Snustad e5 + 36
Chapter 13.35
Recall that nitrous acid deaminates adenine, cytosine, and guanine (adenine hypoxanthine,
which base-pairs with cytosine; cytosine uracil, which base-pairs with adenine; and guanine
xanthine, which base-pairs with cytosine). <In other words, nitrous acid is a bidirectional
transition mutagen but does not produce transversions.>
Would you expect nitrous acid to induce any mutations that result in the substitution of
another amino acid for a glycine residue in a wild-type polypeptide (i. e., glycine another
amino acid) if the mutagenesis were carried out on a suspension of mature (nonreplicating) T4
bacteriophage? (Note: After the mutagenic treatment of the phage suspension, the nitrous
acid is removed. The treated phage are then allowed to infect E. coli cells to express any
induced mutations.)
If so, by what mechanism? If not, why not?
ANS: Yes:
Note: The X at the third position in each codon in mRNA and in each triplet of base pairs in
DNA refers to the fact that there is complete degeneracy at the third base in the glycine
codon. Any base may be present in the codon, and it will still specify glycine.
FEEDBACK: 10.6
DIFF: medium
The specific scenarios for a single mutation are:
GG
GG
AG
U!
C
= Ser
AG
A!
G
= Arg
U!
C!
A!
G
GA
U!
C
= Asp
GA
A!
G
= Glu
U!
C!
A!
G
The third scenario in the book's solution (GC → AT transition at BOTH Position 1 and 2
simultaneously) is very improbable.