Transcription/translation Practice Questions

Q: Given the mRNA sequence shown below, if translation were to start at the first base, what would the tRNA anticodon be for the last amino acid translated in the chain? 5'----GCACCGGCCUGACUAUAA---3'

3' CGG 5'. ***3' CGG 5'*** - The mRNA sequence is 5'-GCACCGGCCUGACUAUAA-3'. We need to identify the **open reading frame** starting from the first base and translate codons until a stop codon is reached. - The codons are **GCA** (Ala), **CCG** (Pro), **GCC** (Ala), **UGA** (Stop). The **last amino acid** translated is Alanine, corresponding to the mRNA codon **GCC**. The tRNA anticodon for GCC is **3'-CGG-5'** because base pairing rules dictate C pairs with G, and G pairs with C, in an antiparallel orientation. *3' GCG 5'* - This anticodon would pair with an mRNA codon of 5'-CGC-3', which codes for Arginine, not the alanine derived from the last amino acid in the given sequence. - It does not correctly reflect the antiparallel binding and base pairing required for the mRNA codon GCC. *5' CGG 3'* - While it contains the correct bases for pairing with GCC, the **orientation is incorrect**. tRNA anticodons are written 3' to 5'. - A 5'-CGG-3' anticodon would pair with an mRNA codon of 3'-GCC-5', which is not consistent with the standard 5' to 3' mRNA codon reading. *3' GAU 5'* - This anticodon would pair with an mRNA codon of 5'-CUA-3', which codes for Leucine. - Leucine is not the last amino acid translated from the given mRNA sequence before a stop codon. *3' CGC 5'* - This anticodon would pair with an mRNA codon of 5'-GCG-3', which codes for Alanine. - However, the last amino acid translated is encoded by 5'-GCC-3', not 5'-GCG-3'.

Q: An investigator is studying collagen synthesis in human fibroblast cells. Using a fluorescent tag, α-collagen chains are identified and then monitored as they travel through the rough endoplasmic reticulum, the Golgi apparatus, and eventually into the extracellular space. Which of the following steps in collagen synthesis occurs extracellularly?

Cleavage of procollagen C- and N-terminals. ***Cleavage of procollagen C- and N-terminals*** - After procollagen is secreted into the extracellular space, specific **proteolytic enzymes** (procollagen peptidases) cleave the bulky N- and C-terminal propeptides. - This cleavage transforms procollagen into insoluble **tropocollagen** molecules, which then spontaneously self-assemble into collagen fibrils. *Triple-helix formation* - This crucial step occurs within the **rough endoplasmic reticulum (RER)**, after hydroxylation and glycosylation of pro-α chains. - The three pro-α chains intertwine to form a stable, rod-like **procollagen molecule**. *Translation of pro-α chains* - The synthesis of pro-α chains (polypeptide chains) takes place on **ribosomes** attached to the **rough endoplasmic reticulum (RER)**. - This process is initiated in the cytosol and completed *into* the lumen of the RER. *Glycosylation of pro-α chains* - The addition of specific **oligosaccharide units** to hydroxylysine residues occurs in the **rough endoplasmic reticulum (RER)** and **Golgi apparatus**. - This modification is important for the stability of the collagen triple helix and for interactions with other extracellular matrix components. *Hydroxylation of proline and lysine* - This post-translational modification, essential for the stability of the collagen triple helix, occurs in the **rough endoplasmic reticulum (RER)**. - Enzymes like **prolyl hydroxylase** and **lysyl hydroxylase** require **vitamin C** as a cofactor for this reaction.

Q: A codon is an mRNA sequence consisting of 3 nucleotides that codes for an amino acid. Each position can be made up of any 4 nucleotides (A, U, G, C); therefore, there are a total of 64 (4 x 4 x 4) different codons that can be created but they only code for 20 amino acids. This is explained by the wobble phenomenon. One codon for leucine is CUU, which of the following can be another codon coding for leucine?

CUA. ***CUA*** - The **wobble hypothesis** allows for non-standard base pairing at the **third position** of the codon. - Since CUU codes for leucine, a change in the third base to **A (CUA)** can often still code for the same amino acid due to degeneracy of the genetic code. *CCC* - This codon codes for **proline**, not leucine. - A change in the **second letter** of the codon almost always results in a different amino acid. *UAA* - This is one of the **stop codons** (UAA, UAG, UGA), which signals the termination of translation. - It does not code for any amino acid. *CCA* - This codon codes for **proline**, not leucine. - Changing the first or second nucleotide typically results in a different amino acid. *AUG* - This codon codes for **methionine** and also serves as the **start codon** for protein synthesis. - It does not code for leucine.

Q: A 4-year-old boy with beta thalassemia requires regular blood transfusions a few times per month because of persistent anemia. He is scheduled for a splenectomy in the next several months. Samples obtained from the boy’s red blood cells show a malformed protein with a length of 160 amino acids (in normal, healthy red blood cells, the functional protein has a length of 146 amino acids). Which of the following best accounts for these findings?

Splice site mutation. ***Splice site mutation*** - A **splice site mutation** can lead to the retention of an **intron** or the **skipping of an exon**, resulting in an abnormal mRNA sequence. - If a cryptic splice site is used or an intron is retained, it can lead to the inclusion of additional amino acids in the final protein, thus increasing its length from 146 to 160 amino acids. *Nonsense mutation* - A **nonsense mutation** results in a **premature stop codon**, which would produce a **truncated protein** shorter than 146 amino acids. - This type of mutation does not explain the observed increase in protein length. *Silent mutation* - A **silent mutation** changes a single nucleotide but does **not alter the amino acid sequence** of the protein due to the redundancy of the genetic code. - This would result in a normal protein length of 146 amino acids and no observed malformation. *Missense mutation* - A **missense mutation** changes a single nucleotide leading to a **different amino acid**, but it typically **does not alter the total length** of the protein. - While it can lead to a *malformed protein*, it wouldn't explain the increased length from 146 to 160 amino acids. *Frameshift mutation* - A **frameshift mutation** is caused by the **insertion or deletion of nucleotides** not divisible by three, leading to a shift in the reading frame downstream. - This often results in a **premature stop codon** and a **shorter, non-functional protein**, or a completely altered sequence that is usually unstable, rather than a longer protein with 160 amino acids.

