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Translation and Regulation Questions and Answers

11.

What is the direct cause of cell death by diphtheria toxin?

Answer: B

Correct answer: Inactivation of a translational elongation factor

Diphtheria toxin ADP-ribosylates elongation factor 2, which is required to move the ribosome along the message. One toxin molecule can inactivate enough EF-2 to halt protein synthesis in the cell entirely, and the cell dies. Its catalytic action is why so tiny a quantity is lethal.

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12.

The development of global regulatory systems in prokaryotes

Answer: D

Correct answer: regulates many genes under varying environmental conditions

A global regulatory system - a regulon or stimulon - controls many genes at once in response to a single condition: heat shock, starvation, oxygen. Coordinating a whole set of responses through one signal is far more efficient than regulating each gene separately.

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13.

The interaction between mRNA and tRNA determines the position of amino acid in a polypeptide sequence. This is called the

Answer: D

Correct answer: adaptor hypothesis

Crick’s adaptor hypothesis proposed that an amino acid cannot recognise a codon directly, so an adaptor molecule must exist that binds the amino acid at one end and pairs with the codon at the other. tRNA turned out to be exactly that. The wobble hypothesis, also Crick’s, is a separate idea about the third base of the codon.

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14.

What are the Svedberg values for the subunits of the 70S ribosomes of E. coli?

Answer: C

Correct answer: 50S and 30S

The prokaryotic 70S ribosome is made of a 50S large subunit and a 30S small one. The figures do not add up arithmetically because S is a sedimentation coefficient, which depends on shape as well as mass. Eukaryotic cytoplasmic ribosomes are 80S, of 60S and 40S subunits - the difference antibiotics exploit.

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15.

Which of the following codons translates as serine (Ser)?

Answer: A

Correct answer: AGU

AGU codes for serine. Serine is unusual in having six codons, and in two separate families - UCN (UCU, UCC, UCA, UCG) and AGU/AGC - which is why it appears twice in a repeating sequence such as (UCAG)n. CGA is arginine, CAU histidine and AUG methionine.

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16.

The role of molecular chaperones is to

Answer: D

Correct answer: aid a newly synthesized polypeptide in folding to its proper shape

Chaperones do not carry folding information - that is in the sequence - but they prevent incorrect folding and aggregation while the chain is still emerging, binding exposed hydrophobic patches until the protein can bury them. Many are heat shock proteins, made in quantity when denaturation threatens.

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17.

What is the action of tetracycline in prokaryotes?

Answer: C

Correct answer: It blocks the binding of amino-acyl tRNA to the A site of ribosomes

Tetracycline binds the 30S subunit and prevents the incoming aminoacyl-tRNA from occupying the A site, so elongation cannot proceed. It is selective because bacterial cells concentrate the drug actively while human cells do not, and because the 70S ribosome differs from our 80S one.

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18.

A quorum sensing system

Answer: B

Correct answer: senses a compound produced by the bacterium itself

Quorum sensing works on a signal the bacteria make themselves - an autoinducer that accumulates as the population grows. When its concentration passes a threshold, the cells switch on genes that are worth expressing only in numbers: bioluminescence, biofilm formation, virulence factors. In effect the population takes a census before acting.

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19.

The Shine-Dalgarno sequence is

Answer: D

Correct answer: a short sequence that acts as a ribosomal binding site

The Shine-Dalgarno sequence is a purine-rich stretch a few bases upstream of the start codon. It base-pairs with a complementary sequence at the 3’ end of the 16S rRNA, which positions the ribosome so that the correct AUG is in the P site. Prokaryotic mRNAs are often polycistronic, and each cistron has its own Shine-Dalgarno sequence.

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20.

Which of the following statements is correct?

Answer: D

Correct answer: Amino acid sequence of a protein can be deduced from the corresponding cDNA nucleotide sequence

cDNA is made from mature mRNA, so it is the coding sequence with the introns already removed - and applying the genetic code to it gives the amino acid sequence directly. The other statements are false: introns are absent from cDNA and so cannot be deduced from it, restriction enzymes cut double-stranded DNA, and RFLPs lie mostly in non-coding regions.

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