Q: Which of the following is the correct formula for calculating the length of a DNA molecule given the number of base pairs?

$$\text{Length (nm)} = \text{Number of base pairs} \times 0.34$$

Question 1

A bacterium has a protective envelope 10 nm thick. If the bacterium is a cylinder with a diameter of 0.8 μm and a length of 2.0 μm, what percentage of the total volume does the envelope occupy? (Assume the envelope is a uniform shell around the cylinder.)

Accepted Answer

About 14%

Question 2

E. coli contains about 15,000 ribosomes, each with a diameter of 18 nm. If the cell is a cylinder with a diameter of 0.8 μm and a length of 2.0 μm, what percentage of the cell volume is occupied by ribosomes? (Assume ribosomes are spheres.)

Accepted Answer

About 7%

Question 3

The molecular weight of an E. coli DNA molecule is about $$3.1 	imes 10^9$ g/mol. The average molecular weight of a nucleotide pair is 660 g/mol, and each nucleotide pair contributes 0.34 nm to the length of DNA. What is the approximate length of the E. coli DNA molecule?

Accepted Answer

$$1.6 mm$$

Question 4

Aspirin is a weak acid with a $$pK_a of 3.5. $$The pH of the stomach is about 1.5, and the pH of the small intestine is about 6.0. Based on this information, where is aspirin more likely to be absorbed into the bloodstream?

Accepted Answer

In the stomach, because it is more non-ionized there

Question 5

Phosphoric acid ($$H_3PO_4) $$has three $$pK_a$$ values: 2.14, 6.86, and 12.4. What is the molar ratio of $$HPO_4$$^{2-}$$ to H_2PO$$_$$4^{-}$$ required to produce a pH of 7.0?

Accepted Answer

1.38:1

Question 6

If a person hyperventilates before a short-distance run, what is the expected effect on blood pH and why?

Accepted Answer

Blood pH increases due to decreased CO$$_2$$ and reduced carbonic acid

Question 7

During intense exercise, muscles produce lactic acid ($$CH_3CH(OH)COOH). $$Given its $$K_a = 1.38 	imes 10$$^{-4}$$, what is the effect of lactic acid accumulation on blood pH?

Accepted Answer

Blood pH decreases due to increased H$$^+$$ from lactic acid dissociation

Question 8

A protein has a molecular mass of 400 kDa by size-exclusion chromatography. SDS-PAGE shows three bands at 180, 160, and 60 kDa. When run with SDS and dithiothreitol, the bands are 160, 90, and 60 kDa. How many subunits does the protein have, and what are their masses?

Accepted Answer

Three subunits: 160, 90, and 60 kDa

Question 9

A column is filled with a cation-exchange resin at pH 7.0. Which peptide will elute last: Peptide A (Ala 30%, Asp 10%, Lys 10%, Ser 15%, Pro 25%, Cys 10%), Peptide B (Ile 25%, Asp 20%, Arg 5%, Tyr 15%, His 5%, Thr 30%), or Peptide C (Ala 40%, Glu 5%, Arg 20%, Ser 5%, His 5%, Trp 25%)?

Accepted Answer

Peptide C

Question 10

Given the following purification steps for an enzyme: crude extract (10,000 mg, 65,000 units), precipitation (salt) (5,000 mg, 65,000 units), precipitation (pH) (4,000 mg, 65,000 units), ion-exchange chromatography (70 mg, 40,000 units), affinity chromatography (12 mg, 42,000 units), and size-exclusion chromatography (8 mg, 40,000 units), which step is least effective in increasing purity?

Accepted Answer

Precipitation (salt)

Question 11

In the Hsp90 chaperone signature sequence YSNKEIFLRE, which amino acid residue is invariant (conserved across all species)?

Accepted Answer

F (Phenylalanine)

Question 12

Which of the following best explains why aspirin is absorbed more efficiently from the stomach than from the small intestine?

Accepted Answer

Aspirin is more non-ionized in the stomach, allowing it to cross membranes more easily

Question 13

Which of the following would migrate most slowly during anion-exchange chromatography?

Accepted Answer

A peptide with a high net negative charge

Question 14

A protein sample contains 682 amino acid residues in a single polypeptide chain. What is the approximate molecular weight of this protein?

Accepted Answer

$$68,200 Da$$

Question 15

If the partial pressure of CO$$_2$$ ($$pCO_2) in $$the lungs increases, what is the effect on blood pH?

Accepted Answer

Blood pH decreases

Question 16

Which of the following best describes the effect of hyperventilation on blood CO$$_2$$ and pH?

Accepted Answer

CO$$_2$$ decreases, pH increases

Question 17

In a cation-exchange chromatography at pH 7.0, which peptide would bind most strongly to the resin?

Accepted Answer

A peptide with a high net positive charge

Question 18

Which of the following is the correct formula for calculating the length of a DNA molecule given the number of base pairs?

Accepted Answer

$$	ext{Length (nm)} = 	ext{Number of base pairs} 	imes 0.34$$

Question 19

Which of the following best explains why the E. coli DNA molecule must be highly compacted to fit inside the cell?

Accepted Answer

The DNA molecule is much longer than the cell itself

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