MCAT Biology : DNA, RNA, and Proteins

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #2 : Dna And Rna Sequencing

Which of the following options include degenerate codons?

Possible Answers:

UCA: Serine

CAC: Histidine

GGG: Glycine

UAA: Stop

ATG: Methionine (start)

AUC: Isoleucine

UCU: Serine

UCA: Serine

AGA: Arginine

UUU: Phenylalanine

ATG: Methionine (start)

CCU: Proline

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 #2 : Dna And Rna Sequencing

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

III, only

II, only

II and III

I 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 #2 : Dna And Rna Sequencing

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

Possible Answers:

It cannot be determined

mRNA

tRNA

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 #1392 : Biology

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'-GUU-5'

3'-ACC-5'

3'-CCA-5'

3'-GGU-5'

3'-CCT-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 #1392 : Biology

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

X-chromosome inactivation

Polygenetic trait

Codominance

Point mutation

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.

Example Question #11 : Dna And Rna Sequencing

A sequence of DNA is written below.

5'-AGCCGTAAG-3'

 

What will be the squence of the strand created from this one by DNA polymerase during replication?

Possible Answers:

5'-AGCCGTAAG-3'

5'-GAATGCCGA-3'

5'-CTTACGGCT-3'

5'-TCGGCATTC-3'

Correct answer:

5'-CTTACGGCT-3'

Explanation:

Remember that DNA polymerase builds the new strand by reading the original DNA strand in a 3' to 5' direction. The newly formed strand will be read 5' to 3' and will have nucleotides that are complementary to the original strand.

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

Which of the following will be the complimentary pairing of this coded gene during transcription?

3’ GCTAGC 5’

Possible Answers:

5’ CGAUCG 3’

3’ CGAUCG 5’

3’ CGATCG 5”

5’ CGATCG 3’

Correct answer:

5’ CGAUCG 3’

Explanation:

During transcription, a complimentary RNA strand is constructed from a DNA template. It is important to realize that in RNA, the nucleotide base uracil is used instead of thymine as a complementary pair with adenine. The complementary strand is constructed 5’ to 3’ , but is lined up anti-parallel to the original strand.

Example Question #51 : Dna, Rna, And Proteins

Choose the transcript created if RNA polymerase transcribes the following template strand.

5'-ACTTGCAGGCC-3' 

Possible Answers:

5'-TGAACGTCCGG-3'

5'-GGCCTGCAAGT-3'

5'-GGCCUGCAAGU-3'

5'-UGAACGUCCGG-3'

Correct answer:

5'-GGCCUGCAAGU-3'

Explanation:

When transcribing from a template strand, here are a few things to remember:

1. RNA polymerase reads the strand in the 3' to 5' direction.

2. In the RNA transcript, thymine is replaced with uracil.

In order to double check, make sure that the two strands are complementary when antiparallel to one another.

Template: 5'-ACTTGCAGGCC-3'

Transcript: 3'-UGAACGUCCGG-5'

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

What is the mRNA transcript of the given DNA sequence?

3' ACCTGTTAC 5'

Possible Answers:

5' TGGACAATG 3'

3' TGGACAATG 5'

3' UGGACAAUG 5'

5' UGGACAAUG 3'

Correct answer:

5' UGGACAAUG 3'

Explanation:

mRNA is transcribed antiparallel to the DNA template, meaning that the 5' end of the mRNA sequence should align with the 3' end of the DNA sequence. Pairing rules dictate that cytosine will pair with guanine and adenine with uracil, rather than thymine. Using these conventions, we can decipher the mRNA sequence that correlates to the given DNA template.

DNA: 3' ACCTGTTAC 5'

RNA: 5' UGGACAAUG 3'

Example Question #12 : Dna And Rna Sequencing

If a gene has a sequence of 5'-AGCTGCCTT-3', what would be the complementary mRNA sequence that leaves the nucleus to be translated?

Possible Answers:

5'-AGCTGCCTT-3'

3'-UCGACGGAA-5'

3'-TCGACGGAA-5'

5'-UCGACGGAA-3'

5'-UCGUCGGAA-3'

Correct answer:

3'-UCGACGGAA-5'

Explanation:

The correct answer is 3'-UCGACGGAA-5'.

In order to arrive at this answer, it is important to note that we are starting with DNA and finding the complementary mRNA. We must remember that there is no thymine in RNA; instead of thymine, RNA has uracil. The last thing to remember is that the mRNA strand will be anti-parallel, meaning that the 5' end of the DNA sequence must match up with the 3' end of the RNA sequence. Cytosine and guanine will form pairs. Adenine bases in DNA will pair to uracil bases in RNA, and thymine bases in DNA will bind to adenine in RNA.

DNA: 5'-AGCTGCCTT-3'

RNA: 3'-UCGACGGAA-5'

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