Gene Amplification
Some genes specify the production of proteins, while others specify the production of RNA. In either case, It sometimes occurs that one copy of a gene, even being transcribed as quickly as the RNA polymerase molecules can work, cannot produce enough of its product to satisfy the needs of the cell. Due to the process of gene amplification, certain genes exist in more than one copy within a single cell. Histone proteins, around which the DNA double helix is wound in the nucleosomes, must be produced in large numbers. In a given cell, the combined weight of the histones is equal to that of the cell's DNA. Each major type of histone, of which there are five, accounts for 0.5 to 1% of the total cell protein. In order to produce histone proteins at a high enough rate, the cells of multicellular organisms contain between 50 and 500 copies of each histone-encoding gene. These gene copies are repeated in tandem, with noncoding spacer DNA separating the copies from each other. Although the DNA sequence of the spacers may vary, the transcribed portions are identical. In the case of tRNA, human cells have between 10 and 100 copies of each tRNA gene. This ensures that each type of tRNA will be produced in abundance. An even more dramatic example is that of rRNA. In humans, for example, a cell must construct about 10 million ribosomes of rRNA to allow for adequate protein synthesis. Cells with high metabolic activity may require even larger numbers. There are about 200 copies of rRNA genes per haploid genome. These are located on the tips of the five acrocentric chromosomes, 13, 14, 15, 21, and 22. These tip regions aggregate during interphase, forming the nucleolus. When condensation occurs in preparation for cell division, the aggregate is dispersed, causing the nucleolus to disappear.
In the amphibian Xenopus laevis, there are about 600 copies of the rRNA gene per haploid genome, clustered on one chromosome.
This produces an interesting visual effect during transcription, as the RNA polymerase molecules line up on the adjacent gene copies, their mRNAs "growing" out from the DNA perpendicular to the genes. These amplified copies of the rRNA genes are made in the oocyte to produce enough ribosomes for a burst of protein synthesis just after fertilization. The extra copies cannot, themselves, be replicated. Rather, they are broken down during embryonic development after they have achieved their function. In Drosophila, a fruit fly, there is a mutation called bobbed, which is the result of the deletion of about 130 rRNA genes. The inadequate amount of rRNA causes low viability, underdeveloped bristles, and reduction in size. The more rRNA genes are deleted, the more pronounced the phenotype will be. The drug methotrexate inhibits the enzyme dihydrofolate reductase (DHFR). If methotrexate is applied to cells in culture, most of the cells will die, but some of them are resistant to methotrexate and continue to grow. In these cells, the DHFR genes are amplified, allowing the cell to overproduce the enzyme, thus overcoming the effect of the drug. By gradually increasing the drug dosage, it is possible to produce successive generations of cells that have more and more copies of the DHFR gene.
Once there are multiple copies of a gene, the number of repeats can be extended by unequal crossover events. Because these crossovers are just as likely to result in fewer copies as in more copies, probably only those repetitions that are beneficial are maintained by the cell. Proto-oncogenes may become oncogenes, causing rapid cell proliferation, as a result of being amplified. Amplification of the myc gene, for example, activates it to form a tumor. Cultured cells in which the myc gene is amplified show reduced differentiation, a hallmark of cancer cells; and have a short life expectancy. Some genes (and their respective proteins) exist because of amplications of pre-existing genes; a well known example of this is hemoglobin. The alpha chain, which existed first, was duplicated. The duplicate eventually mutated, forming the beta chain. The beta chain was later duplicated, its copy mutating into the gamma chain, which is active in the fetus, but inactivated after birth. There are many such "gene families," the members of which were produced by copying existing genes.
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