Although every cell contains the same genetic information as every other cell in an organism, a given cell or type of cell expresses only a distinct subset of its genes at any one time. For example, only red blood cells need to produce hemoglobins and only cells in the retina need to produce light-sensitive proteins. Clearly, cells in the tongue do not need to produce hair. Genes need to be regulated to achieve this diversity of protein function during development. Gene regulation occurs primarily at the level of transcription within a cell that transcribes only a specific set of genes and not others.
The gene regulation theory, proposed by Kanungo (29), hypothesizes that senescence results from changes in the expression of genes after reproductive maturity is reached. Recent evidence that gene expression changes with age was reported by Helfand et al. (30), using a DNA sequence in Drosophilu that interacts with "activator proteins" to stimulate gene expression (i.e., called enhancer traps) and which secretes into the cell a stainable marker substance whenever their gene is active (31,32). These studies enabled the scientists to visualize the gene activity patterns in the antennae of the fly during its adult life. The results revealed that around 10 to 49 genes examined are constant, while the remainder showed a changing expression with age. Of these, slightly over half increase their activities from an initially low level, and around a quarter decrease their activity from an initially high level. The activity patterns of the genes were linked to chronological and not to physiological age (33).
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