Question 1

How does termination of translation take place?

Accepted Answer

Termination of translation occurs when a stop codon (UAA, UAG, or UGA) is encountered on the mRNA molecule. Since there are no corresponding tRNAs that recognize these codons, the ribosome releases the newly synthesized polypeptide chain and dissociates from the mRNA.

Question 2

Ribosomes can attach to prokaryotic messenger RNA

Accepted Answer

Ribosomes can attach to prokaryotic messenger RNA before transcription is complete, allowing for simultaneous transcription and translation to occur in prokaryotic cells. This is known as coupled transcription-translation.

Question 3

Figure 17.3
-Refer to Figure 17.3. What is the function of the AGU on the loop of the tRNA?

Accepted Answer

The AGU on the loop of the tRNA base pairs with the codon of mRNA during translation.

Question 4

Which of the following is NOT one of the steps in initiation of translation?

Accepted Answer

The formation of a polypeptide bond occurs during elongation, not initiation, of translation. The correct steps in initiation of translation are the binding of tRNA carrying formyl methionine to the start codon and small ribosomal subunit, recognition and binding of mRNA by the small ribosomal subunit, and binding of the large ribosomal subunit to the small ribosomal subunit.

Question 5

There are 61 codons that each specify the addition of a specific amino acid, and three stop codons for which there is no corresponding amino acid. However, there are only about 40 tRNA molecules, representing 40 anticodons. How is that possible?

Accepted Answer

The phenomenon described is known as "wobble base pairing," where the anticodon of tRNA molecules can recognize more than one codon for an amino acid due to less strict base pairing rules at the third position of the codon. This allows a smaller number of tRNA molecules to recognize all 61 codons that code for amino acids.

Question 6

A part of an mRNA molecule with the following sequence is being read by a ribosome:
5' CCG- ACG 3'mRNA). The following charged transfer RNA molecules with their anticodons shown in the 3' to 5' direction) are available. Two of them can correctly match the mRNA so that a dipeptide can form.
Table 17.1
$\begin{array} { | l | l | } 
\hline \text { tRNA Anticodon } & \text { Anind Acid } \
\hline \text { FRE } & \text { Prolire } \
\text { CEW } & \text { Alanire } \
\text { WEE } & \text { Thronire } \
\text { EEE } & \text { Glycire } \
\text { AEE } & \text { Cyateine } \
\text { EEE } & \text { Alanire } \
\hline
\end{array}$
What is the anticodon loop of the first tRNA that will complement this mRNA?

Accepted Answer

The anticodon of the first tRNA that will complement the mRNA sequence 5' CCG 3' is 3' GGC 5', which pairs with the codon for proline.

Question 7

A transfer RNA #1) attached to the amino acid lysine enters the ribosome. The lysine binds to the growing poly on the other tRNA #2) in the ribosome already.
Where does tRNA #2 move to after this bonding of lysine to the polypeptide?

Accepted Answer

After a new amino acid is added to the growing polypeptide chain, the tRNA that was in the P site (and is now without an amino acid) moves to the E site (exit site) before being released into the cytosol.

Question 8

The ribosome- binding site of prokaryotes is also known as

Accepted Answer

The ribosome-binding site of prokaryotes is referred to as the Shine-Dalgarno sequence, which is a nucleotide sequence located upstream of the start codon in mRNA. The Shine-Dalgarno sequence binds to the 16S rRNA of the small ribosomal subunit, helping to position the ribosome at the correct start site for translation. The promoter (A) is a region of DNA located upstream of a gene that helps to initiate transcription. The Pribnow box (B) is a specific sequence of DNA found in the promoter region of many prokaryotic genes that helps to recruit RNA polymerase during transcription. The TATA box (D) is a specific sequence of DNA found in the promoter region of eukaryotic genes that helps to recruit RNA polymerase II during transcription.

Question 9

Figure 17.4
-Refer to Figure 17.4. During elongation, which site in the ribosome represents the location where a codon is being read?

