypothesize
what will happen to the telomeres as people age and what effect you think this will eventually have on
coding sequences.
The following is a segment taken from a strand of DNA: 5'-ATGCCTTA-3'. State the complementary strand
for this segment.
Outline how the fact that DNA replicates semiconservatively instead of conservatively decreases the
possibility of errors made during DNA replication. Describe another mechanism that minimizes DNA
replication error.
DNA polymerase III builds complementary DNA strands with free deoxyribonucleoside triphosphates in the
nucleoplasm. DNA structure calls for deoxyribonucleoside monophosphates. Explain why
deoxyribonucleoside triphosphates are required, and what happens to the inorganic phosphate released after
the completion of an elongation reaction.
Despite the fact that the two DNA strands are held together by weak hydrogen bonds, the DNA molecule is
quite stable. Explain how this is possible, given the nature of hydrogen bonding.
Explain why DNA can be classified as a polymer.
Differentiate between a deoxyribose sugar and a ribose sugar.
If a test tube contained a mixture of double-stranded DNA, all four deoxyribonucleoside triphosphates, and
DNA polymerase III, state what additional component would be needed to ensure replication.
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The following is the sequence of the antisense strand of a gene: 5’-GGATCAGGTCCAGGCAATTT-3’.
Copy the DNA sequence and then below it transcribe the sequence into mRNA.
A short fragment of DNA just upstream of a gene includes the following sequence of nucleotides:
3'-TACGGCATGCACCATAATATCGACCTTCGGCACGG-5'
a)
b)
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Deduce the location of the promoter.
A molecular biologist isolated two DNA fragments of a genetic sequence that was 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 strand pulled apart. DNA helix A unwound at
84oC, whereas DNA helix B unwound at 65oC. Explain which of the two strands is most likely to
contain the promoter.
c)
Outline the purpose of the promoter in transcription.
In eukaryotes, mRNA must be modified before it is allowed to exit the nucleus.
a)
Describe the modifications that made to the eukaryotic primary transcript.
b)
Explain how these modifications ensure that mRNA survives in the cytoplasm and is translated into a
functioning protein.
Explain the ramifications for protein production if the following occurs:
a)
The termination sequence of a gene is removed.
b)
Poly-A polymerase is inactivated.
c)
The enzyme that add the 5' cap is non-functional.
d)
Spliceosomes excise exons and join the remaining introns together.
e)
The RNA polymerase fails to recognize the promoter sequence.
Explain why it advantageous to a cell to have some of its genes regulated.
DNA is double-stranded and RNA is single-stranded. Discuss this difference considering the role of DNA
and RNA in protein synthesis.
Describe the relationship between genes and proteins. Explain how an error in a gene results in a change in
protein sequence.
The DNA code is read in groups of 3 nucleotides called codons. Explain why reading the code in pairs of
nucleotides is not sufficient.
Differentiate between a stop codon and a start codon.
Using the genetic code, determine the amino acid sequence indicated by the following mRNA sequence.
5'-GGCAUGGGACAUUAUUUUGCCCGUUGUGGUGGGGCGUGA-3'
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In a hypothetical situation, 85 amino acids exist and there are still only 4 nucleotides found in nucleic acids.
a)
Deduce the minimum number of nucleotides within a codon required to code for this large number of
amino acids.
b)
Calculate the maximum number of amino acids your answer could code for.
The amino acid sequence for a certain protein is leu-tyr-arg-trp-ser.
a)
Calculate the number of nucleotides in the DNA that would be necessary to code for this polypeptide.
b)
Determine a possible DNA sequence for this polypeptide.
c)
Errors in the process of transcription are occasionally made. Explain why an error in the third base of a
triplet may not necessarily result in a mutation. Outline your answer using an example from the DNA
sequence in part (b) above .
A biotechnologist isolated a gene from a human heart cell and spliced it into a bacterium’s genome. The
protein that was synthesized from this gene was useless because it contained many more amino acids than the
protein produced by the heart cell. Explain the biotechnologist’s observations, and explain how this problem
could be corrected.
State a role in protein synthesis for each of the following molecules: ribosomes, mRNA, tRNA and rRNA.
Differentiate between transcription and translation in terms of their purpose and location.
List the possible anticodons for threonine, alanine and proline.
All incoming tRNAs must enter the A site first and then shift to the P site after peptide bond formation.
Explain the exception to this rule.
Explain why the processes of transcription and translation have been appropriately named.
Outline the process of gel electrophoresis.
Explain how it is possible to distinguish the DNA of one person from another using gel electrophoresis.
Describe how restriction enzymes are used in gene splicing.