7-17 Which of the following statements about the genetic code are correct?
(a) All codons specify more than one amino acid.
(b) The genetic code is redundant.
(c) All amino acids are specified by more than one codon.
(d) The genetic code is different in procaryotes and eucaryotes.
(e) All codons specify an amino acid.
NOTE: The following codon table is to be used for Problems Q7-18 – 7-23, Q7-29, and 7-33.
7-18 The following DNA sequence includes the beginning of a sequence coding for a protein.
What would be the result of a mutation that changed the C marked by an asterisk to an A?
5′- AGGCTATGAATGGACACTGCGAGCCC....
*
7-19 Which amino acid would you expect a tRNA with the anticodon 5′-CUU-3′ to carry?
(Refer to Codon table provided above Q7-18.) (a) Lysine
(b) Glutamate (c) Glutamine (d) Leucine (e) Phenylalanine
7-20 Which of the following pairs of codons might you expect to be read by the same tRNA as a result of wobble? (Refer to Codon table provided above Q7-18.)
(a) CUU and UUU (b) GAU and GAA (c) CAC and CAU (d) AAU and AGU (e) CCU and GCU
7-21 Below is a segment of RNA from the middle of an mRNA. If you were told that this segment of RNA was part of the coding region of an mRNA for a large protein, give the amino acid sequence for the protein that is encoded by this segment of mRNA. (Refer to Codon table provided above Q7-18.)
5′- UAGUCUAGGCACUGA -3′
7-22 (Refer to Codon table provided above Q7-18.) One strand of a section of DNA isolated from the bacterium E. coli reads:
5′- GTAGCCTACCCATAGG -3′
A. Suppose that an mRNA is transcribed from this DNA using the complementary strand as a template. What will be the sequence of the mRNA in this region (make sure you label the 5′ and 3′ ends of the mRNA)?
B. How many different peptides could potentially be made from this sequence of RNA, assuming translation initiates upstream of this sequence?
C. What are these peptides? (Give your answer using the one letter amino acid code.) 7-23 A strain of yeast translates mRNA into protein with a high level of inaccuracy. Individual molecules of a particular protein isolated from this yeast have the following variations in the first 11 amino acids compared with the sequence of the same protein isolated from normal yeast cells (Figure Q7-23). What is the most likely cause of this variation in protein sequence? Explain your answer. (Refer to Codon table provided above Q7-18.)
Figure Q7-23 (a) A mutation in the DNA coding for the protein
(b) A mutation in the anticodon of the isoleucine tRNA (tRNAIle)
(c) A mutation in the isoleucyl-tRNA synthetase that decreases its ability to distinguish between different amino acids
(d) A mutation in the isoleucyl-tRNA synthetase that decreases its ability to distinguish between different tRNA molecules
(e) A mutation in a component of the ribosome that allows binding of incorrect tRNA molecules to the A-site
7-24 Which of the following statements is TRUE?
(a) Ribosomes are large RNA structures composed solely of rRNA.
(b) Ribosomes are synthesized entirely in the cytoplasm.
(c) rRNA contains the catalytic activity that joins amino acids together.
(d) A ribosome consists of two equally sized subunits.
(e) A ribosome binds one tRNA at a time.
7-25 Figure Q7-25A shows the stage in translation when an incoming aminoacyl-tRNA has bound to the A-site on the ribosome. Using the components shown in Figure Q7-25A as a guide, show on Figure Q7-25B and Q7-25C what happens in the next two stages to complete the addition of the new amino acid to the growing polypeptide chain.
Figure Q7-25
7-26 A poison added to an in vitro translation mixture containing mRNA molecules with the sequence 5′-AUGAAAAAAAAAAAAUAA-3′ has the following effect: the only product made is a Met-Lys dipeptide that remains attached to the ribosome. What is the most likely way in which the poison acts to inhibit protein synthesis?
(a) It inhibits binding of the small subunit of the ribosome to mRNA.
(b) It inhibits peptidyl transferase activity.
(c) It inhibits movement of the small subunit relative to the large subunit.
(d) It inhibits release factor.
(e) It mimics release factor.
