MCAT Biology : Translation

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #1 : Translation

Which of the following cell organelles is responsible for making proteins?

Possible Answers:

Ribosomes

Endoplasmic reticulum

Lysosomes

Golgi apparatus

Correct answer:

Ribosomes

Explanation:

Ribisomes are the protein creators within the cell. Ribisomes use a process called translation (via the use of mRNA) to form proteins.

The Golgi apparatus and rough endoplasmic reticulum are involved in protein modification and packaging. Lysosomes are involved in detoxification.

Example Question #1206 : Mcat Biological Sciences

Protein is translated from __________ transcripts.

Possible Answers:

other proteins

tRNA

mRNA

DNA

ribosomes

Correct answer:

mRNA

Explanation:

DNA is transcribed into pre-mRNA in the nucleus. To leave the nucleus, the pre-mRNA undergoes modification to remove introns, add a poly-A tail, and add a 5' cap. When modification is complete, the chain is a mature mRNA that is capable of exiting the nucleus. Upon leaving the nucleus, the mRNA is bound to a ribosome for translation into protein product.

Ribosomes are largely comprised of rRNA. tRNA is used during the translation process to transport amino acids through the cytoplasm for addition onto the growing amino acid chain in the ribosome.

Example Question #31 : Dna, Rna, And Proteins

Prions are the suspected cause of a wide variety of neurodegenerative diseases in mammals. According to prevailing theory, prions are infectious particles made only of protein and found in high concentrations in the brains of infected animals. All mammals produce normal prion protein, PrPC, a transmembrane protein whose function remains unclear. 

Infectious prions, PrPRes, induce conformational changes in the existing PrPC proteins according to the following reaction:

PrPC  + PrPRes  → PrPRes + PrPRes

The PrPRes is then suspected to accumulate in the nervous tissue of infected patients and cause disease. This model of transmission generates replicated proteins, but does so bypassing the standard model of the central dogma of molecular biology. Transcription and translation apparently do not play a role in this replication process.

This theory is a major departure from previously established biological dogma. A scientist decides to test the protein-only theory of prion propagation. He establishes his experiment as follows:

Homogenized brain matter of infected rabbits is injected into the brains of healthy rabbits, as per the following table:

Rabbit 1 and 2: injected with normal saline on days 1 and 2

The above trials serve as controls.

Rabbit 3 and 4: injected with homogenized brain matter on days 1 and 2

The above trials use unmodified brain matter.

Rabbit 5 and 6: injected with irradiated homogenized brain matter on days 1 and 2

The above trials use brain matter that has been irradiated to destroy nucleic acids in the homogenate.

Rabbit 7 and 8: injected with protein-free centrifuged homogenized brain matter on days 1 and 2

The above trials use brain matter that has been centrifuged to generate a protein-free homogenate and a protein-rich homogenate based on molecular weight.

Rabbit 9 and 10: injected with boiled homogenized brain matter on days 1 and 2

The above trials use brain matter that have been boiled to destroy any bacterial contaminants in the homogenate. 

Which intermediates of the central dogma of molecular biology below are present in normal cellular replication, but apparently absent in the above model of prion replication?

I. mRNA

II. tRNA

III. Protein

Possible Answers:

I, only

I and II

II, only

I, II, and III

I and III

Correct answer:

I and II

Explanation:

The above reaction shows an actual transfer of information.  The information contaiend in the specific structure of PrPC is converted to that contained in PrPRes; in other words, PrPRes is effectively replicating.  In all other forms of cellular replication, based on the central dogma of molecular biology, replication occurs based on the following pattern:

Parental DNA → mRNA → Protein, carried by tRNA

Example Question #32 : Dna, Rna, And Proteins

Prions are the suspected cause of a wide variety of neurodegenerative diseases in mammals. According to prevailing theory, prions are infectious particles made only of protein and found in high concentrations in the brains of infected animals. All mammals produce normal prion protein, PrPC, a transmembrane protein whose function remains unclear. 

Infectious prions, PrPRes, induce conformational changes in the existing PrPC proteins according to the following reaction:

PrPC  + PrPRes  → PrPRes + PrPRes

The PrPRes is then suspected to accumulate in the nervous tissue of infected patients and cause disease. This model of transmission generates replicated proteins, but does so bypassing the standard model of the central dogma of molecular biology. Transcription and translation apparently do not play a role in this replication process.

