Exam 1: Chapter 7

Which of the following pairs of codons might you expect to be read by the same tRNA as a result of wobble?

CAC and CAU

You have discovered an alien life-form that surprisingly uses DNA as its genetic material, makes RNA from DNA, and reads the information from RNA to make protein using ribosomes and tRNAs, which read triplet codons. Because it is your job to decipher the genetic code for this alien, you synthesize some artificial RNA molecules and examine the protein products produced from these RNA molecules in a cell-free translation system using purified alien tRNAs and ribosomes. You obtain the results shown in in the table. From this information, which of the following peptides can be produced from poly UAUC?

Cys-Pro-Lys-Ala

You have discovered a gene (figure below) that is alternatively spliced to produce several forms of mRNA in various cell types, three of which are shown in the figure. The lines connecting the exons that are included in the mRNA indicate the splicing. From your experiments, you know that protein translation begins in exon 1. For all forms of the mRNA, the encoded protein sequence is the same in the regions of the mRNA that correspond to exons 1 and 10. Exons 2 and 3 are alternative exons used in different mRNA, as are exons 7 and 8. Which of the following statements about exons 2 and 3 is the most accurate?

Exons 2 and 3 must contain a number of nucleotides that when divided by 3, leaves the same remainder (that is, 0, 1, or 2).

Which of the following statements about RNA splicing is FALSE?

For a gene to function properly, every exon must be removed from the primary transcript in the same fashion on every mRNA molecule produced from the same gene.

Consider two genes (see figure below) that are next to each other on a chromosome, as arranged in the figure below. Which of the following statements is TRUE?

Gene A and gene B can be transcribed at different rates, producing different amounts of RNA within the same cell.

Genes in eukaryotic cells often have intronic sequences coded for within the DNA. These sequences are ultimately not translated into proteins. Why?

Intronic sequences are removed from RNA molecules by the spliceosome, which works in the nucleus.

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?

It inhibits movement of the small subunit relative to the large subunit.

What do you predict would happen if you created a tRNA with an anticodon of 5-CAA-3 that is charged with methionine, and added this modified tRNA to a cell-free translation system that has all the normal components required for translating RNAs?

Methionine would be incorporated into proteins at some positions where leucine should be.

Which of the following statements about the proteasome is FALSE?

Misfolded proteins are delivered to the proteasome, where they are sequestered from the cytoplasm and can attempt to refold.

Transcription in bacteria differs from transcription in a eukaryotic cell because

RNA polymerase (along with its sigma subunit) can initiate transcription on its own.

Which of the following statements is FALSE?

RNA polymerase adds bases in a 3′-to-5′ direction; DNA polymerase adds bases in a 5′-to-3′ direction.

Ribozymes are known to catalyze which of the following reactions in cells?

RNA splicing

Which of the following statements about prokaryotic mRNA molecules is FALSE?

Ribosomes must bind to the 5′ cap before initiating translation.

You have a bacterial strain with a mutation that removes the transcription termination signal from the Abd operon. Which of the following statements describes the most likely effect of this mutation on Abd transcription?

The Abd RNA from the mutant strain will be longer than normal.

You have a segment of DNA that contains the following sequence: 5-GGACTAGACAATAGGGACCTAGAGATTCCGAAA-3 3-CCTGATCTGTTATCCCTGGATCTCTAAGGCTTT-5 You know that the RNA transcribed from this segment contains the following sequence: 5-GGACUAGACAAUAGGGACCUAGAGAUUCCGAAA3 Which of the following choices best describes how transcription occurs?

The bottom strand is the template strand; RNA polymerase moves along this strand from 3′ to 5′.

The ribosome is important for catalyzing the formation of peptide bonds. Which of the following statements is TRUE?

The catalytic site for peptide bond formation is formed primarily from an rRNA.

Which of the following statements about the genetic code is CORRECT?

The genetic code is redundant.

Which of the following statements is FALSE?

Unlike DNA, RNA uses a uracil base and a deoxyribose sugar.

A strain of yeast translates mRNA into protein inaccurately. Individual molecules of a particular protein isolated from this yeast have variations in the first 11 amino acids compared with the sequence of the same protein isolated from normal yeast cells, as listed in the figure below. What is the most likely cause of this variation in protein sequence?

a mutation in the isoleucyl-tRNA synthetase that decreases its ability to distinguish between different amino acids

Unlike DNA, which typically forms a helical structure, different molecules of RNA can fold into a variety of three-dimensional shapes. This is largely because RNA

is single-stranded

RNA in cells differs from DNA in that

it is single-stranded and can fold up into a variety of structures.

Which amino acid would you expect a tRNA with the anticodon 5-CUU-3 to carry based on the table below?

lysine

Total nucleic acids are extracted from a culture of yeast cells and are then mixed with resin beads to which the polynucleotide 5-TTTTTTTTTTTTTTTTTTTTTTTTT-3 has been covalently attached. After a short incubation, the beads are then extracted from the mixture. When you analyze the cellular nucleic acids that have stuck to the beads, which of the following is most abundant?

mRNA

According to current thinking, the minimum requirement for life to have originated on Earth was the formation of a

molecule that could catalyze its own replication.

