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Tutor profile: Michael D.

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Michael D.
College senior - Biology Major
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Questions

Subject: Organic Chemistry

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Question:

A linear five-carbon ketone has a boiling point most similar to which of these choices: (A). A five-carbon carboxylic acid. (B). A five-carbon alkane. (C). A branched aldehyde with the same molecular weight. (D). An alcohol with a higher molecular weight.

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Michael D.
Answer:

Boiling point depends on the intermolecular forces holding the molecules together. Stronger intermolecular forces causes a higher boiling point. These intermolecular forces can include London forces, dipole-dipole interactions caused by a dipole moment, and hydrogen bonding. Hydrogen bonding is the strongest, then dipole-dipole interactions, and then London forces. Choice (A) is a carboxylic acid and thus has the same dipole interactions that ketones and aldehydes do with the carbonyl bond; however, carboxylic acids also contain hydrogen bonding between themselves and thus have stronger intermolecular forces and a higher boiling point than the ketone. Choice (B) is an alkane which experiences only London forces, while the ketone experiences London forces and dipole-dipole interactions. Thus, the alkane has weaker intermolecular forces and a lower boiling point than the ketone. Choice (C) is an aldehyde which shares the carbonyl functional group of the ketone with dipole-dipole interactions. The branching of the aldehyde will cause a small reduction in boiling point in the aldehyde due to a reduction in surface area (and thus less available surface for intermolecular interactions) compared to the linear ketone of the same molecular weight; however, these two will have similar boiling points due to their similar functional groups. Choice (D) is an alcohol and, like carboxylic acids, contains a functional group capable of the very strong hydrogen bonding intermolecular interactions and will have a much higher boiling point than the ketone. Therefore, choice (C) is correct as the aldehyde will have the most similar boiling point to the ketone in the question.

Subject: Biochemistry

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Question:

A certain protease cleaves peptides at the N-terminal side of amino acids with branched alkyl side chains. Which of the following peptides, written in a chain based on their one-letter abbreviations, would have the most cleavage sites. (A). DNKGSTVYQRWC (B). GHPTLDQIWCER (C). RTHCQGYNESDK (D). HQICSVGPKLNT

Inactive
Michael D.
Answer:

To answer this question, the one-letter abbreviations for amino acids must be known. Then, an understanding of the structure of each amino acid side chain is required. Leucine (L), isoleucine (I), and valine (V) are the three amino acids with branched alkyl side chains. Knowing this, the number of L, I, or V residues in each chain can be counted. Choice (A) has one V, and thus has one cleavage site for the protease. Choice (B) has one L and one I, and thus has two cleavage sites for the protease. Choice (C) has no L, I, or V residues, and thus has no cleavage sites for the protease. Choice (D) has one L, one I, and one V, and thus has three cleavage sites for the protease. Therefore, the answer is (D) as it has the most cleavage sites for the protease.

Subject: Biology

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Question:

Which of the following mRNA transcripts has the longest open reading frame? A. 5' - CCACGAUGCGUUGAGCGC - 3' B. 3' - UGAUUCUGAAAUGGUAAC - 5' C. 5' - GACGAUGUAACUUGCACU - 3' D. 3' CCGAGUCCUAUGGUAAGA - 5'

Inactive
Michael D.
Answer:

mRNA strands are translated into proteins in codons of three base pairs. They are always translated in the 5' to 3' directions by the ribosome and this translation begins with a start codon (AUG). Three different codons are called 'stop' codons (UAA, UAG, UGA) where translation of the mRNA transcript is terminated. An open reading frame is the portion of an mRNA transcript that can be translated as it starts with (AUG) and ends with one of the stop codons. Choice (A) has a total of 3 codons in its open reading frame: AUG - CGU - UGA Choice (B) need to be read from right to left to follow the 5' to 3' direction and has a total of 5 codons in its open reading frame: AUG - GUA - AAG - UCU - UAG Choice (C) has a total of 2 codons in its open reading frame: AUG - UAA Choice (D) needs to be read from right to left to follow the 5' to 3' direction and has a total of 4 codons in its open reading frame: AUG - GUA - UCC - UGA Therefore, the answer is (B) as this mRNA strands has the longest open reading frame between the answers with 5 codons.

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