This theory is a major departure from previously established biological dogma. A scientist decides to test the protein-only theory of prion propagation. He establishes his experiment as follows:

Homogenized brain matter of infected rabbits is injected into the brains of healthy rabbits, as per the following table:

Rabbit 1 and 2: injected with normal saline on days 1 and 2

The above trials serve as controls.

Rabbit 3 and 4: injected with homogenized brain matter on days 1 and 2

The above trials use unmodified brain matter.

Rabbit 5 and 6: injected with irradiated homogenized brain matter on days 1 and 2

The above trials use brain matter that has been irradiated to destroy nucleic acids in the homogenate.

Rabbit 7 and 8: injected with protein-free centrifuged homogenized brain matter on days 1 and 2

The above trials use brain matter that has been centrifuged to generate a protein-free homogenate and a protein-rich homogenate based on molecular weight.

Rabbit 9 and 10: injected with boiled homogenized brain matter on days 1 and 2

The above trials use brain matter that have been boiled to destroy any bacterial contaminants in the homogenate.

Which of the following findings would most strongly refute the protein-transmission hypothesis of prion propagation?

Possible Answers:

Transmission in the control and transmission in Rabbits 9 and 10

No transmission the control and transmission in Rabbits 5 and 6

Transmission in the control and transmission in Rabbits 7 and 8

No transmission in the control and transmission in Rabbits 9 and 10

Transmission in the control and transmission in Rabbits 5 and 6

Correct answer:

Transmission in the control and transmission in Rabbits 7 and 8

Explanation:

Transmission in the control and a transmission in those rabbits injected with protein free homogenate would refute the above model. A protein-free homogenate would only be able to transmit disease via nucleic acids.

Example Question #33 : Dna, Rna, And Proteins

Cryptosporidium is a genus of gastrointestinal parasite that infects the intestinal epithelium of mammals. Cryptosporidium is water-borne, and is an apicomplexan parasite. This phylum also includes Plasmodium, Babesia, and Toxoplasma. 

Apicomplexans are unique due to their apicoplast, an apical organelle that helps penetrate mammalian epithelium. In the case of cryptosporidium, there is an interaction between the surface proteins of mammalian epithelial tissue and those of the apical portion of the cryptosporidium infective stage, or oocyst. A scientist is conducting an experiment to test the hypothesis that the oocyst secretes a peptide compound that neutralizes intestinal defense cells. These defense cells are resident in the intestinal epithelium, and defend the tissue by phagocytizing the oocysts. 

She sets up the following experiment:

As the neutralizing compound was believed to be secreted by the oocyst, the scientist collected oocysts onto growth media. The oocysts were grown among intestinal epithelial cells, and then the media was collected. The media was then added to another plate where Toxoplasma gondii was growing with intestinal epithelial cells. A second plate of Toxoplasma gondii was grown with the same type of intestinal epithelium, but no oocyst-sourced media was added.

The scientist in the passage develops an effective treatment for cryptosporidium infection. The treatment exploits a difference in the rRNA structure between cryptosporidium and its human hosts. In which organelle is this difference most likely to directly manifest?

Possible Answers:

Golgi body

Ribosomes

Peroxisomes

Smooth endoplasmic reticulum

Cell membrame

Correct answer:

Ribosomes

Explanation:

Ribosomes have rRNA as a critical component, and assisting in protein synthesis is the major role for rRNA in eukaryotic cells.

Example Question #6 : Translation

The genetic code is composed of only four nucleotides. Each codon that is read by a ribosome is composed of a distinct three-nucleotide sequence. 

There are sixty-four total codons that can be made using four different nucleotides, however, there are only twenty different amino acids created by ribosomes. What is the reason for this discrepancy?

Possible Answers:

More than one codon sequence can code for the same amino acid.

Each codon will code for only one amino acid.

Ribosomes are unable to "read" certain codons, and will not translate the mRNA as a result.

Some combinations of nucleotides are not observed in nature.

Correct answer:

More than one codon sequence can code for the same amino acid.

Explanation:

The genetic code is described as degenerative, meaning that more than one codon may code for the same amino acid.

While it is true that a codon will only code for one amino acid, this does not explain why there is a difference in the number of  possible codons and amino acids. All combinations of three nucleotides are observed in nature, and ribosomes are capable of reading all possible codons.

