MCAT Biology : DNA and RNA Sequencing

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #343 : Cell Biology, Molecular Biology, And Genetics

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:

GTGTGATACCCC and CACACTATAAAA

CGGACATGCTGG and GTTACCGCAGGC

CACACTATAAAA and ATCGCTTTGTAC

ATCGCTTTGTAC and CGGACATGCTGG

ATCGCTTTGTAC and GTTACCGCAGGC

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 #1 : Dna And Rna Sequencing

Which piece of DNA has the lowest melting point?

Note: only one strand is shown

Possible Answers:

CCGCGAGGTCCG

TAACCTTGGACA

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 #2 : 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

DNA is translated directly by tRNA linked to amino acids

Ribosomes are important in the synthesis of RNA molecules

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

RNA provides the main storage form of genetic information

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 #34 : 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:

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

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

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

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

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 #346 : Cell Biology, Molecular Biology, And Genetics

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

Possible Answers:

5’ CGTACG 3’

5’ CGCTAT 3’

5’ TGACAT 3’

5’ TTACTA 3’

5’ TTCGAG 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.

Example Question #3 : Dna And Rna Sequencing

Which of the following options include degenerate codons?

Possible Answers:

UCA: Serine

CAC: Histidine

GGG: Glycine

UUU: Phenylalanine

ATG: Methionine (start)

CCU: Proline

UCU: Serine

UCA: Serine

AGA: Arginine

UAA: Stop

ATG: Methionine (start)

AUC: Isoleucine

Correct answer:

UCU: Serine

UCA: Serine

AGA: Arginine

Explanation:

The term "degenerate codons" refers to codons with different nucleotide base sequences that specify the same amino acid. In the provided examples, two codons (UCU and UCA) both specify serine, indicating this is the correct answer.

Example Question #351 : Cell Biology, Molecular Biology, And Genetics

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.

During translation, the genetic code is used to convert a sequence of nitrogenous bases in mRNA to an amino acid sequence. Which of the following is true of the genetic code?

I.  More than one codon sequence codes for a single amino acid

II.  The most 5' position of the codon on mRNA is the wobble position

III.  Each codon sequence only codes for one amino acid

Possible Answers:

I and II

II, only

III, only

I and III

II and III

Correct answer:

I and III

Explanation:

The genetic code is unambiguous, because each codon only codes for one amino acid. It is also degenerate, so that each amino acid can be coded for by multiple codons. Choice 2 is incorrect, as the most 3' position on the mRNA is the wobble position.

Example Question #352 : Cell Biology, Molecular Biology, And Genetics

A short polynucleotide strand with the base sequence of AUCCCUGG must be __________.

Possible Answers:

tRNA

mRNA

It cannot be determined

rRNA

Correct answer:

It cannot be determined

Explanation:

Polynucleotide sequences are nucleic acids, so they must be DNA or RNA. Any sequence containing U (uracil) must be RNA, however there is no way to determine the type of RNA simply by looking at the sequence. This sequence could code for mRNA, rRNA, or tRNA.

mRNA is used to translate proteins. rRNA plays a structural and functional role in composing ribosomes. tRNA carries amino acids to the ribosome during translation.

Example Question #1785 : Mcat Biological Sciences

Which of the following correctly arranges the bases on the anti-codon loop of a tRNA carrying tryptophan?

Tryptophan codon: 3'-UGG-5'

Possible Answers:

3'-ACC-5'

3'-CCA-5'

3'-GGU-5'

3'-CCT-5'

3'-GUU-5'

Correct answer:

3'-CCA-5'

Explanation:

Tryptophan, which is encoded on mRNA as 3'-UGG-5', is going to be transported to the ribosome via tryptophan t-RNA. The anti-codon loop must be complementary to the mRNA strand. Since the code for Tryptophan is 3'-UGG-'5, the anti-codon loop of the t-RNA must read 3'-CCA-5' in order to line up.

mRNA: 3'-UGG-'5

tRNA: 5'-ACC-3'

Example Question #1786 : Mcat Biological Sciences

The codons GGU, GGA, GGC, and GGG all code for the same amino acid, glycine. What biological term is used to describe this phenomenon?

Possible Answers:

Degeneracy

Point mutation

Codominance

Polygenetic trait

X-chromosome inactivation

Correct answer:

Degeneracy

Explanation:

Degeneracy refers to the fact that more than one codon can code for the same amino acid. These codons generally differ in their third or "wobble" base. Degeneracy explains how there can be a total of sixty-four possible codons corresponding to only twenty amino acids.

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