Biochemistry : Translation

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #1 : Translation

Which of the following is a true statement regarding translation in eukaryotes?

Possible Answers:

All translation begins on bound ribosomes attaches to the rough endoplasmic reticulum

tRNA acts as the template for polypeptide synthesis during translation

All translation begins in the cytoplasm on free ribosomes

During translation, the polypeptide is synthesized beginning from its carboxy terminus and ending with its amino terminus

The Shine-Delgarno sequence is a stretch of nucleotides on the RNA to be translated, which helps to initiate polypeptide synthesis by allowing the ribosomes to bind to the RNA

Correct answer:

All translation begins in the cytoplasm on free ribosomes

Explanation:

Translation is a process by which polypeptides are synthesized from a mRNA transcript, which was previously synthesized from the process of transcription. During this process, tRNA acts as a carrier by bringing with it specific amino acids to the ribosome, which are then incorporated into a growing polypeptide chain.

Eukaryotic translation differs in quite a few ways from prokaryotic translation. For one thing, prokaryotic mRNA contains a Shine-Delgarno sequence, which serves as a binding site for prokaryotic ribosomes to assemble on the mRNA. This binding, in turn, helps to initiate translation in prokaryotic cells. Eukaryotic cells do not contain a Shine-Delgarno sequence.

Furthermore, in eukaryotes, translation always begins with the assembly of ribosomal subunits on mRNA in the cytosol. Therefore, translation always begins on free ribosomes in the cytosol! Sometimes, translation will also finish on free ribosomes if the resulting protein is destined to stay within the cytosol where it will serve its function. Alternatively, if the first few amino acids of the polypeptide consists of a specific "signal sequence," translation will be temporarily paused. During this time, the entire ribosome-mRNA-polypeptide complex will be translocated to the rough endoplasmic reticulum. Once attached, polypeptide synthesis will resume and the polypeptide will thread its way into the endoplasmic reticulum. As it does so, additional folding and post-translational modifications are usually done to the polypeptide for it to carry out its proper function. Generally, polypeptides that make their way through the endoplasmic reticulum are destined either to be secreted out of the cell, or to become incorporated into the endomembrane system of the cell. And finally, as polypeptides are synthesized on a ribosome, whether it is free or bound, the amino terminus (aka N-terminus) side of the polypeptide is synthesized first and the carboxy terminus (aka C-terminus) is synthesized last.

Example Question #2 : Translation

A researcher is analyzing a protein that is found on the plasma membrane. What can be concluded about the translation of this protein? 

I. It was translated on a cytoplasmic ribosome

II. It involved the use of all three types of RNA

III. It involved a polymerase enzyme

Possible Answers:

III only

II and III

II only

I and II

Correct answer:

II only

Explanation:

Proteins undergo translation with the help of ribosomes, which can be found in either cytoplasm or on the rough endoplasmic reticulum (rough ER). Proteins synthesized on the ribosomes in cytoplasm are destined for somewhere inside the cell. On the other hand, proteins synthesized on the rough ER are processed in the ER and Golgi apparatus and are transported to the membrane or the extracellular matrix. Since the protein in the question is found on a membrane, it must have been synthesized on the ribosomes on the rough ER.  

Recall that all three types of RNA are used in translation. mRNA is the template strand used to synthesize the protein molecule. It contains the information regarding the sequence of amino acids in the protein molecule. tRNA is involved in transporting the amino acid to the growing polypeptide chain. rRNA molecules make up the ribosomes, the location of translation.

Polymerase enzymes are used in DNA replication (DNA polymerase) and transcription (RNA polymerase). They are not involved in translation.

Example Question #3 : Translation

Upon completion of translation, all proteins have __________ starting amino acid(s) and __________ ending amino acid(s).

Possible Answers:

different . . . the same

the same . . . different

the same . . . the same

different . . . different

Correct answer:

the same . . . different

Explanation:

Translation begins when a start codon is recognized in the mRNA molecule. The start codon is AUG, which codes for the amino acid methionine; therefore, all proteins begin with methionine. There are multiple stop codons; therefore, the ending of proteins could be different from one another.

Note that the question is asking about the state of a protein molecule after the completion of translation. A protein can undergo further processing events in the rough ER and Golgi apparatus during which the starting methionine may be cleaved; therefore, the ultimate end product of proteins might have a different starting amino acid.

Example Question #4 : Translation

Which of the following are the same regarding prokaryotic and eukaryotic translation?

