Microbiology, 2021

464 11 • Review Questions
67. Label the following in the figure: ribosomal E, P, and
A sites; mRNA; codons; anticodons; growing
polypeptide; incoming amino acid; direction of
translocation; small ribosomal unit; large ribosomal
unit.
68. Prior to the elucidation of the genetic code,
prominent scientists, including Francis Crick,
had predicted that each mRNA codon, coding
for one of the 20 amino acids, needed to be at
least three nucleotides long. Why is it not
possible for codons to be any shorter?
69. Below are several DNA sequences that are
mutated compared with the wild-type
sequence: 3’-T A C T G A C T G A C G A T C-5’.
Envision that each is a section of a DNA
molecule that has separated in preparation for
transcription, so you are only seeing the
template strand. Construct the complementary
DNA sequences (indicating 5’ and 3’ ends) for
each mutated DNA sequence, then transcribe
(indicating 5’ and 3’ ends) the template strands,
and translate the mRNA molecules using the
genetic code, recording the resulting amino acid
sequence (indicating the N and C termini). What
type of mutation is each?
Mutated DNA Template Strand #1: 3’-T A C T
G T C T G A C G A T C-5’
Complementary DNA sequence:
mRNA sequence transcribed from template:
Amino acid sequence of peptide:
Type of mutation:
Mutated DNA Template Strand #2: 3’-T A C G
G A C T G A C G A T C-5’
Complementary DNA sequence:
mRNA sequence transcribed from template:
Amino acid sequence of peptide:
Type of mutation:
Mutated DNA Template Strand #3: 3’-T A C T
G A C T G A C T A T C-5’
Complementary DNA sequence:
mRNA sequence transcribed from template:
Amino acid sequence of peptide:
Type of mutation:
Mutated DNA Template Strand #4: 3’-T A C G
A C T G A C T A T C-5’
Complementary DNA sequence:
mRNA sequence transcribed from template:
Amino acid sequence of peptide:
Type of mutation:
70. Why do you think the Ames test is preferable to
the use of animal models to screen chemical
compounds for mutagenicity?
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