Definition
Unsurprisingly, Webster (1913) doesn't mention the word telomere. It's a fairly new term from the realm of biochemistry. However, Webster's modern descendent, the Merriam-Webster OnLine Dictionary1 has this to say:
Main Entry: telo·mere
Pronunciation: 'te-l&-"mir, 'tE-
Function: noun
Etymology: International Scientific Vocabulary
Date: 1940
: the natural end of a eukaryotic chromosome
A minimally more eloquent definition comes from another source2:
Telomeres are the physical ends of linear eukaryotic chromosomes.
Explanation
Recursing into the components of this definition brings us most of the way toward understanding the topic:
Chromosomes are strands of genetic information. Consider a chromosome as something like an open necklace, a long kind of rope with genes attached to it like totems on a pole or beads on a string.
Each human cell contains 23 chromosomes in its nucleus, or inner core. Microphotographs of most chromosomes look like fuzzy, bent little X's, or like pairs of curved sticks siamese-twinned in the middle.
- Genes are sequences of nucleic acids, perhaps a few dozen to a few hundred of them. The particular combination of nucleic acids in a gene is a kind of biological code. Each of these codes, or genes, is not just a representation of a particular protein, it actually serves as a chemical template for its construction. The presence of one or more genes results in a particular feature in the physical makeup, or phenotype, of a living being.
- proteins are the building blocks of all living cells. Cell membranes are made of particular proteins, and there are many different proteins inside. Different cells are made up of different proteins; this makes the difference between muscle fibres, hair follicles and bone cells. Viruses are made of proteins, and eggs are mostly proteins as well. The selection of proteins which make up a cell is determined by the composition of the genes on the cell's chromosomes. All proteins are made up of amino acids.
- amino acids are the organic molecules that make up all living matter. Comprised of between a dozen and a few dozen atoms, amino acids are fairly complex as far as molecules go, but they are just the building blocks of super-complex molecules called proteins.
- linear means essentially straight. In the context of chromosomes, this means not necessarily straight as an arrow, but it's essential that this precludes a closed loop, such as a circle. Thus, linear chromosomes are straight like sticks: Some are really straight, others are a bit bent.
- eukaryotic is a term which biologists apply to cells which are well-organized, such as those of humans, plants and animals. The cells which make up the "higher lifeforms" have a clearly defined nucleus, held together and protected by a membrane, and also functional units called organelles. In more primitive cells, genetic material and other components of the cell just drift around in the cell's protoplasm.
- The ends of chromosomes are, then, exactly what you'd imagine if you think of linear chromosomes as more-or-less straight sticks: Simply short pieces of the chromosomes near their two tips.
Significance
Given this background, it becomes possible to understand the significance of the ends of chromosomes.
Chromosomes are the structures that hold our genetic material together, Nature's blueprints for all the proteins in all living things, and how they connect together. The process of protein construction is called expression, and it essentially works by means of cellular proteins specialized as "construction machines" which inch along on the chromosome and use its structure as a template for constructing new proteins. I'm skipping some steps here that aren't too relevant to this explanation.
Although the protein "construction" business is essentially chemical, chromosomes are big enough to be considered mechanical entities too, and like the corners of furniture, the ends of chromosomes are most likely to get "bumped into" by the other contents of a cell, and damaged. Also, chromosomes aren't fully covered end-to-end in genes; there are bare spots, and inactive genes. In particular, the "construction machines" are adapted to constructing proteins from somewhere along the length of a chromosome; the end has a different geometry. So the ends are essentially "dead" zones as far as genetic information is concerned, and protective end pieces to the structure of each chromosome. They also work like bookends, holding the information between them together.
The significance of these genetic bookends becomes dramatically apparent when cells reproduce: Before a cell splits into two new ones, it must duplicate all of its chromosomal material so that there are (nearly) identical copies for both of the new cells. In this duplication process, the chromosomes are not perfectly copied. Partly because of the geometric particularity of the tips, part of the end pieces, or telomeres, is lost with each duplication. Thus, over the course of (cell) generations, the chromosomes become shorter and shorter until at some point the "book ends" are gone. At some point, genetic information from the ends of the chromosome is lost in duplication and the cells inheriting these defective chromosomes lose some of their function, eventually becoming defunct, or more simply: dead. What this all leads up to is this: The gradual shortening of telomeres is the process we know as aging.
Research is still ongoing, of course, but there are clear signs that telomeres, along with some essential cellular mechanisms that deal with them, are the keys to aging and longevity. With their physical length, telomeres measure out the lifespan of most living things.
Naïve thinking might lead us to believe that a bit of chemical refurbishing is all that's needed to make humans immortal; but like everything in nature, the truth is more complex than this.
Philosophically, death of old age is Nature's way of recycling living protein and of keeping the planet from getting overcrowded. In some ways, it's simply not healthy to keep on living forever. For striking evidence of this statement, consider cancer. Yes, malignant tumors are simply masses of cells that multiply without bound. The body's natural tendencies in this direction are usually checked by the shortening of telomeres, but in cancer this mechanism somehow fails. What's good for individual cells is not always good for the organism as a whole.
But if telomeres shorten each time a cell divides, how can cancer come about? Again, the paths of evolution have led to some interesting mechanisms. In many kinds of cells, an enzyme, a cellular chemical tool called telomerase, has the job of patching up chromosomes damaged in duplication.
Apart from cancerous cells, whose life cycle isn't yet fully understood, there are also some very clear examples of cells which employ this mechanism:
So, in their ongoing research for the secrets of longevity while avoiding the hazards of cellular explosion, scientists may yet have a lot to learn from the humblest of organisms.
Results, present and future
On another front3, scientists have discovered a protein they call TRF2 which somehow bonds to telomers in the cells of higher animals and keeps them from harm. They found that so long as this substance is produced and available in abundance in cells, telomeres do not shorten and aging does not set in. So at least in some organisms, aging seems to be a precise mechanism of checks and balances intended to measure out the life span of living beings.
These findings, though, probably represent no more than the tip of an iceberg of exhaustive scientific research that will be necessary before eternal youth will be a finished, labelled, FDA approved product sold at your local drugstore. In the meantime, a sensible diet and moderate exercise will probably be most effective in helping you attain a healthy old age.
References
- http://www.m-w.com/cgi-bin/dictionary
- http://www.genlink.wustl.edu/teldb/tel.html
was unfortunately not available when I tried it, except for a brief excerpt from a search engine's cache.
- http://www.wissenschaft.de/sixcms/detail.php?id=119818
Further reading
- http://opbs.okstate.edu/~melcher/MG/MGW1/MG1352.html