1. How do ultraviolet light, X rays, and gamma rays cause mutations?
Damage to DNA is caused by photons of ultraviolet light, X rays, and gamma rays (but not by photons of lower energy, such as visible light, microwaves, and radio/TV waves). The highest energy photons (X rays and gamma rays) cause the breakage of covalent bonds, leading to a variety of damaging effects on DNA. Photons of ultraviolet light get absorbed by DNA, causing a rearrangement of covalent bonds to give the production of "pyrimidine dimers" (usually involving two thymines), as shown in Figure 14.27.
DNA damage caused by UV light, X rays, or gamma rays may kill a cell unless the damage is repaired before or during the next round of DNA replication. The mutagenic effects of these forms of radiation are primarily due to errors that occur during the repair process; i.e., the repair job is good enough to keep the cell alive but may not return the DNA to exactly the same state.
2. Has "transposition" ever been a significant
cause of mutations in humans?
Our genome is loaded with "transposable elements", as shown in Table 14.2 (page 611), accounting for most of the repetitive DNA present in the genome. This suggests strongly that transposition played a major role in mutation production earlier in our evolution. Now, however, as your text states on page 612, "only about 1 in 600 new mutations in the human genome is due to transposition."
3. How can crossing-over within a gene be involved in the formation of new alleles beyond those formed directly from mutation?
Think about how a combination of mutations and crossing-over (with the cross-over occurring within the gene of interest) could lead to the creation of an allele containing multiple mutations. Consider a man whose two alleles a1 and a2 of gene alpha (on chromosome #1) are both "mutant", but where the mutation in a1 is somewhere in the first half of the gene and the mutation in a2 is somewhere in the second half of the gene. During the many millions of meiosis events that occur during sperm production each week, there might be a few instances of crossing-over occuring right in the middle of gene alpha. For a cell in which such a cross-over occurs, two of the four resulting sperm cells will have "non-parental" versions of gene alpha. For the situation as described, one of these will be the wild-type allele of alpha, while the other will be a new allele of gene alpha containing both of the original mutations. If the sperm cell carrying this "double mutant" allele fertilizes an egg, the resulting baby will be carrying a new allele of gene alpha that never existed before.