MCAT Biology : DNA, RNA, and Proteins

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #41 : Dna, Rna, And Proteins

Which of the following statements about translation is incorrect?

Possible Answers:

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

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

The mRNA first attaches to the small ribosomal subunit

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

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 #42 : Dna, Rna, And Proteins

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:

III, only

II, only

I, II, and III

I and III

II 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 #11 : 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

Cells undergo apoptosis

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

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

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 #11 : 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

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

Translation relies on membrane-bound ribosomes

Translation converts rRNA into protein

rRNA is an important component of the translation machinery

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 #1221 : Mcat Biological Sciences

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

Possible Answers:

smooth endoplasmic reticulum

nucleolus

rough endoplasmic reticulum

cytoplasmic ribosomes

Golgi apparatus

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.

Example Question #46 : Dna, Rna, And Proteins

An important part of creating DNA primers when performing a PCR or other quantitative analysis is the melting point of the primer.  Which set of primers would most likely work well together as the forward and reverse primers of a PCR?

Possible Answers:

CGGACATGCTGG and GTTACCGCAGGC

CACACTATAAAA and ATCGCTTTGTAC

GTGTGATACCCC and CACACTATAAAA

ATCGCTTTGTAC and GTTACCGCAGGC

ATCGCTTTGTAC and CGGACATGCTGG

Correct answer:

CGGACATGCTGG and GTTACCGCAGGC

Explanation:

The melting point of a strand of DNA can be predicted by the bases that make it up.  Cytosine and guanine have three hydrogen bonds to each other, so they bond more strongly than adenine and thymine's two hydrogen bonds.  This means that strands containing the same amount of Cs and Gs would work best together.  There is only answer choice in which both strands have the same amount of Cs and Gs (or Ts and As).

Example Question #1391 : Biology

Which piece of DNA has the lowest melting point?

Note: only one strand is shown

Possible Answers:

TAACCTTGGACA

CCGCGAGGTCCG

CCGGATGGTCCT

TTACGTGGCCGC

CGTAGGTATTAT

Correct answer:

CGTAGGTATTAT

Explanation:

Cytosine and guanine bond more strongly to each other than adenine and guanine because they have three hydrogen bonds as opposed to two.  Therefore, a piece of DNA with a high concentration of Ts and As will have a low melting point.  The correct choice has 8 Ts and As, while the rest have less than that.

Example Question #1 : Dna And Rna Sequencing

Human chromosomes are divided into two arms, a long q arm and a short p arm.  A karyotype is the organization of a human cell’s total genetic complement.  A typical karyotype is generated by ordering chromosome 1 to chromosome 23 in order of decreasing size. 

When viewing a karyotype, it can often become apparent that changes in chromosome number, arrangement, or structure are present.  Among the most common genetic changes are Robertsonian translocations, involving transposition of chromosomal material between long arms of certain chromosomes to form one derivative chromosome.  Chromosomes 14 and 21, for example, often undergo a Robertsonian translocation, as below.

1

A karyotype of this individual for chromosomes 14 and 21 would thus appear as follows:

Pic2

Though an individual with aberrations such as a Robertsonian translocation may be phenotypically normal, they can generate gametes through meiosis that have atypical organizations of chromosomes, resulting in recurrent fetal abnormalities or miscarriages.

 

The principal chemical component of chromosomes is nucleic acid, though proteins are also important elements. Which of the following is true of nucleic acids?

Possible Answers:

DNA contains uracil residues, while RNA contains thymine

Guanine-cytosine rich regions have higher melting points than adenine-thymine rich regions

Ribosomes are important in the synthesis of RNA molecules

RNA provides the main storage form of genetic information

DNA is translated directly by tRNA linked to amino acids

Correct answer:

Guanine-cytosine rich regions have higher melting points than adenine-thymine rich regions

Explanation:

Guanine-cytosine pairing forms three hydrogen bonds, instead of the two bonds formed by adenine and thymine. The other choices are all tempting, but subtly wrong. RNA contains uracil, DNA is the main storage form for information, mRNA is directly translated by tRNA, and ribosomes are important in the synthesis of proteins. It is worth noting that the 2' hydroxyl group of RNA's pentose sugar backbone is lost in DNA, which increases the stability and allows DNA to serve as a stable storage medium.

Example Question #32 : Cell Biology

Pick the reason that is least likely to explain why two purines will never be seen attached to each other in a DNA helix.

Possible Answers:

The bulky two-ring structure of purines would cause too much hindrance in the inside of the helix.

Purine bases will never be found on opposite DNA strands, so they do not have the ability to pair with one another.

The functional groups at the end of one purine would not correctly match with the other purine. 

Two purines could cause a bump in the DNA, causing problems with transcription and replication.

Correct answer:

Purine bases will never be found on opposite DNA strands, so they do not have the ability to pair with one another.

Explanation:

DNA strands are composed of millions of nucleotides. As a result, it would be virtually impossible to find a single strand that did not have all four nucleotides.

Nucleotides combine in purine-pyrimidine pairs due to the sterically appropriate fit of the bases, as well as the preferred combination of hydrogen bonds between the two nucleotides. As a result, two purines would not be seen combined. This is due to both being too large when together, and the incorrect hydrigen bonding between their functional groups.

Example Question #1 : Dna And Rna Sequencing

Which segment of DNA would have the highest melting point when paired with its complimentary strand?

Possible Answers:

5’ TTACTA 3’

5’ TTCGAG 3’

5’ CGCTAT 3’

5’ TGACAT 3’

5’ CGTACG 3’

Correct answer:

5’ CGTACG 3’

Explanation:

DNA nucleotide base pairs are held together by hydrogen bonding. Cytosine and guanine are held together by three hydrogen bonds, where adenine and thymine are held together by only two. Increased hydrogen bonding within a strand of DNA will increase the melting point. The DNA segment with the most guanine-cytosine base pairs will have the highest melting point.

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