Question 1

Use the following model of a eukaryotic transcript to answer the next few questions.
5' UTR E₁ I₁ E₂ I₂ E₃ I₃ E₄ UTR 3'
-Which components of the previous molecule will also be found in mRNA in the cytosol?

Accepted Answer

The untranslated regions (UTRs) and all of the exons (E₁-E₄) will be found in mRNA in the cytosol as they are necessary for translation. The introns (I₁-I₃) are removed during RNA splicing and are not present in the final mRNA product that is translated in the cytosol.

Question 2

An experimenter has altered the 3' end of the tRNA corresponding to the amino acid methionine in such a way as to remove the 3' AC. Which of the following hypotheses describes the most likely result?

Accepted Answer

The 3' AC of tRNA is important for binding the amino acid to the tRNA. Without this group, the amino acid methionine will not be able to bind to the tRNA, which means it will not be available for incorporation into a growing protein chain. This would result in a loss of function and likely have serious consequences for the organism. The other options listed are not as likely to occur as a result of this specific alteration to the tRNA.

Question 3

Use the following model of a eukaryotic transcript to answer the next few questions.
5' UTR E₁ I₁ E₂ I₂ E₃ I₃ E₄ UTR 3'
-When the spliceosome binds to elements of this structure, where can it attach?

Accepted Answer

The answer of Use the following model of a eukaryotic...

Question 4

The process of translation, whether in prokaryotes or eukaryotes, requires tRNAs, amino acids, ribosomal subunits, and which of the following?

Accepted Answer

The process of translation requires energy in the form of GTP, which is used by polypeptide factors to assist in the formation of the polypeptide chain. ATP is used in other cellular processes, but not specifically in translation. Polymerases are involved in transcription, not translation. SRP (signal recognition particle) and chaperones are involved in protein targeting and folding, respectively, but not directly in translation. Signal peptides and release factor are also involved in protein targeting, but not in the actual process of translation.

Question 5

What is the function of the release factor (RF)?

Accepted Answer

The release factor (RF) binds to the stop codon in the A site in place of a tRNA, causing the ribosome to release the newly synthesized polypeptide.

Question 6

What is the function of GTP in translation?

Accepted Answer

GTP energizes the formation of the initiation complex, using initiation factors. During translation, the small ribosomal subunit binds to the mRNA and moves along it until it reaches the start codon. At this point, the initiator tRNA carrying methionine (or N-formylmethionine in bacteria) binds to the start codon, with the help of initiation factors. The initiation factors require GTP to carry out this process. Once the initiation complex is formed, GTP hydrolysis releases the initiation factors and allows for the binding of the large ribosomal subunit to complete the formation of the functional ribosome.

Question 7

In the structural organization of many eukaryotic genes, individual exons may be related to which of the following?

Accepted Answer

Exons are coding regions of DNA that are transcribed into mRNA and ultimately translated into proteins. Each exon codes for a specific domain or functional unit of the protein. Therefore, individual exons are related to the various domains of the polypeptide product.

Question 8

When the ribosome reaches a stop codon on the mRNA, no corresponding tRNA enters the A site. If the translation reaction were to be experimentally stopped at this point, which of the following would you be able to isolate?

Accepted Answer

The answer of When the ribosome reaches a stop codon...

Question 9

Which of the following is the first event to take place in translation in eukaryotes?

Accepted Answer

The first step in translation in eukaryotes involves the small subunit of the ribosome recognizing and attaching to the 5' cap of the mRNA. This is crucial for the initiation of translation, as it positions the ribosome correctly to start reading the mRNA from the correct end.

Question 10

When the function of the newly made polypeptide is to be secreted from the cell where it has been made, what must occur?

Accepted Answer

If the newly made polypeptide is to be secreted from the cell where it has been made, it must contain a signal sequence that targets it to the ER. The signal sequence is recognized by a signal recognition particle (SRP), which causes the ribosome to attach to the ER membrane. The polypeptide is then translocated into the ER lumen, where it undergoes further processing and modification before being transported to the Golgi and eventually secreted from the cell.

Question 11

Use the following model of a eukaryotic transcript to answer the next few questions.
5' UTR E₁ I₁ E₂ I₂ E₃ I₃ E₄ UTR 3'
-Which of the following is a useful feature of introns for this model?

