ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Chapter Outline		AP BIOLOGY: Chapter Eighteen Review Answers 1. A mutation is a change in the genetic message, usually involving the alteration of DNA. If one or a few nucleotides are altered, it is called a point mutation. 2. Ionizing radiation causes electrons to be ejected from atoms, leaving behind ionized atoms with unpaired electrons: free radicals. These ionized atoms react violently with other molecules, DNA in particular, causing damage. 3. 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. 4. 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. 5. Mutations in germline cells can be passed on to a subsequent generation; mutations in somatic cells cannot. 6. 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. 7. 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 check points regulating cell division, however, development of cancer would require mutations in the genes for each check point, or mutation of multiple genes. 8. 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. 9. 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. 10. Insertional inactivation is the loss of activity in a gene as a result of the insertion of a mobile element (transposon) within that gene. Transposons can also contribute to the formation of composite plasmids in bacteria, bringing distantly located genes in the genome closer together. 11. 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. 12. Unequal crossing over occurs when two chromosomes exchange segments of unequal length. It usually occurs when more than one copy of a gene exists on a chromosome and the homologue lines up at the wrong one. The result is that one gamete's chromosome has hundreds of copies of a particular gene, while the other has very few, if any. 13. Satellite DNA is short nucleotide sequences that are repeated several million times. It is usually clustered around the centromere or near the ends of chromosomes. It is usually highly condensed, tightly coiled, and untranscribed. Its function is most likely structural. 14. Tandem clusters encode products that the cell requires in large amounts. This occurs as the cell rapidly transcribes all of the numerous copies simultaneously, typically rRNA molecules. Since rRNA is synthesized in the nucleolus, the nucleolus is readily visible during this time, creating visible areas called nucleolar organizer regions. 15. 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. 16. Pseudogenes are silent copies of a gene that have been inactivated by mutation. Dispersed pseudogenes might have accumulated enough fortuitous mutations to produce a "new" single copy gene, and therefore may have contributed to the evolution of the eukaryotic genome.