Chapter 16

8.  You need to know the reading frame of the possible message.

11.  With the assumption of single-base-pair substitutions, CGG can be changed to CGU, CGA, CGC, or AGG and will still encode arginine.

14.  The following list of observations argues “cancer is a genetic disease”:

  1. Certain cancers are inherited as highly penetrant simple Mendelian traits.

  2. Most carcinogenic agents are also mutagenic.

  3. Various oncogenes have been isolated from tumor viruses.

  4. A number of genes that lead to the susceptibility to particular types of cancer have been mapped, isolated, and studied.

  5. Dominant oncogenes have been isolated from tumor cells.

  6. Certain cancers are highly correlated to specific chromosomal rearrangements.

17.  The mismatched T would be corrected to C and the resulting ACG, after transcription, would be 5′ UGC 3′ and encode cysteine. Or, if the other strand were corrected, ATG would be transcribed to 5′ UAC 3′ and encode tyrosine.

24.  Many repair systems are available: direct reversal, excision repair, transcription-coupled repair, and nonhomologous end joining.

25.  Yes, it is mutagenic. It will cause CG-to-TA transitions.

35. 

  1. A lack of revertants suggests either a deletion or an inversion within the gene.

  2. To understand these data, recall that half the progeny should come from the wild-type parent.

    Prototroph A: Because 100 percent of the progeny are prototrophic, a reversion at the original mutant site may have occurred.

    Prototroph B: Half the progeny are parental prototrophs, and the remaining prototrophs, 28 percent, are the result of the new mutation. Notice that 28 percent is approximately equal to the 22 percent auxotrophs. The suggestion is that an unlinked suppressor mutation occurred, yielding independent assortment with the nic-2 mutant.

    841

    Prototroph C: There are 496 “revertant” prototrophs (the other 500 are parental prototrophs) and four auxotrophs. This suggests that a suppressor mutation occurred in a site very close to the original mutation and was infrequently separated from the original mutation by recombination [100%(4 × 2)/1000 = 0.8 m.u.].

38.  Xeroderma pigmentosum is a heterogeneous genetic disorder and is caused by mutations in any one of several genes taking part in the process of NER (nucleotide excision repair). As the discovery of yet another protein in the NHEJ pathway through research on cell line 2BN attests, this patient could have a mutation in an as yet unknown gene that encodes a protein necessary for NER.