Background

Although yeast cultures are immortal, individual yeast cells display a finite life span (3,4). S. cerevisiae divides asymmetrically, giving rise to a larger mother cell and a smaller daughter cell with each cell division. A circular bud scar on the mother cell's surface is left at the site of division, serving as a physical marker of the number of cell divisions a yeast cell has undergone. The cell wall and many other constituents of daughter cells are synthesized de novo, and most daughters have the potential for a full life span. Thus, the yeast population is immortal. The number of cell divisions that the mother cell undergoes varies somewhat between yeast strains. By a laborintensive process, the number of divisions a mother cell undergoes can be determined by microscopic observation of individual large mother cells and manual separation by micromanipulation of the smaller daughter cells. Yeast aging has most often been defined as the number of times the yeast mother cell divides. Although the median life span varies among different laboratory yeast strains (in the range of 18-30 cells divisions), it remains relatively constant for a given strain, thus supporting a strong genetic influence on the life span (4-6). The mortality rate for mother cells increases exponentially with the number of cell divisions (7,8). These characteristics have allowed for the use of yeast as a simple system for the genetic study of aging and senescence.

Additional features that make a yeast an attractive experimental model include its relatively uncomplicated and short life cycle; its small genome size (about 6000 genes), many of which have been shown to have orthologs in the human genome; its well-developed system of homologous recombination allowing for relatively simple knockout or knockin of individual genes; and its economy of cell culture—it is inexpensive to grow in large quantities on simple medium.

The ease with which mutants in nearly any biologically interesting phenomenon can be isolated in yeast has been difficult to translate directly to the study of yeast replicative aging. In any growing culture of yeast cells in logarithmic phase, approximately one-half the cells will be virgin cells, one-quarter will have completed one division, one-eighth will have completed two divisions, and so on. In a growing culture, cells near the end of their replicative life span (~20 divisions) are present as a minute fraction, and direct genetic approaches based on the isolation of mutant colonies are not applicable.

How To Reduce Acne Scarring

How To Reduce Acne Scarring

Acne is a name that is famous in its own right, but for all of the wrong reasons. Most teenagers know, and dread, the very word, as it so prevalently wrecks havoc on their faces throughout their adolescent years.

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