7-27 In eucaryotes, but not procaryotes, ribosomes find the start site of translation by (a) binding directly to a ribosome-binding site preceding the initiation codon.
(b) scanning along the mRNA from the 5′ end.
(c) recognizing an AUG codon as the start of translation.
(d) binding an initiator tRNA.
7-28 Figure Q7-28 shows an mRNA molecule.
Figure Q7-28
A. Match the labels given in the list below with the label lines in Figure Q7-28.
(a) ribosome-binding site (b) initiator codon
(c) stop codon (d) untranslated 3′ region (e) untranslated 5′ region (f) protein-coding region
B. Is the mRNA shown procaryotic or eucaryotic? Explain your answer.
7-29 A tRNA for the amino acid lysine is mutated such that the sequence of the anticodon is 5′-UAU-3′ (instead of 5′-UUU-3′). Which of the following aberrations in protein synthesis might this tRNA cause? (Refer to Codon table provided above Q7-18.) (a) Read through of stop codons
(b) Substitution of lysine for isoleucine (c) Substitution of lysine for tyrosine (d) Substitution of lysine for phenylalanine
(e) Substitution of lysine for the amino-terminal methionine
7-30 You have discovered a protein that inhibits translation. When you add this inhibitor to a mixture capable of translating human mRNA and centrifuge the mixture to separate polyribosomes and single ribosomes, you obtain the results shown in Figure Q7-30.
Which of the following interpretations are consistent with these observations?
Figure Q7-30
(a) The protein binds to the small ribosomal subunit and increases the rate of initiation of translation.
(b) The protein binds to sequences in the 5′ region of the mRNA and inhibits the rate of initiation of translation.
(c) The protein binds to the large ribosomal subunit and slows down elongation of the polypeptide chain.
(d) The protein binds to sequences in the 3′ region of the mRNA and prevents termination of translation.
7-31 The concentration of a particular protein X in a normal human cell rises gradually from a low point, immediately after cell division, to a high point, just before cell division, and then drops sharply. The level of its mRNA in the cell remains fairly constant throughout this time. Protein X is required for cell growth and survival, but the drop in its level just before cell division is essential for division to proceed. You have isolated a line of human cells that grow in size in culture but cannot divide, and on analyzing these mutants, you find that levels of X mRNA in the mutant cells are normal. Which of the following mutations in the gene for X could explain these results?
(a) The introduction of a stop codon that truncates protein X at the fourth amino acid.
(b) A change of the first ATG codon to CCA.
(c) The deletion of a sequence that encodes sites at which ubiquitin can be attached to the protein.
(d) A change at a splice site that prevents splicing of the RNA.
7-32 For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once.
Once an mRNA is produced, its message can be decoded on ribosomes.
The ribosome is composed of two subunits: the __________________
subunit, which catalyzes the formation of the peptide bonds that link the amino acids together into a polypeptide chain, and the
__________________ subunit, which matches the tRNAs to the codons of the mRNA. During the chain elongation process of translating an mRNA into protein, the growing polypeptide chain attached to a tRNA is bound to the __________________ -site of the ribosome. An incoming aminoacyl-tRNA carrying the next amino acid in the chain will bind to the
__________________ -site by forming base pairs with the exposed codon in the mRNA. The __________________ enzyme catalyzes the formation of a new peptide bond between the growing polypeptide chain and the newly arriving amino acid. The end of a protein-coding message is signaled by the presence of a stop codon, which binds the
__________________ called release factor. Eventually, most proteins will be degraded by a large complex of proteolytic enzymes called the __________________.
A medium proteosome
central P RNA
DNA peptidyl transferase small
E polymerase T
large protein ubiquitin
7-33 After treating cells with a mutagen, you isolate two mutants. One carries alanine and the other carries methionine at a site in the protein that normally contains valine. After treating these two mutants again with mutagen, you isolate mutants from each that now carry threonine at the site of the original valine. Assuming that all mutations caused by the mutagen involve single nucleotide changes, deduce the codons that are used for valine, alanine, methionine, and threonine at the affected site. See Figure Q7-33. (Refer to Codon table provided above Q7-18.)
Figure Q7-33