Chapter Summary
15.1 Mutations Are Heritable Changes in DNA
Mutations are stable, heritable changes in DNA. Somatic mutations are passed on to daughter cells, but only germ line mutations are passed on to sexually produced offspring. Review Focus: Key Figure 15.1, Activity 15.1, Activity 15.2
Point mutations result from alterations in single base pairs of DNA. Silent mutations can occur in noncoding DNA or in coding regions of genes and do not affect the amino acid sequences of proteins. Missense, nonsense, and frame-
shift mutations all cause changes in protein sequences. Review Figure 15.2 Chromosomal mutations (deletions, duplications, inversions, and translocations) involve large regions of chromosomes. Review Figure 15.3
Spontaneous mutations occur because of instabilities in DNA or chromosomes. Induced mutations occur when a mutagen damages DNA. Review Figure 15.4
Mutations can occur in “hot spots” where cytosine has been methylated to 5-
methylcytosine. Review Figure 15.5 Mutations, although often detrimental to an individual organism, are the raw material of evolution.
15.2 Mutations in Humans Can Lead to Diseases
Abnormalities in proteins have been implicated in genetic diseases.
While a single amino acid difference can be the cause of disease, amino acid variations have been detected in many functional proteins. Review Figures 15.6, 15.7
Point mutations, deletions, and chromosome abnormalities are associated with genetic diseases. Review Figure 15.8
The effects of fragile-
X syndrome worsen with each generation. This pattern is the result of an expanding triplet repeat. Review Figure 15.9 A series of genetic mutations can lead to colon cancer. Review Figure 15.10
Multifactorial diseases are caused by the interactions of many genes and proteins with the environment. They are much more common than diseases caused by mutations in a single gene.
15.3 Mutations Can Be Detected and Analyzed
Restriction enzymes, which are made by microorganisms as a defense against viruses, bind to and cut DNA at specific recognition sequences (also called restriction sites), producing smaller fragments of DNA. This cutting process is known as restriction digestion. Restriction enzymes can be used in the laboratory to produce small fragments of DNA for study. Review Figure 15.11
DNA fragments can be separated by size using gel electrophoresis. Review Figure 15.12, Animation 15.1
DNA fingerprinting is used to distinguish among specific individuals or to reveal which individuals are most closely related to one another. It involves the detection of DNA polymorphisms, including single nucleotide polymorphisms (SNPs) and short tandem repeats (STRs). Review Figure 15.13, Activity 15.3
It is possible to isolate both the mutant genes and the abnormal proteins responsible for human diseases. Review Figure 15.14, Investigating Life: How Was the BRCA1 Gene Identified?
15.4 Genetic Screening Is Used to Detect Diseases
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Genetic screening is used to detect human genetic diseases, alleles predisposing people to those diseases, or carriers of those disease alleles.
Genetic screening can be done by looking for abnormal protein expression. Review Figure 15.15
DNA testing is the direct identification of mutant alleles. Any cell can be tested at any time in the life cycle. Review Figure 15.16, Animation 15.2
15.5 Genetic Diseases Can Be Treated
There are three ways to modify the phenotype of a genetic disease: restrict the substrate of a deficient enzyme, inhibit a harmful metabolic reaction, or supply a missing protein. Review Figure 15.17
Cancer sometimes can be treated with metabolic inhibitors.
In gene therapy, a mutant gene is replaced with a normal gene. Both ex vivo and in vivo therapies are being developed. Review Figure 15.18
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