20.1 Recombination alters gene location.
Gene Transfer
• Some genes move because they are part of plasmids that transfer copies of bacterial genes from one bacterium to another, while others move within transposons that jump from one genomic position to another at random. (pp. 406-408)
Reciprocal Recombination
• Unequal crossing over occurs when homologues exchange segments of unequal length. One chromosome gains extra copies of the multicopy sequences, while the other chromosome loses them. (p. 409)
• Gene conversion occurs as error correction systems in a cell convert one of the mismatched sequences to resemble the other. (p. 409)
20.2 Mutations are changes in the genetic message.
Kinds of Mutation
• Mutations are changes in the hereditary message of an organism, and germ-line mutations provide raw material for evolutionary change. (p. 410)
• DNA mutational alterations arise in at least five ways: base substitution, chemical modification, DNA breaks, slipped mispairing, and triplet expansion. (p. 410)
• Some chromosomal alterations, such as duplications, deletions, aneuploidy, and polyploidy, can change the number of gene copies an individual possesses. (p. 411)
DNA Repair
• Cells can repair many potential genetic mutations by recognizing pairing mismatches created by a base substitution or by recognizing inappropriate bases. (p. 412)
20.3 Most cancer results from mutation of growth-regulating genes.
What Is Cancer?
• Cancer occurs when a cell begins to grow in an uncontrolled and invasive way. It can result in a cluster of cells (tumor) that might metastasize and form new tumors at distant sites. (p. 413).
• Cancer is a gene disorder of somatic tissue, in which damaged genes fail to properly control cell proliferation. (p. 413)
Causes of Cancer
• Agents thought to cause cancer are called carcinogens; for example, some chemicals are carcinogens. (p. 414)
• Chemicals in cigarette smoke, such as tar, have clearly been demonstrated to cause cancer. (p. 415)
Cancer and the Cell Cycle
• Oncogenes are genes that, when introduced, cause a normal cell to become cancerous. (p. 416)
• Proto-oncogenes are normal forms of genes that can be mutated to produce a tumor-forming oncogene. (p. 416)
• Chromosomal abnormalities can lead to the activation of proto-oncogenes. (p. 416)
• Cell division is normally turned off in healthy cells by tumor-suppressor genes. (pp. 416-417)
• Cells control proliferation at several checkpoints, and all of these controls must be inactivated for cancer to be initiated. Therefore, the induction of cancer typically involves the mutation of four to six genes. (p. 419)
Smoking and Cancer
• About one-third of all cancer cases in the United States are directly attributable to cigarette smoking. (p. 420)
• Introducing mutagens into the lungs causes damage to the genes of the epithelial cells that line the lungs and are directly exposed to the chemicals. (p. 420)
• The destruction of p53 in lung epithelial cells hastens the onset of lung cancer; p53 is mutated to an inactive form in over 70% of lung cancers. (p. 421)
Curing Cancer
• Cancer therapies are being developed on many fronts, which can be consolidated into two broad categories: preventing the start of cancer and preventing the spread of cancer. (pp. 422-423)
20.4 Reproductive cloning of animals, once thought impossible, isn't.
The Challenge of Cloning
• Cloning differentiated mammalian tissue is possible, but most attempts have failed due to improper gene reprogramming. (pp. 424-425)
20.5 Therapeutic cloning is a promising but controversial possibility.
Stem Cells
• Each embryonic stem cell is capable, by itself, of developing into a healthy individual. (p. 426)
• Because embryonic stem cells can develop into any tissue, they offer the possibility of replacing damaged or lost tissues. (p. 426)
• Stem cell research is very controversial because embryonic stem cells are typically isolated from embryos discarded by reproductive clinics. (p. 426)
• Tissue-specific stem cells may allow the replacement of damaged or lost tissues while avoiding the ethical considerations involved in using embryonic tissues. (p. 427)
Therapeutic Cloning
• Therapeutic cloning involves initiating blastocyst development from a patient's tissue, and then using these embryonic stem cells to replace the patient's damaged or lost tissue. (pp. 428-429)
Grappling with the Ethics of Stem Cell Research
• One of the most controversial issues involving embryonic stem cell research is deciding when human life begins. (p. 430)