Accepted Answer

The A site in the ribosome represents the location where a codon is being read during elongation. The A site is where the incoming aminoacyl-tRNA binds to complement the mRNA codon in the ribosomal RNA.

Question 10

Post- translational modifications include all of the following EXCEPT

Accepted Answer

Post-translational modifications refer to any changes made to a polypeptide after it has been synthesized. Therefore, the removal of introns, which occurs during the pre-mRNA splicing process prior to translation, is not a form of post-translational modification. The other options (formation of covalent bonds, addition of carbohydrates to make a glycoprotein, and formation of hydrogen bonds) are all examples of post-translational modifications.

Question 11

Which of the following occurs in prokaryotes, but not eukaryotes?

Accepted Answer

Concurrent transcription and translation occurs in prokaryotes, but not in eukaryotes. In prokaryotes, the lack of a nuclear membrane allows for mRNA to be immediately translated by ribosomes while it is still being transcribed. In contrast, in eukaryotes, transcription occurs in the nucleus and mature mRNA must be transported out to the cytoplasm for translation to occur. Gene regulation, translation in the absence of a ribosome, and post-transcriptional splicing occur in both prokaryotes and eukaryotes.

Question 12

Once a peptide has been formed between the amino acid attached to the tRNA in the P site and the amino acid associated with the tRNA in the A site, which process on the list occurs next?

Accepted Answer

After a peptide bond is formed between the amino acids on the P and A site tRNAs, the ribosome undergoes translocation to shift the tRNAs to the P and E sites respectively, and expose the next codon of mRNA for decoding by a new tRNA in the newly vacant A site.

Question 13

What is the function of the 3' end of the tRNA?

Accepted Answer

The 3' end of tRNA binds covalently to the amino acid, forming the aminoacyl-tRNA complex. This complex is then delivered to the ribosome for translation.

Question 14

The start codon is the same for prokaryotes and eukaryotes. What is the start codon?

Accepted Answer

The start codon is the first three-letter sequence of mRNA that signals the ribosome to start translation. AUG is the start codon and codes for the amino acid methionine. This codon is universal in all organisms, from bacteria to humans. Anticodons are found on tRNA molecules and bind to codons during translation. Ribozymes are RNA molecules that function as enzymes. GGC is a triplet nucleotide sequence that codes for the amino acid glycine but is not a start codon.

Question 15

Which of the following does not play a role in the process of translation?

Accepted Answer

ATP is primarily involved in energy transfer and is not directly used in the translation process, which involves mRNA, tRNAs, ribosomes, and other factors.

Question 16

A mutant bacterial cell has a defective aminoacyl- tRNA synthetase that attaches a lysine to tRNAs with the anticodon AAA instead of the normal phenylalanine. What would the consequence of this be for the cell?

Accepted Answer

The mutant bacterial cell will attach lysine instead of phenylalanine to tRNAs with the anticodon AAA. When the ribosome encounters the codon UUU, it will pair it with the anticodon AAA carrying lysine instead of phenylalanine. As a result, proteins in the cell will include lysine instead of phenylalanine at amino acid positions specified by the codon UUU. This could potentially alter the structure and function of the resulting proteins.

Question 17

Put the following events of elongation in prokaryotic translation in chronological order.
1) Binding of mRNA with small ribosomal subunit.
2) Recognition of initiation codon
3) Complementary base pairing between initiator codon and anticodon of initiator tRNA
4) Base pairing of the mRNA codon following the initiator codon with its complementary tRNA
5) Attachment of the large subunit

Accepted Answer

The first step in prokaryotic elongation is the binding of mRNA with the small ribosomal subunit (1). Then, the recognition of the initiation codon occurs (2). The complementary base pairing between the initiator codon and the anticodon of the initiator tRNA happens next (3). After that, the mRNA codon following the initiator codon base pairs with its complementary tRNA (4). Finally, the attachment of the large subunit occurs, and translation can begin (5). Therefore, the correct order is 1, 2, 3, 5, 4, which is option C.

Quiz 17: Transcription, Rna Processing, and Translation