Cystic fibrosis results from a mutation. A mutation is a change in the nucleotide-base sequence of a gene or DNA molecule. Germ-cell mutations occur in an organism's gametes. Germ-cell mutations do not affect the organism itself, but they can be passed on to offspring. Somatic-cell (soh-MAT-ik SEL) mutations take place in an organism's body cells and can therefore affect the organism. For example, certain types of human skin cancer and leukemia result from somatic-cell mutations. Somatic-cell mutations cannot be inherited.

Lethal mutations cause death, often before birth. Some mutations, however, result in phenotypes that are beneficial to the individual. Organisms with beneficial mutations have a better chance of surviving and reproducing, and therefore have an evolutionary advantage. Mutations provide the variation upon which natural selection acts. Mutations can involve an entire chromosome or a single DNA nucleotide.

Chromosome Mutations

Chromosome mutations involve changes in the structure of a chromosome or the loss or gain of a chromosome. Three types of chromosome mutations are shown in Figure 12-6. A deletion is the loss of a piece of a chromosome due to breakage. In an inversion, a chromosomal segment breaks off, flips around backward, and reattaches. In a translocation, a piece of one chromosome breaks off and reattaches to a nonhomologous chromosome. In nondisjunction (NAHN-dis-JUNGK-shuhn), a chromosome fails to separate from its homologue during meiosis. One gamete receives an extra copy of a chromosome, and another gamete receives no copies. An example of non-disjunction that results in Down syndrome is shown in Figure 12-7.

figure 12-6

In some types of mutations, chromosomes break. In a deletion, a piece of a chromosome is lost. In an inversion, a piece flips and reattaches. In a translocation, a broken piece attaches to a nonhomologous chromosome.

Some chromosome mutations are the loss or gain of entire chromosomes. The mutation that gives a person three copies of chromosome 21 results in Down Syndrome.
figure 12-8

(a) In a substitution mutation, one nucleotide replaces another, forming a new codon that may signal the insertion of the wrong amino acid.

(b) Deleting a nucleotide causes all subsequent codons to be incorrectly read, resulting in a frameshift mutation. Adding a nucleotide shifts the codon grouping too, and causes misreading.

Gene Mutations

The substitution, addition, or removal of a single nucleotide is a point mutation, which is a change that occurs within a single gene or other segment of DNA on a chromosome. In a substitution, one nucleotide replaces another, as shown in Figure 12-8a. If this substitution occurs in a codon, the amino acid can be changed. In a deletion mutation, one or more nucleotides in a gene are lost. This loss can cause incorrect grouping of the remaining codons, called a frameshift mutation, making all amino acids downstream change. A frameshift mutation is shown in Figure 12-8b. This mutation, in turn, can have a disastrous effect on the protein's function. In insertion mutations, one or more nucleotides are added to a gene, which can also result in a frameshift mutation.

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