MCAT Biology : DNA, RNA, and Proteins

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #11 : Dna, Rna, And Proteins

Which base pair requires the most amount of energy to break?

Possible Answers:

A-T

G-T

T-C

G-C

T-G

Correct answer:

G-C

Explanation:

The guanine and cytosine base pairing forms 3 hydrogen bonds. Both adenine and thymine form only 2 hydrogen bonds. Thus the G-C base pair has the strongest interactions, and requires the most amount of energy to break.

Example Question #12 : Dna, Rna, And Proteins

Which of the following choices represents the structure of DNA from least organized to most organized?

Possible Answers:

Deoxyribose, nucleoside, nucleotide, DNA double helix, nucleosome, chromatin 

None of these

Deoxyribose, nucleoside, nucleotide, DNA double helix, chromatin, nucleosome

Nucleotide, deoxyribose, nucleotide, DNA double helix, chromatin, nucleosome

Deoxyribose, nucleoside, chromatin, DNA double helix, nucleotide, nucleosome

Correct answer:

Deoxyribose, nucleoside, nucleotide, DNA double helix, nucleosome, chromatin 

Explanation:

Deoxyribose is the sugar in the backbone of DNA. Next, a nitrogenous base is attached to the deoxyribose, which together, is called a nucleoside. A nucleoside with one or more phosphate groups attached is called a nucleotide. The double stranded DNA coils around histone proteins; this complex is called a nucleosome. Lastly the DNA gets further packaged and condensed into chromatin.

Example Question #1161 : Biology

The central nervous system consists of the brain and the spinal cord. In general, tracts allow for the brain to communicate up and down with the spinal cord. The commissures allow for the two hemispheres of the brain to communicate with each other. One of the most important commissures is the corpus callosum. The association fibers allow for the anterior regions of the brain to communicate with the posterior regions. One of the evolved routes from the spinal cord to the brain is via the dorsal column pathway. This route allows for fine touch, vibration, proprioception and 2 points discrimination. This pathway is much faster than the pain route. From the lower limbs, the signal ascends to the brain via a region called the gracile fasciculus. From the upper limbs, the signal ascends via the cuneate fasciculus region in the spinal cord.

The dorsal column pathway is a fast and advanced signaling system. Some researchers believe that this pathway evolved not to long ago. If this transformation required the change in the DNA, how might this have occurred? 

I. Point mutation

II. Frame shift

III. Nonsense mutation

Possible Answers:

II only

I only

I and II

I, II, and III

III only

Correct answer:

I, II, and III

Explanation:

All of these choices may cause a change in the DNA. A point mutation involves a single base pair change in the DNA or the RNA. A frameshift mutation involves a deletion or insertion, in which any number of base pairs are inserted or deleted, which causes a shift in the reading frame of the DNA (not RNA). A nonsense mutation involves the change in DNA that results in a premature stop codon in the mRNA. 

Example Question #1 : Transcription

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.

 

In chromosome 21, parts of the DNA are converted to protein, while other parts are interspersed, but do not correlate to the final protein sequence. The portions of the DNA that code for final amino acid sequence are called __________.

Possible Answers:

Okazaki fragments

introns

template strands

exons

coding strands

Correct answer:

exons

Explanation:

In the splicing model of DNA expression, certain regions of DNA are converted to proteins while intervening portions are cut out.  The portions of "junk DNA" are known as introns, while exons are the sequences actually converted to protein. Okazaki fragments may appear tempting, but actually refers to fragments of DNA synthesized during replication of the lagging strand.

Example Question #11 : Dna, Rna, And Proteins

Which of the following is not true about RNA?

Possible Answers:

RNA contains a five-carbon sugar

The sole function of RNA is translation to a protein product

RNA can be double stranded

RNA contains uracil

RNA contains adenine

Correct answer:

The sole function of RNA is translation to a protein product

Explanation:

There are a few different types of RNA, each serving different purposes. Messenger RNA (mRNA) is transcribed to a protein product, but transfer RNA (tRNA) acts as a carrier for amino acids while ribosomal RNA (rRNA) forms some structures of the ribosome. Micro RNA (miRNA) can be used to regulate transcription.

RNA can be single or double stranded, leading to both ssRNA and dsRNA viruses. The ribose sugar in RNA forms a five-carbon ring, much like deoxyribose in DNA. RNA contains both adenine and uracil, though thymine is not found in RNA.

Example Question #1 : Transcription

What enzyme is required for transcription of mRNA in eukaryotes?

Possible Answers:

RNA polymerase I

RNA polymerase II

DNA polymerase

RNA polymerase III

Correct answer:

RNA polymerase II

Explanation:

RNA polymerase II is required for transcription of mRNA, snRNA, and miRNA. Alternatively, RNA polymerase I transcribes some rRNA and RNA polymerase III transcribes tRNA, some rRNA, and other small RNAs. DNA polymerase is required for DNA replication but does not play a role in transcription.

Example Question #1 : Transcription

Which of the following statements is FALSE regarding transcription in eukaryotic cells?

Possible Answers:

RNA polymerase only transcribes one side of the DNA strand.

Transcription results in strands that contain uracil instead of thymine.

Transcription takes place in the cytoplasm.

RNA polymerase requires a sequence on DNA to show it where to bind.

Correct answer:

Transcription takes place in the cytoplasm.

Explanation:

Remember that DNA is found in the nucleus of cells. As a result, it would not make sense that transcription would take place in the cytoplasm.

RNA polymerase replaces thymine with uracil, and a specific DNA sequence (the promoter) shows RNA polymerase where to begin transcription on the template strand. RNA polymerase will only transcribe one DNA strand (the template strand) because the complementary strand would result in a different mRNA product.

Example Question #1 : Transcription

Which of the following statements is true concerning DNA replication and transcription?

Possible Answers:

Both strands act as templates in replication and transcription.

Both use DNA as the template.

Both are performed at the same rate.

Both require a primer to begin.

Correct answer:

Both use DNA as the template.

Explanation:

In both replication and transcription, DNA is the template used to create the desired product. RNA polymerase uses a promoter, and DNA polymerase uses an RNA primer, in order to determine where to begin. DNA polymerase moves much more quickly than RNA polymerase, and RNA polymerase only uses one of the two strands in order to make the desired product.

Example Question #1 : Transcription

An operon is a section of DNA that is composed of a promoter, an operator, and all of the structural genes that will be transcribed in one mRNA. The lac operon is perhaps the most famous example of how prokaryotic organisms will simultaneously transcribe all of the structural genes necessary to accomplish a certain function in the cell. In the absence of glucose, the lac operon will be transcribed so that lactose can be metabolized in the cell for energy.

Which of the following conditions would result in an increase in transcription of the structural genes found on the lac operon?

Possible Answers:

A very low amount of lactose available.

An increase in cyclic AMP.

Low CAP levels.

A sufficient amount of glucose available.

Correct answer:

An increase in cyclic AMP.

Explanation:

When glucose levels are low, there is an increase in cyclic AMP. The cAMP wil then activate CAP, which will attach upstream to the promoter of the lac operon and promote transcription.

Example Question #1 : Transcription

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 contrast to translation, transcription __________.

Possible Answers:

produces an rRNA transcript

takes place in the nucleus

takes place in the cytosol

uses tRNA, not rRNA

relies on soluble factors 

Correct answer:

takes place in the nucleus

Explanation:

Transcription takes place in the nucleus, where DNA access is ensured. DNA is then turned into an mRNA transcript, which can exit the nucleus and move to the cytosol for translation.

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