Molecular Biology Questions and Answers – Post-Translational Modifications of Proteins

This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “Post-Translational Modifications of Proteins”.

1. Which of the following is not a type of post translational modification?
a) Proteolysis
b) Protein folding
c) Glycosylation
d) Lipid addition
View Answer

Answer: b
Explanation: Post translational modifications are the chemical changes to the Proteins after they are synthesized and this includes three types of modification that is proteolysis, glycosylation and lipid addition; Whereas Protein folding is the process by which a newly synthesized protein acquires its mature 3D structure, by folding the polypeptide, with the help of chaperons. This folding happens during translation on the ribosome, not after as a separate process.

2. Proteolytic modifications of the polypeptide are an important process in the mechanism for protein sorting and transport.
a) True
b) False
View Answer

Answer: a
Explanation: Proteolytic modifications of the amino terminus play a part in the translocation of many proteins across the membranes, including secretory proteins in both bacteria and eukaryotes as well as proteins destined for incorporation into the plasma membrane, lysozomes, mitochondria and chloroplasts of eukaryotic cells. These proteins are targeted for transport to their destinations by amino terminal sequences that are removed by proteolytic cleavage as the protein crosses the membrane.

3. The amino acid is the signal sequence in any polypeptide chain for ____________
a) Protein activity
b) Glycosylation site
c) Proteolytic site
d) Site for lipid addition
View Answer

Answer: c
Explanation: The amino acid is the signal sequence that is usually about 20 amino acids long and targets many secretory proteins to the plasma membrane of bacteria or the ER of eukaryotic cells while translation is still in progress. The signal sequence, which consists predominantly of hydrophobic amino acids, is inserted into a membrane channel as it emerges from the ribosome. This signal is cleaved off to produce the mature polypeptide.
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4. The first step in protein targeting is ___________
a) Synthesis of protein
b) Translocation to Golgi body
c) Translocation to nucleus
d) Translocation to ER
View Answer

Answer: d
Explanation: The first step in protein targeting is its translocation to the ER after its production from the ribosome. Only after this translocation proteolytic cleavage occurs producing the mature protein which undergoes many modifications before it is targeted to its destination.

5. How many Proteolytic cleavages produce mature insulin?
a) 1
b) 2
c) 3
d) 4
View Answer

Answer: b
Explanation: Insulin forms by two cleavages. The initial precursor, preproinsulin, contains an amino terminal signal sequence that targets the polypeptide chain to the ER. Removal of the signal sequence during transfer to the ER yields a second precursor, called proinsulin. This precursor is then converted to insulin by proteolytic removal of an internal peptide.
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6. Glycosylation is the addition of ___________ to the protein.
a) Carbohydrate
b) Lipid
c) Fat
d) Minerals
View Answer

Answer: a
Explanation: Glycosylation is the addition of carbohydrate moieties to the protein. The proteins to which the carbohydrate groups are added are called glycoproteins. They can be added in but amino and carboxy terminals of the amino acids depending on the type of amino acid.

7. In the N-linked glycoprotein, the carbohydrates are attached to which of the following bases?
a) Valine
b) Threonine
c) Asparagine
d) Serine
View Answer

Answer: c
Explanation: In the N-linked glycoprotein the carbohydrates are attached to the amino terminus of the amino acid. An example of such an amino acid to which N-linked glycosylation occurs is the nitrogen atom in the side chain of Asparagine.
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8. In O-linked glycoproteins, the carbohydrates may be attached to how many amino acids?
a) 1
b) 2
c) 3
d) 4
View Answer

Answer: b
Explanation: In O-linked glycoproteins, the carbohydrates may be attached to two of the 20 amino acids found generally. These carbohydrate moieties are added to the oxygen atom in the side chain of serine or Threonine.

9. Initiation of glycosylation occurs in the Golgi body.
a) True
b) False
View Answer

Answer: b
Explanation: The proteins are usually transferred to the ER as soon as they are produced by the ribosome. Thus most of the modification such as proteolysis and glycosylation occurs within the lumen of the ER.
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10. How many types of modification are possible in eukaryotes by addition of lipids?
a) 1
b) 2
c) 3
d) 4
View Answer

Answer: d
Explanation: Three general types of lipid additions – N-myristoylation, prenylation and palmitoylation – are common in eukaryotic proteins associated with the cytosolic face of the plasma membrane. A fourth type of modification, the addition of glycolipids, plays an important role in anchoring some cell surface proteins to the extracellular face of the plasma membrane.

11. Prenylation adds prenyl groups to the ___________ amino acid residues.
a) Methionine
b) Cystine
c) Threonine
d) Arginine
View Answer

Answer: b
Explanation: Prenylation adds prenyl groups to the Cystine residue. The prenyl group is added to the sulphur atom in the side chains of cystine residues located near the C-terminus of the polypeptide chain.

12. Frame shift mutation can increase or decrease the length of a polypeptide.
a) True
b) False
View Answer

Answer: a
Explanation: Frame shift mutation is a type of point mutation that introduces alterations in the genetic code. Thus frame shift mutation introduces insertion or deletions of one or a small number of base pairs that alter the reading frame.

13. Consider a tandem repeat of the sequence GCU. Now if insertion of an A residue occurs in the message as follows
Find the type of mutation has taken place in case of tandem repeat of the sequence GCU
What type of mutation has taken place in this case?
a) Missense mutation
b) Stop mutation
c) Frame shift mutation
d) Reverse mutation
View Answer

Answer: c
Explanation: GCU generally codes for alanine but the insertion of A in the genetic message changes the reading frame thus generates a serine codon (AGC) at the site of insertion. This results in the frame shift in the open reading frame from alanine to cystine in the downstream of the insertion. Thus the insertion of a single base alters the coding capacity of the message.

14. If the insertions of three nucleotides occur in a consecutive manner it causes which of the following type of mutation?
a) Missense mutation
b) Stop mutation
c) Frame shift mutation
d) Reverse mutation
View Answer

Answer: c
Explanation: If the insertions of three nucleotides occur in a consecutive manner it causes frame shift mutation. This is because frame shift mutation introduces insertion or deletions of one or a small number of base pairs that alter the reading frame.

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