Q: An investigator is studying the modification of newly formed polypeptides in cultured eukaryotic cells. After the polypeptides are released from the ribosome, a chemically-tagged protein attaches covalently to lysine residues on the polypeptide chain, forming a modified polypeptide. When a barrel-shaped complex is added to the cytoplasm, the modified polypeptide lyses, resulting in individual amino acids and the chemically-tagged proteins. Which of the following post-translational modifications has most likely occurred?

Ubiquitination. ***Ubiquitination*** - The description of a **chemically-tagged protein** attaching to **lysine residues** on a newly formed polypeptide strongly suggests **ubiquitin**, a small protein that marks other proteins for degradation. - The subsequent lysis by a **barrel-shaped complex** (the **proteasome**) into amino acids and the chemically-tagged proteins is the hallmark of the **ubiquitin-proteasome pathway**, a major mechanism for targeted protein degradation. *Glycosylation* - Involves the **covalent attachment of carbohydrate moieties** to proteins, typically at asparagine, serine, or threonine residues. - While it is a common post-translational modification, it does not involve a "chemically-tagged protein" marking for proteasomal degradation. *Acylation* - Refers to the addition of an **acyl group** (e.g., fatty acids like myristate or palmitate) to a protein, often impacting membrane association. - This process is distinct from the described mechanism of protein tagging and subsequent degradation by a barrel-shaped complex. *Carboxylation* - Involves the **addition of a carboxyl group** to a protein, most notably to glutamate residues in clotting factors, requiring vitamin K. - This modification is not involved in marking proteins for degradation and does not utilize a specific "chemically-tagged protein" for this purpose. *Phosphorylation* - Refers to the **addition of a phosphate group** to a protein, typically at serine, threonine, or tyrosine residues, to regulate protein activity, signaling, and interactions. - While it is a common regulatory mechanism, it does not involve a "chemically-tagged protein" targeting the protein for complete degradation into amino acids by a proteasome.

Question 1

Given the mRNA sequence shown below, if translation were to start at the first base, what would the tRNA anticodon be for the last amino acid translated in the chain?
5'----GCACCGGCCUGACUAUAA---3'

Accepted Answer

3' CGG 5'

Answer

3' GCG 5'

Answer

3' CGC 5'

Answer

5' CGG 3'

Answer

3' GAU 5'

Question 2

An investigator is studying collagen synthesis in human fibroblast cells. Using a fluorescent tag, α-collagen chains are identified and then monitored as they travel through the rough endoplasmic reticulum, the Golgi apparatus, and eventually into the extracellular space. Which of the following steps in collagen synthesis occurs extracellularly?

Accepted Answer

Cleavage of procollagen C- and N-terminals

Answer

Triple-helix formation

Answer

Translation of pro-α chains

Answer

Glycosylation of pro-α chains

Answer

Hydroxylation of proline and lysine

Question 3

A codon is an mRNA sequence consisting of 3 nucleotides that codes for an amino acid. Each position can be made up of any 4 nucleotides (A, U, G, C); therefore, there are a total of 64 (4 x 4 x 4) different codons that can be created but they only code for 20 amino acids. This is explained by the wobble phenomenon. One codon for leucine is CUU, which of the following can be another codon coding for leucine?

Accepted Answer

CUA

Answer

CCC

Answer

UAA

Answer

CCA

Answer

AUG

Question 4

A pharmaceutical company has modified one of its existing antibiotics to have an improved toxicity profile. The new antibiotic blocks protein synthesis by first entering the cell and then binding to active ribosomes. The antibiotic mimics the structure of aminoacyl-tRNA. The drug is covalently bonded to the existing growing peptide chain via peptidyl transferase, thereby impairing the rest of protein synthesis and leading to early polypeptide truncation. Where is the most likely site that this process occurs?

Accepted Answer

A site

Answer

E site

Answer

30S small subunit

Answer

40S small subunit

Answer

P site

Question 5

A 4-year-old boy with beta thalassemia requires regular blood transfusions a few times per month because of persistent anemia. He is scheduled for a splenectomy in the next several months. Samples obtained from the boy’s red blood cells show a malformed protein with a length of 160 amino acids (in normal, healthy red blood cells, the functional protein has a length of 146 amino acids). Which of the following best accounts for these findings?

Accepted Answer

Splice site mutation

Answer

Nonsense mutation

Answer

Silent mutation

Answer

Missense mutation

Answer

Frameshift mutation

Question 6

An investigator is studying the modification of newly formed polypeptides in cultured eukaryotic cells. After the polypeptides are released from the ribosome, a chemically-tagged protein attaches covalently to lysine residues on the polypeptide chain, forming a modified polypeptide. When a barrel-shaped complex is added to the cytoplasm, the modified polypeptide lyses, resulting in individual amino acids and the chemically-tagged proteins. Which of the following post-translational modifications has most likely occurred?

Accepted Answer

Ubiquitination

Answer

Glycosylation

Answer

Acylation

Answer

Carboxylation

Answer

Phosphorylation

Transcription/translation — MCQs

Transcription/translation — MCQs

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