Look at the friends and acquaintances of your older friends or parents and you will see that, even though they are of similar ages, some people look much older than others. So what determines how quickly an individual grows old?
There are many factors at play here, some inherited, some environmental. One mechanism that has come to the fore recently in studies of variation in ageing rates is differences in telomere length. Telomeres are specialised caps that occur at the ends of the linear chromosomes of almost all multicellular animals (see Figure 2). They consist of a variable number of repeats of a short sequence of DNA bases, and are not part of the gene coding sequence. Every time a cell divides, the chromosome ends are not completely replicated, so the telomeres shield the coding DNA from this erosion but at the same time themselves become shorter. Telomeres also enable the cell machinery to identify the true chromosome ends and prevent any end-to-end joining of chromosomes.
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