Example Question #7 : Translation

A mutation causes the insertion of a single nucleotide into a template strand of DNA. How will the mutant protein compare to the wild type protein?

Possible Answers:

The mutant protein will be completely different from the original protein, differing in both length and amino acid sequence.

The mutant protein will differ from the original protein by one amino acid.

The ribosome will only translate the original mRNA strand. It will stop translating the mutant strand once it reaches the mutation.

Both of the proteins will be the same. No change will be witnessed.

Correct answer:

The mutant protein will be completely different from the original protein, differing in both length and amino acid sequence.

Explanation:

The insertion or deletion of one base pair to a strand of mRNA results in a frameshift mutation. Because ribosomes read mRNA by reading three nucleotides at a time, the addition of another nucleotide can completely alter the reading frame, thus the name of the mutation. The ribosome will now be reading completely different codons, and the mutant protein will look entirely different from the original protein. The shift will also affect the "stop" codon, likely leading to early termination and a shorter protein.

The alteration of only one amino acid is typically seen when one nucleotide is replaced by another nucleotide, so this would not be seen in this case.

Example Question #34 : Dna, Rna, And Proteins

Most scientists subscribe to the theory of endosymbiosis to explain the presence of mitochondria in eukaryotic cells. According to the theory of endosymbiosis, early pre-eukaryotic cells phagocytosed free living prokaryotes, but failed to digest them. As a result, these prokaryotes remained in residence in the pre-eukaryotes, and continued to generate energy. The host cells were able to use this energy to gain a selective advantage over their competitors, and eventually the energy-producing prokaryotes became mitochondria.

In many ways, mitochondria are different from other cellular organelles, and these differences puzzled scientists for many years. The theory of endosymbiosis concisely explains a number of these observations about mitochondria. Perhaps most of all, the theory explains why aerobic metabolism is entirely limited to this one organelle, while other kinds of metabolism are more distributed in the cellular cytosol.

Scientists studying endosymbiosis often support the theory by referencing the differences between mitochondria and other membrane-bound organelles, as the passage discusses. Which of the following is NOT a membrane-bound organelle?

Possible Answers:

Endosome

Rough endoplasmic reticulum

Golgi apparatus

Ribosomes

Nuclei

Correct answer:

Ribosomes

Explanation:

Ribosomes are the only answer choices not bound by a membrane. Prokaryotes do not have membrane bounded organelles, but do have ribosomes to translate their RNA into proteins.

Example Question #35 : Dna, Rna, And Proteins

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:

Untitled

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 ribosomes central to translation, as discussed in the passage?

Possible Answers:

The rRNA that makes up ribosomes is transmitted from the nucleolus, through nuclear pores, to the cytosol

Ribosomes are only found in eukaryotes

Each ribosome can harbor up to five tRNA molecules at a time

Ribosomes are the site of extensive protein modification conicident with translation

Ribosomes bind to tRNA first, followed by mRNA binding

Correct answer:

The rRNA that makes up ribosomes is transmitted from the nucleolus, through nuclear pores, to the cytosol

Explanation:

Ribosomes are the main place where rRNA is used in cells. The rRNA makes up a portion of the ribosome structure, and this rRNA is made in the nucleolus before leaving the nucleus via nuclear pores

The remaining choices are all false. Ribosomes are found in both prokaryotes and eukaryotes and can hold two tRNA molecules (though they have three binding sites). Ribosomes bind to mRNA during translation, but most protein modification occurs later in the endoplasmic reticulum.

Example Question #36 : Dna, Rna, And Proteins

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:

Untitled

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.

In which type of cell would a factor such as eIF2 be most active?

Possible Answers:

Keratinocyte

Apoptotic cell

Necrotic cell

Sperm cell

Secretory cell

Correct answer:

Secretory cell

Explanation:

We know that inhibition fo eIF2 represses traslation. The question asks for the cells where eIF2 is most active, thus where translation is likely at its higest.

Secretory cells make protein secretions, such as Goblet cells or pancreatic acinar cells. These protein secretions are the product of translation, which requires eIF2 to function, based on the passage.

Keratinocytes form the outermost layer of the dermis, sperm cells are essential to reproduction, and apoptotic and necrotic cells are in the process of cell death; none of these will put significant energy into the translation of proteins.

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