Question 1

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

Accepted Answer

They found that each mutant strain could grow only when supplemented with the specific amino acid or vitamin it could not synthesize.

Question 2

What was the key finding from Vernon Ingram's analysis of sickle-cell hemoglobin?

Accepted Answer

Sickle-cell hemoglobin differs from normal hemoglobin by a single amino acid substitution.

Question 3

Why is the genetic code described as 'degenerate'?

Accepted Answer

Because multiple codons can specify the same amino acid.

Question 4

A mutation causes the insertion of a single nucleotide into a coding sequence. What is the most likely consequence?

Accepted Answer

The reading frame will shift, altering all downstream amino acids (frameshift mutation).

Question 5

Which codon acts as the start codon for translation in most organisms?

Accepted Answer

$$\mathrm{AUG}$$

Question 6

How did cell-free systems help decipher the genetic code?

Accepted Answer

They enabled the synthesis of proteins from synthetic RNAs of known sequence, revealing which codons specify which amino acids.

Question 7

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 8

Which of the following statements about the genetic code is FALSE?

Accepted Answer

It is ambiguous, with some codons specifying more than one amino acid.

Question 9

What is the effect of adding or removing three nucleotides from a coding sequence?

Accepted Answer

It adds or removes one amino acid, but the reading frame is preserved.

Question 10

Which strand of DNA is used as the template for mRNA synthesis?

Accepted Answer

The template strand

Question 11

A patient with sickle-cell anemia has a mutation in the hemoglobin gene. What is the molecular basis of this disease?

Accepted Answer

A single nucleotide substitution resulting in a valine replacing a glutamic acid

Question 12

Which of the following best describes a frameshift mutation?

Accepted Answer

A mutation that inserts or deletes nucleotides, altering the reading frame

Question 13

Why was the one gene–one enzyme hypothesis refined to the one gene–one polypeptide theory?

Accepted Answer

Because some genes encode non-enzymatic proteins, such as hemoglobin

Question 14

Which of the following codons is a stop codon?

Accepted Answer

$$\mathrm{UAA}$$

Question 15

What is the minimum number of nucleotide bases required to code for all 20 amino acids?

Accepted Answer

Three bases per codon, giving $$4^3 = 64$ combinations

Question 16

Which experimental approach allowed assignment of codons to specific amino acids using alternating copolymers?

Accepted Answer

Synthesizing RNA molecules with alternating sequences, such as UAUAUAU...

Question 17

What is the consequence of the genetic code being nonoverlapping?

Accepted Answer

Each nucleotide is read only once, and codons are read in discrete groups of three.

Question 18

In a cell-free system, adding synthetic RNA composed only of cytosine ($$\mathrm{C}) $$results in a polypeptide containing which amino acid?

Accepted Answer

Proline

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 RNA

Question 20

A mutation in a gene causes a change from $$\mathrm{GAA} to$$ $$\mathrm{GUA} in $$the mRNA. What is the effect on the encoded protein?

Accepted Answer

Glutamic acid is replaced by valine, which can cause diseases such as sickle-cell anemia.

Gene Expression II: The Genetic Code and Protein Synthesis – Study Notes

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