Transcription is similar to DNA replication in that

nucleotide polymerization occurs only in the 5′-to-3′ direction.

In eukaryotes, but not in prokaryotes, ribosomes find the start site of translation by

scanning along the mRNA from the 5′ end.

A mutation in the tRNA for the amino acid lysine results in the anticodon sequence 5-UAU-3 (instead of 5-UUU-3). Which of the following aberrations in protein synthesis might this tRNA cause?

substitution of lysine for isoleucine

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?

the deletion of a sequence that encodes sites at which ubiquitin can be attached to the protein

The figure below shows a ribose sugar. RNA bases are added to the part of the ribose sugar pointed to by arrow number?

3

The figure below shows a ribose sugar. The part of the ribose sugar where a new ribonucleotide will attach in an RNA molecule is pointed to by arrow number?

4

You are studying a disease that is caused by a virus, but when you purify the virus particles and analyze them you find they contain no trace of DNA. Which of the following molecules are likely to contain the genetic information of the virus?

RNA

An extraterrestrial organism (ET) is discovered whose basic cell biology seems pretty much the same as that of terrestrial organisms except that it uses a different genetic code to translate RNA into protein. You set out to break the code by translation experiments using RNAs of known sequence and cell-free extracts of ET cells to supply the necessary protein-synthesizing machinery. In experiments using the RNAs below, the following results were obtained when the 20 possible amino acids were added either singly or in different combinations of two or three: RNA 1: 5-GCGCGCGCGCGCGCGCGCGCGCGCGCGC-3 RNA 2: 5-GCCGCCGCCGCCGCCGCCGCCGCCGCCGCC-3 Using RNA 1, a polypeptide was produced only if alanine and valine were added to the reaction mixture. Using RNA 2, a polypeptide was produced only if leucine and serine and cysteine were added to the reaction mixture. Assuming that protein synthesis can start anywhere on the template, that the ET genetic code is nonoverlapping and linear, and that each codon is the same length (like the terrestrial triplet code), how many nucleotides does an ET codon contain?

5

The figure below shows a ribose sugar. The part of the ribose sugar that is different from the deoxyribose sugar used in DNA is pointed to by arrow number?

5

You have a piece of DNA that includes the following sequence: 5-ATAGGCATTCGATCCGGATAGCAT-3 3-TATCCGTAAGCTAGGCCTATCGTA-5 Which of the following RNA molecules could be transcribed from this piece of DNA?

5′-AUAGGCAUUCGAUCCGGAUAGCAU-3′

Below is the sequence from the 3 end of an mRNA. 5-CCGUUACCAGGCCUCAUUAUUGGUAACGGAAAAAAAAAAAAAA-3. If you were told that this sequence contains the stop codon for the protein encoded by this mRNA, what is the anticodon on the tRNA in the P site of the ribosome when release factor binds to the A site?

5′-CCA-3′

Which of the following molecules of RNA would you predict to be the most likely to fold into a specific structure as a result of intramolecular base-pairing?

5′-CCCUAAAAAAAAAAAAAAAAUUUUUUUUUUUUUUUUAGGG-3′

Which of the following statements is FALSE?

Eukaryotic mRNA molecules do not have lifetimes longer than 30 minutes.

Which of the following methods is not used by cells to regulate the amount of a protein in the cell?

Nuclear pore complexes can regulate the speed at which newly synthesized proteins are exported from the nucleus into the cytoplasm.

There are several reasons why the primase used to make the RNA primer for DNA replication is not suitable for gene transcription. Which of the statements below is NOT one of those reasons?

Primase can initiate RNA synthesis without the need for a base-paired primer.

Which of the following molecules is thought to have arisen first during evolution?

RNA

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 the figure. Which of the following interpretations is consistent with these observations?

The protein binds to sequences in the 5′ region of the mRNA and inhibits the rate of initiation of translation.

Which of the following does not occur before a eukaryotic mRNA is exported from the nucleus?

The ribosome binds to the mRNA.

Which one of the following is the main reason that a typical eukaryotic gene is able to respond to a far greater variety of regulatory signals than a typical prokaryotic gene or operon?

The transcription of a eukaryotic gene can be influenced by proteins that bind far from the promoter.

Which of the following might decrease the transcription of only one specific gene in a bacterial cell?

a mutation that introduced extensive sequence changes into the DNA that precedes the gene's transcription start site

The piece of RNA below includes the region that codes for the binding site for the initiator tRNA needed in translation. 5-GUUUCCCGUAUACAUGCGUGCCGGGGGC-3 in the table Which amino acid will be on the tRNA that is the first to bind to the A site of the ribosome?

arginine

snRNAs

can bind to specific sequences at intron-exon boundaries through complementary base-pairing.

Which of the following statements is TRUE?

rRNA contains the catalytic activity that joins amino acids together.

The sigma subunit of bacterial RNA polymerase

recognizes promoter sites in the DNA.

When using a repeating trinucleotide sequence (such as 5-AAC-3) in a cell-free translation system, you will obtain

three different types of peptides, each made up of a single amino acid.