Accepted Answer

Introns allow for exon shuffling, which can produce different combinations of exons in mRNA through alternative splicing. This can increase genetic diversity and create different protein isoforms with different functions.

Question 12

In an experimental situation, a student researcher inserts an mRNA molecule into a eukaryotic cell after he has removed its 5' cap and poly-A tail. Which of the following would you expect him to find?

Accepted Answer

The 5' cap and poly-A tail protect the mRNA from degradation by exonucleases. Without them, the mRNA is vulnerable to exonucleases and likely to be digested. Therefore, we would expect the molecule to be digested by exonucleases since it is no longer protected at the 5' end.

Question 13

A particular triplet of bases in the coding sequence of DNA is AAA. The anticodon on the tRNA that binds the mRNA codon is

Accepted Answer

The anticodon on the tRNA that binds the mRNA codon is complementary to the mRNA codon, not the DNA sequence. The mRNA codon for AAA in DNA is UUU, so the tRNA anticodon that pairs with this mRNA codon is AAA's complement, which is UUU.

Question 14

Suppose that an induced mutation removes most of the 5' end of the 5' UTR. What might result?

Accepted Answer

The 5' cap, which is necessary for translation initiation, is located at the 5' end of the 5' UTR. Without the cap, the mRNA will not be recognized by the ribosome and will be rapidly degraded.

Question 15

There are 61 mRNA codons that specify an amino acid, but only 45 tRNAs. This is best explained by the fact that

Accepted Answer

The phenomenon described is due to the "wobble" hypothesis, which explains that the base pairing rules between the third base of a codon and the anticodon of tRNA are less stringent, allowing a single tRNA molecule to recognize and bind to more than one codon.

Question 16

Which of the following is a function of a signal peptide?

Accepted Answer

The function of a signal peptide is to direct the translocation of newly synthesized polypeptides across the endoplasmic reticulum membrane. This is accomplished by binding to the signal recognition particle (SRP) and the SRP receptor, which facilitates transport to the translocon channel in the ER membrane. Once the polypeptide is fully translocated into the ER lumen, the signal peptide is cleaved off by a signal peptidase enzyme.

Question 17

Accuracy in the translation of mRNA into the primary structure of a polypeptide depends on specificity in the

Accepted Answer

Accuracy in translation depends on both the specificity in the bonding of the anticodon to the codon as well as the attachment of the correct amino acid to the corresponding tRNA molecule. Both of these events must occur accurately in order for the correct amino acid to be incorporated into the growing polypeptide chain. The other choices may play a role in translation efficiency but are not directly related to accuracy of translation.

Question 18

A mutant bacterial cell has a defective aminoacyl synthetase that attaches a lysine to tRNAs with the anticodon AAA instead of the normal phenylalanine. The consequence of this for the cell will be that

Accepted Answer

The mutant bacterial cell has a defective aminoacyl synthetase that attaches a lysine to tRNAs with the anticodon AAA instead of the normal phenylalanine. This means that during translation, instead of a phenylalanine being added to the growing polypeptide chain when a UUU codon is encountered, a lysine will be added instead. Therefore, proteins in the cell will include lysine instead of phenylalanine at amino acid positions specified by the codon UUU.

Question 19

Suppose that exposure to a chemical mutagen results in a change in the sequence that alters the 5' end of intron 1 (I₁). What might occur?

Accepted Answer

Altering the 5' end of intron 1 could potentially lead to the inclusion of intron 1 in the mRNA, as the spliceosome would recognize a different splice site. This could result in a truncated or nonfunctional protein. However, none of the other choices necessarily follow from this specific mutation.

Question 20

When translating secretory or membrane proteins, ribosomes are directed to the ER membrane by

Accepted Answer

Ribosomes are directed to the ER membrane by a signal-recognition particle (SRP) that recognizes and binds to the signal sequence of the newly synthesized polypeptide chain. The SRP/ribosome complex is then directed to a receptor protein in the ER membrane, where the polypeptide chain is translocated across the membrane and into the ER lumen.

Quiz 17: From Gene to Protein

Use the following model of a eukaryotic transcript to answer the next few questions. 5' UTR E₁ I₁ E₂ I₂ E₃ I₃ E₄ UTR 3' -Which components of the previous molecule will also be found in mRNA in the cytosol?