Overview
1. Many of the gene products involved in DNA synthesis were ini- tially defined by studying mutant E. coli strains that could not syn- thesize DNA.
(a) The dnaE gene encodes the a subunit of DNA polymerase III.
What effect is expected from a mutation in this gene? How could the mutant strain be maintained?
(b) The dnaQ gene encodes the subunit of DNA polymerase. What effect is expected from a mutation in this gene?
2. Why might we predict that the organization of eukaryotic genetic material will be more complex than that of viruses or bacteria?
3. Assuming the genetic code is a triplet, what effect would the addi- tion or loss of two nucleotides have on the reading frame? The addition or loss of three, six, or nine nucleotides?
4. In studies of the amino acid sequence of wild-type and mutant forms of tryptophan synthetase in E. coli, the following changes have been observed: Arg
Thr
Gly
Ser
Ile
Glu
Val
Ala
Determine a set of triplet codes in which only a single-nucleotide change produces each amino acid change.
5. The synthesis of flower pigments is known to be dependent on enzymatically controlled biosynthetic pathways. For the crosses shown here, postulate the role of mutant genes and their products in producing the observed phenotypes:
(a) P₁: white strain Ax white strain B
F₁: all purple
F2: 9/16 purple: 7/16 white
(b) P₁: white x pink
F₁: all purple
F2: 9/16 purple: 3/16 pink: 4/16 white
6. Several amino acid substitutions in the and ẞ chains of human hemoglobin are shown in the following table.
Hb Type
Hb Toronto
Substituted Amino Acid
HbJ Oxford
Hb Mexico
Hb Bethesda
Hb Sydney
Normal Amino Acid
Ala
Gly
Gln
Tyr
HbM Saskatoon
Val
His
Asp (a-5)
Asp (a-15)
Glu (a-54)
His (β-145)
Ala (β-67)
Tyr (β-63)
Using the code table (Figure 13.7), determine how many of them can occur as a result of a single-nucleotide change
7. Why are frameshift mutations likely to be more detrimental than point mutations, in which a single pyrimidine or purine has been substituted?