Question 1

The following is a short segment of an mRNA molecule. The polypeptide it codes for is also shown:
5 ' AUGGUGCUGAAG 3 ': methionine-valine-leucine-lysine
A mutation in the DNA occurs so that the fourth base (counting from the 5 ' end) in the messenger RNA now reads A rather than G. What sequence of peptides will the mRNA now code for? (You do not need a copy of the genetic code to answer the question.)
A) methionine-lysine-leucine-lysine
B) methionine-leucine-leucine-lysine
C) methionine-valine-methionine-lysine
D) methionine-methionine-leucine-lysine

Accepted Answer

The original sequence AUGGUGCUGAAG codes for methionine-valine-leucine-lysine. Changing the fourth base from G to A changes the codon from GUG (valine) to AUG (methionine), resulting in the sequence methionine-methionine-leucine-lysine.

Question 2

The genetic code is said to be "degenerate" because

Accepted Answer

The genetic code is considered degenerate because multiple codons can code for the same amino acid. This means that a single tRNA, which recognizes a specific codon through its anticodon, can be used for different codons that code for the same amino acid, reflecting the redundancy in the genetic code.

Question 3

Which statement about the genetic code is NOT correct?

Accepted Answer

The sense strand of DNA does not serve as the template for RNA synthesis; rather, the antisense (or template) strand does. The sense strand has the same sequence as the mRNA (except for thymine being replaced by uracil in RNA), and it is the antisense strand that is used as a template by RNA polymerase to synthesize mRNA.

Question 4

_____________ different genetically-encoded amino acids are found in the proteins of cells; they are distinguished by___________________ .

Accepted Answer

There are 22 genetically-encoded amino acids, including the standard 20 plus selenocysteine and pyrrolysine, and they are distinguished by the composition of their side chains, or R-groups.

Question 5

Which of the following are rare amino acids that are sometimes encoded by stop codons?

Accepted Answer

Selenocysteine and pyrrolysine are known as the 21st and 22nd amino acids, respectively, and are unique in that they are encoded directly in the genome by stop codons under specific conditions, making them rare compared to the standard 20 amino acids.

Question 6

The modified nucleotide inosine (I) found in tRNA can pair with

Accepted Answer

Inosine (I) in tRNA can pair with three different bases: uracil (U), cytosine (C), and adenine (A), due to its unique structure that allows it to form hydrogen bonds with these bases.

Question 7

The modified base 2-thiouracil found in tRNA can pair with

Accepted Answer

2-thiouracil in tRNA specifically pairs with adenine (A), which is a modification that enhances the accuracy of protein synthesis by improving the fidelity of codon-anticodon pairing.

Question 8

The "wobble hypothesis" states that

Accepted Answer

The "wobble hypothesis" suggests that the base pairing between the codon on the mRNA and the anticodon on the tRNA is less strict for the third base, allowing for some flexibility or "wobble" in the pairing, which can lead to alternative (non-Watson-Crick) base-pairing at this position.

Question 9

The frequencies with which different codons are used vary significantly between different organisms and between proteins expressed at high or low levels within the same organism. This is referred to as:

Accepted Answer

Codon bias refers to the phenomenon where certain codons are used more frequently than others in the genome of an organism or in specific genes. This variation in codon usage can affect gene expression levels and is influenced by factors such as the organism's evolutionary history and the availability of tRNAs.

Question 10

At the pH found in cells (about 7.0), what happens to the amino group on an amino acid?

Accepted Answer

At a pH of around 7.0, typical of intracellular conditions, the amino group (-NH2) of an amino acid acts as a base by accepting a proton (H+), thus becoming -NH3+ and carrying a positive charge.

Question 11

At the pH found in cells (about 7.0), what happens to the carboxyl group on an amino acid?

Accepted Answer

At a pH of about 7.0, the carboxyl group (-COOH) acts as an acid and loses a proton (H+), resulting in a negatively charged carboxylate ion (-COO-).

Question 12

Suppose you have discovered a new amino acid. Its R-group contains only carbon and hydrogen atoms. Predict the behavior of this amino acid.

Accepted Answer

Since the R-group contains only carbon and hydrogen atoms, it is nonpolar and hydrophobic, meaning it does not interact well with water.

Question 13

Amino acids are joined by a condensation reaction to form a polypeptide chain with a ___________ backbone and ___________ side chain.

Accepted Answer

Amino acids are joined by a condensation reaction to form a polypeptide chain, which has a common backbone consisting of the repeating sequence of the nitrogen-carbon(carbonyl)-carbon (N-C-C) pattern. The side chains (R groups) attached to the central carbon vary among different amino acids, giving each its unique properties.

Question 14

The peptide bond acts as a partial double bond as a result of

Accepted Answer

The peptide bond acts as a partial double bond due to resonance, where the electrons are shared between the carbonyl oxygen and the amide nitrogen, giving the bond characteristics of both single and double bonds. This resonance stabilizes the peptide bond and restricts its rotation, contributing to the protein's structure.

Question 15

The primary structure of a protein is

Accepted Answer

The primary structure of a protein is defined by the sequence of amino acids in the chain, which are linked together by peptide bonds.

Question 16

The C-terminus of a protein corresponds to

Accepted Answer

The C-terminus of a protein refers to the end of the amino acid chain where the carboxyl group (-COOH) of the last amino acid is located, hence option D is correct.

Question 17

You have just sequenced a new protein found in Tetrahymena and observe that sulfur-containing cysteine residues occur at regular intervals in the sequence. Which hypothesis is supported by this observation?

Accepted Answer

The regular occurrence of cysteine residues suggests that disulfide bridges (covalent bonds between sulfur atoms of cysteine residues) play a significant role in stabilizing the protein's tertiary structure. Disulfide bridges are a common way to stabilize the three-dimensional structure of proteins, especially those secreted outside of cells.

Question 18

Which amino acid shown below, glutamine or valine, would be more likely to be found within the membrane region of a transmembrane protein, and less likely to be found in the intracellular region?

Accepted Answer

The answer of Which amino acid shown below, glutamine or...

Question 19

You have discovered an enzyme that has three active sites. When you denature the enzyme and study its composition, you find that each active site occurs on a different polypeptide. Which of the following hypotheses does this observation support?

Accepted Answer

The presence of active sites on different polypeptides indicates that the enzyme is made up of multiple polypeptide chains, which is characteristic of proteins with quaternary structure.

Question 20

D- and L- amino acids

Accepted Answer

D- and L- amino acids are mirror images of each other, meaning they are enantiomers. This is due to their spatial arrangement around the chiral carbon, not differences in R groups, proportions in proteins, or backbone carbon atoms.

Quiz 4: Protein Structure and Folding