Biochemistry : Regulating Protein Synthesis

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Regulating Protein Synthesis

Which of the following statements is false about the human genome?

Possible Answers:

None of the other answers are false

There are about 20,000 proteins that are coded for by the genome

Repeated sequences make up about 5% of the genome

The sequences which code for proteins make up about 2% of the genome

Paralogs are homologous gene sequences caused by duplication events within the genome

Correct answer:

Repeated sequences make up about 5% of the genome

Explanation:

Scientists have indeed counted about 20,000 proteins coded for by the genome. Coding sequences are only about 2% or less of the genome. The definition of paralogs is genes related by duplication within a genome. Within the genome, not about 5%, but rather about 50%, of DNA sequences are repeated.

Example Question #2 : Regulating Protein Synthesis

Where does the pentose phosphate pathway primarily take place? 

Possible Answers:

The brush border of the small intestine

The mitochondria of the kidney

The mitochondria of the liver

The cytosol of the liver

Correct answer:

The cytosol of the liver

Explanation:

The pentose phosphate pathway (also known as the hexose monophosphate shunt or HMS), which mainly serves to produce  for anabolic reduction reactions and ribose-5-phosphate for nucleic acid production, takes place in the cytosol of hepatic cells.

Example Question #3 : Regulating Protein Synthesis

Which of the following amino acids can be created from the carbon skeleton of oxaloacetate?

Possible Answers:

Valine

Glutamine

Histidine

Leucine

Methionine

Correct answer:

Methionine

Explanation:

From the carbon skeleton of oxaloacetate, methionine can be created.  However, glutamine comes from alpha ketoglutarate, valine and leucine come from pyruvate, and histidine comes from ribose-5-phosphate.

Example Question #4 : Regulating Protein Synthesis

Which of the following molecules is not necessary to create glutamate from alpha-ketoglutarate?

Possible Answers:

Alpha-ketoglutarate

ATP

NADPH

Carbon dioxide

Correct answer:

Carbon dioxide

Explanation:

The reaction for the conversion of glutamine into glutamate is:

As seen in the reaction above, carbon dioxide is uninvolved.

Example Question #5 : Regulating Protein Synthesis

Which of the following correctly lists the severity of damage done by mutations in DNA from most severe to least?

Possible Answers:

Missense, nonsense, silent

Silent, missense, nonsense

Nonsense, silent, missense 

Nonsense, missense, silent

Correct answer:

Nonsense, missense, silent

Explanation:

When a change results in an early stop codon, nonsense mutation occurs and the protein is done being read early, often resulting in a nonfunctional protein. When a base change results into a different amino acid, this is a missense mutation. When a base change occurs but results in the same amino acid being read, this is considered a silent mutation. 

Example Question #6 : Regulating Protein Synthesis

Which of the following correctly describes the function of a signal sequence with respect to proteins?

Possible Answers:

Allows the protein to be recognized by the ubiquitination pathway for degradation

Marks the protein to be destined inside the nucleus

Transports proteins that are destined for the secretory pathway within cells

Serves as a signal for chaperone proteins to help fold the protein into the correct conformation

Allows the protein or peptide to act as a signaling molecule by recognizing its associated receptor

Correct answer:

Transports proteins that are destined for the secretory pathway within cells

Explanation:

To answer this question, it's essential to have an understanding of what a signal sequence is.

A signal sequence (also sometimes called a signal peptide) is a specific sequence of amino acids on a polypeptide that appears near the beginning of translation. When this signal sequence is present, it causes a temporary halt in the translation process. Meanwhile, another protein called a signal recognition particle (SRP) comes along and binds to the ribosome that is translating the polypeptide. Together, this polypeptide-ribosome-SRP complex is transferred from the cytosol to the surface of the endoplasmic reticulum (ER). Once there, the complex allows the polypeptide to resume synthesis, but in doing so, causes it to be synthesized into the inner lumen of the endoplasmic reticulum. Consequently, this polypeptide will go on to be modified within the ER and also the Golgi apparatus. Afterwards, it will be sent off within a vesicle, where is will either be A) secreted outside of the cell or B) incorporated into the endomembrane system of the cell (in other words, the peptide will be inserted into a membrane such as the plasma membrane, ER membrane, Golgi membrane, etc.). Lastly, it is the nuclear localization sequence (NLS) that, when added to a protein, allows it to enter the nucleus through the nuclear membrane.

Example Question #7 : Regulating Protein Synthesis

Which of of the following are the termination signals for translation?

Possible Answers:

GUA, GAU, UAA

UAA, UGA, UAG

TUA, UAG, GAU

UAG, UAA, UTA

GUA, UAA, UAG

Correct answer:

UAA, UGA, UAG

Explanation:

Just as there is an initiation codon regulating translation, there are termination codons that code for the end of translation. The three termination codons are UAA, UAG, and UGA. 

A helpful mnemonic for these are the phrases:

You are annoying (UAA)

You are gross (UAG)

You go away (UGA)

Example Question #8 : Regulating Protein Synthesis

What are some post-translational modifications collagen goes thru before attaining its final structure?

I. The precursor collagen molecule undergoes hydroxylation of selected proline and lysine amino acids.

II. The procollagen precursor is glycosylated by the addition of galactose and glucose.

III. Procollagen has amino and carboxy procollagen extension propeptides that make it soluble.

IV. Procollagen proteinases remove extension peptides from the ends of the molecule to form collagen.

Possible Answers:

I, III, and IV

I and II

I, II, III, and IV

I, II, and III

I and IV

Correct answer:

I, II, III, and IV

Explanation:

Procollagen has amino and carboxy procollagen extension propeptides that make it soluble. The preprocollagen undergoes both hydroxylation and glycosylation at specific aminoacid residues to form procollagen. Once secreted extracellularly, proteinases remove the extension peptides from procollagen to form the final collagen molecule.

Example Question #9 : Regulating Protein Synthesis

Which of the following enzyme cofactors transfer methyl groups?

Possible Answers:

S-adenylosyl methionine, tetrahydrofolate

B12 cobalamin, S-adenylosyl methionine, tetrahydrofolate, biotin

Tetrahydrofolate, biotin

B12 cobalamin, S-adenylosyl methionine, tetrahydrofolate

Correct answer:

B12 cobalamin, S-adenylosyl methionine, tetrahydrofolate

Explanation:

Biotin moves carboxyl groups in the enzyme acetyl-CoA carboxylase. Tetrahydrofolate and S-adenylosyl methionine move methyl groups in amino acid synthesis and post-translational modifications such as DNA methylation. B12 cobalamin is a cofactor in the reactions producing succinyl-CoA and methionine, where it transfers methyl groups to complete the products.

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