Question 1

Which experimental evidence led Beadle and Tatum to propose the one gene–one enzyme hypothesis?

Accepted Answer

They showed that specific gene mutations prevented the synthesis of particular enzymes, and growth was restored only when the missing metabolic product was supplemented.

Question 2

Which of the following best describes the refined 'one gene–one polypeptide' theory?

Accepted Answer

Each gene determines the amino acid sequence of a single polypeptide chain.

Question 3

A frameshift mutation is most likely to occur when:

Accepted Answer

A single nucleotide is inserted or deleted, causing the reading frame to shift.

Question 4

Why is a doublet genetic code insufficient to encode all 20 amino acids?

Accepted Answer

A doublet code would only allow for $$4^2 = 16$ possible combinations, which is less than the number of amino acids.

Question 5

Which statement about the genetic code is correct?

Accepted Answer

It is degenerate, meaning multiple codons can specify the same amino acid.

Question 6

In the sickle-cell anemia case studied by Pauling and Ingram, what was the molecular basis for the disease?

Accepted Answer

A substitution of valine for glutamic acid in the hemoglobin polypeptide.

Question 7

Which codon acts as the start codon in most organisms?

Accepted Answer

AUG

Question 8

If a DNA coding strand has the sequence $$5'	ext{ATG GGC TCC}\ 3'$$, what is the corresponding mRNA sequence?

Accepted Answer

$$5'	ext{AUG GGC UCC}\ 3'$$

Question 9

What is the effect of inserting three nucleotides into a coding region of a gene?

Accepted Answer

It adds one extra amino acid to the polypeptide, but the reading frame is preserved.

Question 10

Which experimental approach did Nirenberg and Matthaei use to decipher the genetic code?

Accepted Answer

They used cell-free systems with synthetic RNAs of known sequence to determine which codons specify which amino acids.

Question 11

Which of the following is true about the template and coding strands during mRNA synthesis?

Accepted Answer

The template strand is used to synthesize mRNA, which is complementary to it and similar to the coding strand (except U replaces T).

Question 12

Which of the following best explains why the genetic code is considered 'nearly universal'?

Accepted Answer

Most organisms use the same genetic code, but exceptions exist in mitochondria and some bacteria.

Question 13

A mutation changes the codon $$GAA to$$ $$GAG in a $$gene. Both codons specify glutamic acid. What type of mutation is this?

Accepted Answer

Silent mutation

Question 14

Which property of the genetic code allows for some mutations to have no effect on the amino acid sequence of a protein?

Accepted Answer

Degeneracy

Question 15

What is the consequence of a frameshift mutation caused by the insertion of a single nucleotide in a nonoverlapping genetic code?

Accepted Answer

All codons downstream of the mutation are read incorrectly, resulting in a completely altered polypeptide sequence.

Question 16

Which of the following statements about codons is correct?

Accepted Answer

Codons are triplets of nucleotides written in the $$5' \rightarrow 3'$$ direction.

Question 17

Which of the following is NOT a stop codon?

Accepted Answer

AUG

Question 18

How did the use of copolymers by H. Gobind Khorana help assign codons to specific amino acids?

Accepted Answer

Copolymers allowed the synthesis of RNA molecules with alternating sequences, narrowing down which codons specified which amino acids.

Question 19

Which of the following best defines a gene according to modern understanding?

Accepted Answer

A functional unit of DNA that encodes one or more polypeptides or functional RNAs.

Gene Expression II: The Genetic Code and Protein Synthesis

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