DNA and RNA Sequencing
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MCAT Biology › DNA and RNA Sequencing
Which of the following will be the complimentary pairing of this coded gene during transcription?
3’ GCTAGC 5’
5’ CGAUCG 3’
5’ CGATCG 3’
3’ CGATCG 5”
3’ CGAUCG 5’
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.
If a gene has a sequence of 5'-AGCTGCCTT-3', what would be the complementary mRNA sequence that leaves the nucleus to be translated?
3'-UCGACGGAA-5'
5'-AGCTGCCTT-3'
3'-TCGACGGAA-5'
5'-UCGACGGAA-3'
5'-UCGUCGGAA-3'
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'
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?
CGGACATGCTGG and GTTACCGCAGGC
ATCGCTTTGTAC and CGGACATGCTGG
ATCGCTTTGTAC and GTTACCGCAGGC
GTGTGATACCCC and CACACTATAAAA
CACACTATAAAA and ATCGCTTTGTAC
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).
The codons GGU, GGA, GGC, and GGG all code for the same amino acid, glycine. What biological term is used to describe this phenomenon?
Degeneracy
Point mutation
Codominance
Polygenetic trait
X-chromosome inactivation
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.
Which segment of DNA would have the highest melting point when paired with its complimentary strand?
5’ CGTACG 3’
5’ TGACAT 3’
5’ TTACTA 3’
5’ CGCTAT 3’
5’ TTCGAG 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.
Which of the following correctly arranges the bases on the anti-codon loop of a tRNA carrying tryptophan?
Tryptophan codon: 3'-UGG-5'
3'-CCA-5'
3'-GGU-5'
3'-CCT-5'
3'-ACC-5'
3'-GUU-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'
Which of the following options include degenerate codons?
UCU: Serine
UCA: Serine
AGA: Arginine
UCA: Serine
CAC: Histidine
GGG: Glycine
UUU: Phenylalanine
ATG: Methionine (start)
CCU: Proline
UAA: Stop
ATG: Methionine (start)
AUC: Isoleucine
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.
The base sequence of one strand of tRNA is v. What is the corresponding sequence of DNA?
5'-AGTCGATCTAGC-3'
5'-AGUCGAUCUAGC-3'
5'-TCAGCTAGATCG-3'
5'-GATCGATCGATA-3'
3'-AGUCGAUCUAGC-5'
Explanation
Both tRNA and DNA are complementary to mRNA, meaning that they will have the exact same sequence in the exact same direction with only one distinction: tRNA will use uracil where DNA uses thymine.
The given tRNA strand is 5'-AGUCGAUCUAGC-3'.
The corresponding DNA strand will be 5'-AGTCGATCTAGC-3'.
You are a student researcher cloning a gene that is around 1500 bases long into a vector for recombinant expression. Starting with cDNA, you succesfully clone and transfect bacterial cells to propogate the plasmid. You sequence the plasmid to check and make sure that the target gene has been succesfully incoorporated into the vector. After checking the sequence, you notice that there is a single nucleotide that has been switched from an A to a G several hundred basepairs after the start codon. You express the protein anyway. After subjecting the protein to SDS-page, you learn that it exists at its expected length, but in a functional asssay, the protein seems to have lost its function. What type of mutation is this called?
Missense mutation
Nonsense mutation
Frameshift mutation
Silent mutation
Explanation
Nonsense mutations result in a premature stop codon, which would have resulted in a shorter peptide. A silent mutation means that there is a change in the nucleotide sequence, but not in the amino acid sequence. Thus, this is not likely the case seeing as there is a loss of function in the cloned and recombinantly expressed protein. We have no reason to believe that a frameshift mutation occurred. Rather, what the student observed was a point mutation that resulted in a missense reading of the gene.
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:
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
I and III
I and II
II, only
II and III
III, only
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.