Question 1

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

Accepted Answer

They found that mutants unable to grow on minimal medium could survive when supplemented with specific amino acids or vitamins.

Question 2

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

Accepted Answer

Sickle-cell hemoglobin contains a valine instead of a glutamic acid, resulting in a neutral amino acid replacing a negatively charged one.

Question 3

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

Accepted Answer

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

Question 4

Which statement best describes a frameshift mutation?

Accepted Answer

A mutation that inserts or deletes one or more base pairs, causing the reading frame to shift and altering all downstream codons.

Question 5

What evidence did Crick and Brenner provide for the triplet nature of the genetic code?

Accepted Answer

Frameshift mutations that added or removed three nucleotides restored the original reading frame, indicating codons are read in groups of three.

Question 6

Which property of the genetic code allows multiple codons to specify the same amino acid?

Accepted Answer

Degeneracy

Question 7

In mRNA synthesis, which DNA strand is copied and what is it called?

Accepted Answer

The template strand is copied to produce mRNA.

Question 8

Which codon acts as the start codon in most organisms?

Accepted Answer

AUG

Question 9

How did Nirenberg and Matthaei use cell-free systems to decipher the genetic code?

Accepted Answer

They added synthetic RNAs of known sequence to cell-free extracts and observed which amino acids were incorporated into polypeptides.

Question 10

If a mutation changes the DNA coding strand from 5'-ATG GGC TCC-3' to 5'-ATG GGC TGC-3', what is the effect on the encoded protein?

Accepted Answer

The third codon changes from Serine (TCC) to Cysteine (TGC), resulting in an amino acid substitution.

Question 11

Which of the following best explains why the genetic code is described as nonoverlapping?

Accepted Answer

Each nucleotide is part of only one codon, and the reading frame advances three nucleotides at a time.

Question 12

A mutation in the template strand of DNA changes a codon from 3'-TAC-5' to 3'-TAT-5'. What is the corresponding change in the mRNA and the resulting amino acid?

Accepted Answer

mRNA changes from 5'-AUG-3' (Met) to 5'-AUA-3' (Ile), resulting in a methionine to isoleucine substitution.

Question 13

Which statement about the universality of the genetic code is correct?

Accepted Answer

The genetic code is nearly universal, with only a few exceptions such as in mitochondria and some bacteria.

Question 14

What is the consequence of a frameshift mutation caused by the insertion of a single nucleotide in a coding region?

Accepted Answer

It alters the reading frame, changing all downstream amino acids and often resulting in a nonfunctional protein.

Question 15

Which experimental approach allowed H. Gobind Khorana to assign codons to specific amino acids?

Accepted Answer

He synthesized RNA molecules with alternating sequences, allowing assignment of codons to amino acids based on the resulting polypeptide sequence.

Question 16

Which of the following is true about the relationship between codons and amino acids?

Accepted Answer

Each codon specifies only one amino acid, but some amino acids are specified by more than one codon.

Question 17

A mutation changes the mRNA codon from UUU to UUC. What is the effect on the protein sequence?

Accepted Answer

No effect; both UUU and UUC code for phenylalanine due to the degeneracy of the genetic code.

Question 18

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.

Question 19

Which of the following statements about alternative splicing is correct?

Accepted Answer

Alternative splicing allows coding sequences to be read in various combinations, producing unique polypeptide chains from a single gene.

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

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