MCAT Biology : Translation

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #11 : Translation

In the crusade to create a vaccine for Poliomyelitis, Jonas Salk and Albert Sabin created two separate vaccines that proved to be successful in preventing Polio onset.

The Salk vaccine, which is given by standard injection, contained virus particles inactivated by an organic solvent. This method has the advantage of inactivating each of the three Polio strains with no bias.

Albert Sabin's vaccine, given by oral inoculation via sugar water, contained live virus particles that had been genetically attenuated. With this method, each of the three Polio strains acquired separate mutations that made them unable to infect the human host cells. Strain 2 in particular contained one single nucleotide polymorphism in the internal ribosomal entry site (IRES) that prevented successful viral replication.

What is the function of the internal ribosomal entry site (IRES) utilized by the Poliovirus? 

Possible Answers:

The IRES provides an alternate site of entry that allows loading of viral mRNA onto the ribosome for translation

The IRES facilitates interaction between the large and small ribosomal subunits, which contributes to viral mRNA translation

The IRES is encoded by the host and prevents loading of viral mRNA onto the ribosome

The IRES allows the viral ribosome to out-compete the host ribosome, thus allowing for greater translation of viral proteins

Correct answer:

The IRES provides an alternate site of entry that allows loading of viral mRNA onto the ribosome for translation

Explanation:

Poliovirus mRNA is not capped, and therefore cannot be loaded onto host ribosomes for translation. To overcome this, viral mRNA contains an internal ribosomal entry site (IRES) sequence that allows it to bind and bring the translational machinery in close contact with the start codon to initiate translation.

Example Question #11 : Translation

Which of the following statements about translation is incorrect?

Possible Answers:

The mRNA is "read" by the ribosome in a 5' to 3' direction

The tRNAs that have given up their amino acid can exit the ribosome at the E site

New tRNAs with an amino acid enter the ribosomal complex at the P site

The mRNA first attaches to the small ribosomal subunit

Correct answer:

New tRNAs with an amino acid enter the ribosomal complex at the P site

Explanation:

Make sure that you know the three steps of translation: initiation, elongation, and termination. The order of the three sites available to a tRNA in the ribosome are A, P, and E. New tRNAs with an amino acid attached enter the ribosome at the A site. The tRNA that previously occupied the A site is pushed to the P site, with the growing polypeptide attached to it. It then gives the polypeptide to the next tRNA in the A site, and the now amino acid deficient tRNA can exit at the E site.

All other answer choices are true.

Example Question #13 : Translation

Type 1 diabetes is a well-understood autoimmune disease. Autoimmune diseases result from an immune system-mediated attack on one’s own body tissues. In normal development, an organ called the thymus introduces immune cells to the body’s normal proteins. This process is called negative selection, as those immune cells that recognize normal proteins are deleted. If cells evade this process, those that recognize normal proteins enter into circulation, where they can attack body tissues. The thymus is also important for activating T-cells that recognize foreign proteins.

As the figure below shows, immune cells typically originate in the bone marrow.  Some immune cells, called T-cells, then go to the thymus for negative selection. Those that survive negative selection, enter into general circulation to fight infection. Other cells, called B-cells, directly enter general circulation from the bone marrow. It is a breakdown in this carefully orchestrated process that leads to autoimmune disease, such as type 1 diabetes.

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When activated, T-cells use a number of proteins to kill cells that they recognize as foreign. A scientist develops an experimental drug to treat autoimmune disease by disrupting one of these proteins. The drug degrades the cytosolic mRNA for this protein in a T-cell. Which of the following is true if this drug is used successfully?

I. The protein is synthesized, but in an inactive form

II. The protein gene is transcribed

III. The total complement tRNA used for synthesis of the protein is not mobilized to active ribosomes

Possible Answers:

II and III

II, only

I, II, and III

III, only

I and III

Correct answer:

II and III

Explanation:

The successful use of the drug implies that the mRNA is degraded before it can be used in translation. As a result, the tRNAs used for the translation would not be mobilized for use on translating ribosomes, but transcription of mRNA would be unimpeded. The protein gene would be transcribed, the mRNA would be modified and leaves the nucleus, and would then be degraded before any synthesis could occur.

Example Question #14 : Translation

Temperature sensitive (Ts) mutations are a powerful genetic tool in yeast and fruit flies. Ts mutations allow researchers to examine biological functions of specific genes at permissive (phenotypically normal) and restrictive (phenotypically abnormal) temperatures. What is the likely result of the Ts mutation at the restrictive temperature?

Possible Answers:

The protein arising from the mutated gene does not fold properly

The gene that is mutated is only required at the restrictive temperature

Biological processes, such as transcription and translation, are globally impaired

Cells undergo apoptosis

Correct answer:

The protein arising from the mutated gene does not fold properly

Explanation:

The temperature sensitive (Ts) mutation to a given gene results in a less stable protein product. At higher temperatures the protein does not fold properly or "melts," resulting in an improper structure of the protein. In turn, this improper structure will inhibit its function.

The Ts mutation will only affect a single gene, and is unlikely to affect global functions or cause apoptosis. Very few genes are only required at specific temperatures (genes required for stress response is an example). It is possible that the gene in question is required only at the restricted temperature, however, this is not the likely cause.

Example Question #15 : Translation

In 2013, scientists linked a cellular response called the unfolded protein response (UPR) to a series of neurodegenerative diseases, including such major health issues as Parkinson’s and Alzheimer’s Disease. According to their work, the unfolded protein response is a reduction in translation as a result of a series of enzymes that modify a translation initiation factor, eIF2, as below:

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In the above sequence, the unfolded protein sensor binds to unfolded protein, such as the pathogenic amyloid-beta found in the brains of Alzheimer’s Disease patients. This sensor then phosphorylates PERK, or protein kinase RNA-like endoplasmic reticulum kinase. This leads to downstream effects on eIF2, inhibition of which represses translation. It is thought that symptoms of neurodegenerative disease may be a result of this reduced translation.

Which of the following is true of the process of translation discussed in the passage?

Possible Answers:

Translation takes place in the nucleus

rRNA is an important component of the translation machinery

Translation relies on membrane-bound ribosomes

Translation converts rRNA into protein

Translation relies on anticodons in mRNA to bind to codons on tRNA

Correct answer:

rRNA is an important component of the translation machinery

Explanation:

rRNA is an important building block of ribosomes, which synthesize proteins.

The anticodons of tRNA (not mRNA) bind to codons of mRNA (not rRNA), allowing ribosomes to tie together amino acids shuttled in on tRNA molecules. Many ribosomes are bound to the rough endoplasmic reticulum, but are not commonly bound to the membrane of the cell.

Example Question #16 : Translation

A surface protein would most likely be translated from mRNA by the __________.

Possible Answers:

nucleolus

Golgi apparatus

smooth endoplasmic reticulum

cytoplasmic ribosomes

rough endoplasmic reticulum

Correct answer:

rough endoplasmic reticulum

Explanation:

Proteins are translated from mRNA by ribosomes. Ribosomes are located on the surface of the rough endoplasmic reticulum, as well as in the cytoplasm. Inter-membrane proteins are constructed by the rough endoplasmic reticulum and sent off in a vesicle, which later becomes part of the cell surface. Generally, proteins created by cytoplasmic ribosomes are destined to serve as cytoplasmic proteins.

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