I. Location of translation of prokaryotic proteins and eukaryotic membrane proteins

II. The start codon

III. Coupling of transcription with translation

Possible Answers:

I and III

II only

I and II

I only

Correct answer:

II only

Explanation:

Like transcription, there are slight differences between prokaryotic and eukaryotic translation. In prokaryotes transcription and translation are coupled and occur in the cytoplasm. Recall that in eukaryotes, translation can occur either in the cytoplasm or on the rough ER. Membrane and secretory proteins are synthesized in ribosomes on the rough ER whereas the cytosolic proteins are synthesized in ribosomes in cytoplasm.

The start codon for both prokaryotic and eukaryotic translation is AUG. This codes for the amino acid methionine, which is usually the first amino acid added to a growing polypeptide chain.

As mentioned, coupling of transcription and translation only occurs in the prokaryotes. Eukaryotic transcription occurs in the nucleus and the products need to undergo post-transcriptional modification before entering the cytoplasm for translation; therefore, the two processes aren’t coupled in eukaryotes.

Example Question #4 : Translation

What amino acid is found in all proteins that have just completed translation?

Possible Answers:

Methionine

Cysteine

Valine

Phenylalanine

Correct answer:

Methionine

Explanation:

Every protein begins with methionine, therefore, this will be found in all proteins upon completion of translation. Recall that this methionine might be excised when the protein is further processed in eukaryotes.

Example Question #6 : Translation

During translation, AUG corresponds to which amino acid?

Possible Answers:

Methionine

Isoleucine

Alanine

Serine

Valine

Correct answer:

Methionine

Explanation:

5' AUG 3' is the start codon for polypeptide synthesis and corresponds to the amino acid methionine.

Example Question #7 : Translation

Which of the following is false about the genetic code?

Possible Answers:

Codons are traditionally written with the 5' terminal on the left.

Each codon signifies either an amino acid, or a translation stop.

There are  = 64 three-letter combinations of A, C, T, and U.

Some nucleotide triplets are never used in translation.

Almost all organisms (except mitochondria) use basically the same genetic code.

Correct answer:

Some nucleotide triplets are never used in translation.

Explanation:

All nucleotide triplets can theoretically occur in translation. The genetic code is basically universal to all species, except for mitochondria, which create proteins independently from the cell. Each codon translates to an amino acid, a stop codon, or a start codon (which is also an amino acid, methionine). There are indeed 64 possible combinations of the nucleotides, by rules of combinatorics. The 5' terminal is written on the left, as a convention among biologists.

Example Question #8 : Translation

Which of the following is false about ribosomal binding sites?

Possible Answers:

The set of three binding sites is labelled A, P, and E.

The mRNA is shifted three nucleotides' length through the ribosome for each amino acid added.

Ribosomes have a set of three mRNA binding sites, and one tRNA binding site.

Peptidyl transferase creates the bond between neighboring amino acids using tRNAs stationed at the P and A sites.

If a tRNA molecule's anticodon is complementary to an mRNA codon, the tRNA is held at the P and A sites.

Correct answer:

Ribosomes have a set of three mRNA binding sites, and one tRNA binding site.

Explanation:

There are three tRNA binding sites -- A, P, and E (for Aminoacyl, Peptidyl, and Exit) -- but only one mRNA binding site. The enzyme which bonds amino acids carried by tRNAs at A and P is indeed called peptidyl transferase. tRNA is held at A and P when its anticodon matches the codon of the mRNA to be translated. Because each codon is three codons long, per amino acid, the mRNA is indeed shifted three nucleotides' length through the ribosome, each time an amino acid is added to the growing chain.

Example Question #9 : Translation

Which of the following amino acids has only one possible codon that codes for it?

Possible Answers:

Tyrosine

Phenylalanine

Asparagine

Aspartic acid

Methionine

Correct answer:

Methionine

Explanation:

Among the amino acids, there are two which only have one codon that code for them: tryptophan (UGG), and methionine. Methionine, is, of course, special among them, because the same codon is also the start codon -- AUG. Aspartic acid, asparagine, tyrosine, and phenylalanine all each have two possible corresponding codons (respectively: GAC/GAU, AAC/AAU, UAC/UAU, and UUC/UUU).

Example Question #5 : Translation

Which of the following translation initiation factors is incorrectly matched with its function?

Possible Answers:

None of these

eIF2B is a GEF for eIF1

eIF4G is a scaffolding protein

eIF4E is a cap binding protein

eIF4A is a helicase

Correct answer:

eIF2B is a GEF for eIF1

Explanation:

eIF2B is a GEF (guanine nucleotide exchange factor) for eIF1 eIF2. 

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