Chapter 20: Altering the genetic message – Summary
All of the DNA within a cell originated from other DNA via multitudes of replications, not an error-free process. Eukaryotes may have evolved meiosis to help correct serious replication errors. Any change in a cell’s genetic information is a mutation. There are three basic kinds: point mutations, transpositions, and chromosomal rearrangement. DNA may be damaged as a result of ionizing or ultraviolet radiation, or chemical mutagens. Damage also results from spontaneous mutation of nucleotide bases. Occasionally portions of a DNA strand may loop out, a transitory occurrence that usually corrects itself. Problems occur when repair enzymes act too quickly and excise the yet unreverted loop, resulting in the deletion of perhaps hundreds of nucleotides and creating a frameshift mutation. Chromosomal rearrangements result from physical changes to the chromosome that alter a gene’s location. They include translocations, which have important consequences on gene expression and inversions, which rarely alter expression. Regardless of the cause, mutation is necessary as it is the basis for evolution. Life as we know it would not exist without mutation.
Among the more damaging kinds of DNA mutation are those associated with cancer, a general disorder that causes uncontrollable cell growth. There are two broad causes for cancer, viral infections, and carcinogens. Oncogenes are associated with cancer, and are actually growth promoting genes lacking normal regulation. They may also cause changes in cellular activities, activating receptor sites under improper conditions. Induction of cancer may involve the action of two or more genes. One of the reasons that cancers generally develop in later years is that significant time is required for a number of different mutations to occur. Although many forms of cancer are yet incurable by means other than removal of damaged tissue, it is possible to lessen one’s chances for developing certain cancers by reducing exposure to known mutagens, most importantly cigarette smoke. New molecular therapies associated with cell growth and cycling are being developed to combat cancer.
While genetic change is dependent upon mutation, genetic variation results from the recombination of genetic material. Gene transfer occurs in both prokaryotes and eukaryotes, while reciprocal recombination and chromosome assortment occur only in eukaryotes. Auxiliary chromosomes called plasmids exist in bacteria and are a means of gene transfer. The material in a plasmid may exist as a small separate circle of DNA or it may be incorporated into the main chromosome. A bacterium with a special F plasmid can pass a replicate of it into another bacterium, and even pass a copy of its entire genome if the F plasmid is incorporated within the main chromosome. The positions at which plasmid genes are separated and reincorporated are strictly controlled through specific recognition sites. Transposons are genes that move randomly to other locations on a chromosome. Transposition results in mutation when a transposon inserts itself within another gene, destroying or altering its function. Gene mobilization can result in the formation of plasmids containing several genes with similar functions. Separation of alleles during meiosis shuffle entire chromosomes of information. The most common example of reciprocal recombination is crossing over. It results in the production of gametes with new allele combinations and increases the chance of producing chromosomes with a variety of mutations. Unequal crossing over resulting from duplicate sequences at different locations can produce chromosomes with too much or too little DNA. Chromosomal alteration can also be caused by gene conversion when the proofreading enzymes excise one of a mismatched pair of nucleotide sequences between homologues. Trinucleotide repeats are a form of genetic change that simply involves increasing the number of
