Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 18 (p. 388)

1. A pyrimidine dimer is a cross-link between adjacent pyrimidines of the DNA strand. It harms a cell by blocking DNA replication. To repair this damage, the cell cleaves the bond between pyrimidines or excises the entire dimer and fills in the gap using the other strand as a template. If the dimer portion is not repaired, the problem area is filled in later, but this is very prone to error.

2. A slipped mispairing is when chromosomes pair and sequences misalign, looping out a portion of one strand. Possible deletion of several hundred nucleotides from one of the DNA strands may result. Some of the deletions are frameshifts, in which the code is misread so that the protein made is useless.

3. In transfection, DNA is removed from a cancer cell and chopped up into pieces. Each piece is then inserted into a cell to see which piece is actually responsible for causing the disease. This technique has shown that genes involving regulation of cell division (mitosis) are responsible for the development of tumors.

4. Mutations in very few genes actually cause cancer, but it is the mutation in those genes that regulate cell-cycle check points that can be devastating in any kind of cancer. Since there are usually several checkpoints regulating cell division, however, development of cancer would require mutations in the genes for each checkpoint, or mutation of multiple genes.

5. Genetic recombination is the change in the chromosomal position of a gene or a fragment of a gene. The three kinds are: (1) reciprocal recombination, wherein two chromosomes trade segments; (2) gene transfer, wherein one chromosome donates a piece to another; and 3) chromosome assortment in meiosis. All occur in eukaryotes, but only reciprocal recombination also occurs in prokaryotes.

6. A plasmid is a small auxiliary chromosome. It can insert on a main chromosome where a nucleotide sequence occurs that matches one on the plasmid DNA. A transposon is a small gene fragment that can move from place to place at random. Plasmids are found in bacteria; transposons are found in bacteria and eukaryotes.

7. A mismatched pair is a region along the length of the paired chromosomes where nucleotides of one homologue are not complementary with those of the other. If during replication enzymes detect that this has occurred, they excise one of the mismatched regions and correct it to match the other. This is called gene conversion.

8. The genes in multigene families are those that are groups of related but distinctly different genes. These families are thought to have evolved from a single ancestral gene that altered and multiplied through a series of unequal crossing overs.

9. Pseudogenes are silent copies of genes inactivated by mutation. Reactivated pseudogenes are believed to become single copy genes which, in time, will be duplicated and become "new genes" which could impact evolution.