Question 1

A newly discovered protein is found to be modified by a lipid. Where is this protein most likely located?

Accepted Answer

Lipids are a major component of cell membranes, and therefore a protein that is modified by a lipid is most likely located in the cell membrane.

Question 2

Which antibiotic functions by inhibiting cell wall synthesis?

Accepted Answer

Penicillin inhibits cell wall synthesis by preventing the cross-linking of peptidoglycan chains in bacterial cell walls, leading to cell lysis and death.

Question 3

If a protein is covalently modified by ubiquitin, it will be

Accepted Answer

Ubiquitination is a post-translational modification in which ubiquitin, a small protein, is covalently attached to a target protein. This modification usually directs the protein to the proteasome, a cellular machinery responsible for protein degradation. Therefore, if a protein is covalently modified by ubiquitin, it is most likely targeted for degradation.

Question 4

A nucleophilic amino group of the amino acid bound to the 3 ^' terminus of A-site tRNA attacks the electrophilic carbonyl carbon in the ester bond between the 3 ^' terminus of P-site tRNA and its bound amino acid during the step of translation.

Accepted Answer

The step described in the sentence is the peptidyl transferase reaction, which occurs during elongation. During this step, the amino acid at the A site attacks the carbonyl carbon of the ester bond between the P-site tRNA and its bound amino acid, forming a peptide bond and transferring the growing polypeptide chain to the amino acid on the A-site tRNA.

Question 5

After the initiation complex is complete, what occurs next in the process of translation?

Accepted Answer

After the initiation complex is complete, the next step in the process of translation is the binding of EF-Tu•GTP to the AA-tRNA, followed by the AA-tRNA binding to the ribosome at the A site. This allows for the formation of the peptide bond and the continuation of protein synthesis.

Question 6

Release factor hydrolyzes GTP during the __________ step of translation.

Accepted Answer

Release factor binds to the stop codon during the termination step of translation and hydrolyzes GTP to release the polypeptide chain from the ribosome.

Question 7

Gene expression can be controlled by the modification of histones by all of the following EXCEPT

Accepted Answer

Gene expression can be controlled by modifications of histones, such as phosphorylation, methylation and acetylation. However, glycosylation is not known to directly modify histones and therefore does not control gene expression in this way.

Question 8

In the image below, which group will be attacked by the nucleophilic amino group of the next amino acid brought into the ribosome?

Accepted Answer

The answer of In the image below, which group will...

Question 9

Place the following steps in the initiation phase in a prokaryote in their proper order. 
A. mRNA binds to the complex.
B. GTP is hydrolyzed.
C. IF1, IF2, and IF3 bind the 30S subunit.

Accepted Answer

In prokaryotic initiation, IF1, IF2, and IF3 bind the 30S subunit first (C), followed by the hydrolysis of GTP (B), and then mRNA binds to the complex (A).

Question 10

Place the following steps in the elongation phase of translation in their proper order. A. Peptide bond formation occurs. B. GTP is hydrolyzed, and the ribosome moves one codon in the 3 ^' direction. C. GTP is hydrolyzed, and EF-Tu•GDP is released. D. tRNA is released from the E site.

Accepted Answer

The correct sequence for the elongation phase of translation involves the binding of a tRNA to the A site facilitated by EF-Tu and GTP (C), followed by peptide bond formation (A), then the ribosome translocates which involves GTP hydrolysis (B), and finally, the now uncharged tRNA is released from the E site (D).

Question 11

Which of the following is NOT a component that is released from the ribosome on translation termination?

Accepted Answer

The nascent polypeptide, RF2, and tRNA are all components that are released from the ribosome on translation termination. However, Met-tRNA^fMet is not released as it is the initiator tRNA and remains bound to the ribosome to initiate the next round of translation.

Question 12

Enzyme-mediated protein modification in eukaryotic cells is a means of regulating the activity of the protein. Which of the following is UNLIKELY to result from such a modification?

Accepted Answer

Hydration is a process of adding water molecules to a molecule and is not typically involved in enzyme-mediated protein modification in eukaryotic cells. Phosphorylation, methylation, and glycosylation are common modifications that can alter protein activity by adding or removing chemical groups to specific residues in the protein.

Question 13

Which antibiotic would be a good choice to use as a research tool if investigating the role of DNA gyrase in a cell?

Accepted Answer

Coumermycin inhibits the activity of DNA gyrase, which is an enzyme that is involved in DNA replication, recombination, and repair. Therefore, it would be a good choice to use as a research tool to investigate the role of DNA gyrase in a cell. The other options (penicillin, tetracycline, and bacitracin) do not specifically target DNA gyrase.

Question 14

In the ER, prenylation can occur. Prenylation is the attachment of an isoprenoid group to a __________ residue via a(n) _.

Accepted Answer

Prenylation involves the attachment of an isoprenoid group to a cysteine residue via a thioester bond.

Question 15

Which antibiotic functions by inhibiting DNA gyrase?

Accepted Answer

Coumermycin is the only antibiotic on the list that inhibits DNA gyrase. Vancomycin targets cell wall synthesis, aminoglycosides inhibit protein synthesis, and clindamycin inhibits protein synthesis by binding to the 50S subunit of the bacterial ribosome.

Question 16

Secondary downstream of the cap structure in the mRNA is unwound during the step of translation.

Accepted Answer

During the initiation step of translation, the mRNA is bound by the small ribosomal subunit and the cap structure is recognized by the eukaryotic initiation factor 4E (eIF4E). The downstream secondary structure in the mRNA is unwound at this stage to facilitate ribosome binding and subsequent translation.

Question 17

All glycoproteins have __________ covalently added posttranslationally.

Accepted Answer

Glycoproteins are proteins that have carbohydrates covalently attached to them post-translationally. This addition of carbohydrates is known as glycosylation and is important for many biological functions such as cell signaling and recognition. Glycoproteins can be found in a variety of cellular locations such as the cell surface, extracellular matrix, and secretory vesicles.

Question 18

How many GTP are hydrolyzed during a single round of translation elongation in a eukaryote?

Accepted Answer

Two GTP molecules are hydrolyzed during a single round of translation elongation in eukaryotes. The first GTP is hydrolyzed during the initiation of translation, and the second GTP is hydrolyzed during the elongation phase, specifically when the ribosome advances one codon along the mRNA.

Question 19

An experiment with bacteria that has been exposed to an antibiotic reveals that the translocation step has been inhibited. Which antibiotic was likely used?

Accepted Answer

Erythromycin is known to inhibit the translocation step in protein synthesis by binding to the 50S subunit of the bacterial ribosome, preventing the movement of the ribosome along the mRNA.

Question 20

The antibiotics streptomycin, tetracycline, and chloramphenicol all interfere with

Accepted Answer

Streptomycin, tetracycline, and chloramphenicol all inhibit protein synthesis in bacterial cells, making them effective antibiotics against infections caused by bacteria.

Quiz 22: Protein Synthesis, Posttranslational Modification, and Transport