In nerve cells, microtubule-associated proteins (MAPs), most notably MAP2 and MAP tau, act to stabilize microtubules. In a mouse model, a mutant is developed that vastly reduced function across all families of MAPs, leading to increased microtubule degradation. Which cellular activity would likely be most affected?
Answer(s): D
Microtubules are a component of the cytoskeleton and central to a number of cellular processes that involve movement, such as mitosis and meiosis. They provide the framework for the organization and separation of chromosomes and sister chromatids during that process. Thus (D) is the correct answer.A: The movement created by muscle cells depends on actin and myosin, rather than microtubules.B: Transcription is primarily regulated by enzymes, not microtubules.C: The Krebs cycle depends primarily on mitochondria, not microtubules.
Which of the following are steroid-based molecules?I). TestosteroneII). TriglyceridesIII). ProgesteroneIV. DNA
Answer(s): C
Common steroid-based molecules commonly seen on the MCAT include testosterone, progesterone, estrogen, cortisol, and aldosterone.A: I only, incorrect.B: I, II, and III, incorrect. Triglycerides consist of three fatty acids bound by ester linkages to a glycerol backbone.C: I and III, correct.D: I, III, and IV, incorrect. DNA is not a steroid-based molecule; DNA consists of deoxyribose, a phosphodiester linkage, and a nitrogenous base.
The amino acid histidine has a nitrogen-containing side chain. The pKa for the carboxylic acid group, imidazole side chain, and the primary amine has a value of 1.8, 6.0, and 9.0, respectively. Which of the following structures accurately reflects the predominate form of histidine at a normal physiological pH of 7.4?
This question requires the examinee to apply the Henderson-Hasselbalch equation to determine the most common structure of the amino acid histidine at a normal physiological pH. pH = pKa + log[A-/HA] To answer this question, you must figure out the predominate form of each acidic substituent group. Carboxylic acid group:7.4 = 1.8 + log[A-/HA]. The logarithm of the ratio of the conjugate base to the acid must equal 5.6. That means the concentration of the conjugate base is 5.6 orders of magnitude greater (105.6) than the concentration of the acid. Thus, the carboxylic acid will be in the conjugate base form. Imidazole side chain: 7.4 = 6.0 + log[A-/HA].The concentration of the conjugate base is 1.4 orders of magnitude greater than the concentration of the acid.Therefore, the conjugate base will be the predominate form. Primary amine group: 7.4 = 9.0 + log[A-/HA]. The log[A-/HA] is equal to -1.6, which means the concentration of the protonated form is 1.6 orders of magnitude greater than the concentration of the conjugate base. Accordingly, histidine will have its carboxylic acid and imidazole side chain in their conjugate base form while the primary amine will remain as neutral R-NH2. Thus, C is the correct answer.
Diisopropylfluorophosphate (DFP) binds to the active site of acetylcholinesterase (ACE) in the synapses of neurons. When DFP binds to ACE, the ACE enzyme is rendered permanently inactive. This makes DFP a potent toxin, with lethal amounts at less than 100 mg. The interaction between DFP and ACE can best be characterized as:
A: Competitive inhibitors bind to the active site but bind reversibly: they do not permanently disable the enzyme.By adding very high levels of the normal substrate, the effects of competitive inhibitors can be overcome.B: Noncompetitive inhibitors bind to a site other than the active siteD: Partially competitive inhibitors bind to a site other than the active site and allow the enzyme to still have some catalytic activity (typically reduced, but possible to be higher).
DNA polymerase creates new DNA by adding complimentary nucleotides to a template strand from the original double-stranded DNA. If a section of the template strand had a ratio of 3:2 of A:T bases, what is the ratio of A:T in the newly synthesized complimentary strand of DNA?
DNA polymerase matches complimentary bases to each base in the template strand. If the template strand has an A:T ratio of 3:2, then the complimentary strand must have an A:T ratio of 2:3 in order to match the complimentary bases with their correct paired nucleotide. That is, each A in the original strand will be matched with a T and vice versa.A: 3:2, incorrect. This would be true if the complimentary strand matched A:A and T:T.B: 1:1, incorrect. Complimentary strand bases are paired in the same ratio, but with T in the place of A.C: 2:3, correct.D: it cannot be determined, incorrect. A:T must be matched with T:A on the complimentary strand.
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