1 – The inaccurate replication.
We carry in each of our cells the chromosomes, long strands of DNA wound up into proteins. The ends of these strands are made of short-sequence repeats, and their assembly is known as telomere. When cells replicate in mitosis (the rate of replication differs from tissue to tissue – skin and gut cells renew every week or so, while neurons may last a lifetime), the DNA must be copied and inherited by daughter cells. Every cell carries in DNA the information which stands for the whole body.
But for molecular reasons, in each mitosis the telomeres shorten up. Imagine the telomeres are cushions protecting genes between one another. These cushions get thinner and thinner as replications go by, to the point that genes can be affected. If genes are affected, further replication and survival of the cells becomes jeopardized (what can be observed in elders, e. g., in scarceness of their defense cells).
If genes are deteriorated because they lack telomere protection, problems emerge. For instance, the cell may lose control over replication and it brakes loose. Maddened promiscuous replication of cells is famously known as cancer. This is one of the reasons why elderly people are more susceptible to cancer.
For a tumor to appear, it is not strictly necessary that replication-related genes are affected. The process may be triggered by mutation in specific receptor genes alone, those which allow the cell to perceive its neighbors. That is to say, a cell in solitude is a dangerous cell.
2 – The decay of biological structures
This second reason is intimately related to the first one. The very phenomenon of breathing (and by obvious extension, eating, since breathing is connected to food energy intake) causes decay to the tissues. By decay I mean mutation. Every woman who buys anti-wrinkle cosmetics has heard about free radicals – and these are produced due to bad reception of electrons at the end of respiratory chain in mitochondria. Free radicals are so damaging that natural selection favored cells owning peroxisomes, organelles capable of tackling them. These radicals interact with DNA and change it, causing mutations, spoiling genes which control cells and tissues. Mutations can also be caused by many other circumstances.
As our body is always renewing itself, and needs energy (food and respiration) in order to do so, it has no means to avoid gradual gathering of mutations throughout the tissues in the course of decades. This means stem cells (which give birth to themselves and to tissue-function-specialized cells all over the body) diverge from the ancestral type, and this ancestral type is the fertilized egg from our mothers.
When diverging from their ancestor, these cells may start misproducing elastin and collagen, then wrinkles thrive and skin slackens. They may overproduce melanin and bring about dark spots on the skin. May damage the brain and provoke diseases such as Parkinson’s and Alzheimer’s. And so cells keep up the noble art of aging.
It is well known that our bodies are largely a result of natural selection, so there have been also genes selected during evolution because they diminish this biological decay in some ways. But they are not perfect, for evolution mainly privileges effective means to surviving until reproduction happens and offspring grants another chance to the lineage. Old age is not common in nature, so not many ways to restore genetic integrity to longer lasting have appeared. Some genes “detect” virus infection and cancer conspiracy, and oblige the cell to cease its activity, brake its DNA and dissolve into vesicles later “eaten” by defense cells. This is called apoptosis. Furthermore, defense cells can also detect carcinogenic activity in other cells and induce this programmed death to them. There are ways to thicken telomeres (namely, reconstruction from telomerase activity), mastered by stem cells such as those that produce sperms and eggs (gametes). Thus, although gametes are vulnerable to mutations, it is an evolutionary advantage that individuals are born from a single cell, for this grants at least in youth and reproductive age minimal resemblance among copies of genes in the various cells of the organism, so that they “agree” with each other, and work in harmony as a choir to be judged by natural selection, until the inexorable forces of aging hijack and shatter this agreement once again.
“Nothing in Biology makes sense except in the light of evolution”, said Dobzhansky in 1973, and now the statement has never been so truthful. It may be concluded that cancer is a microevolutionary process, in which the selection unit is the cell.
As Homo sapiens descended from bacteria, and bacteria are “immortal” beings (for there is no senility among them), cancerous cells from people can return to this condition of perpetual replication. This may happen because the only “purpose” of living beings, if it may be called this way, is to copy themselves.
HeLa stands for Henrietta Lacks, a woman who died in 1951. Scientists extracted tumor cells from Henrietta and bred them in culture medium. Nowadays, these cells sum up to tons, spread all over the world in laboratories. They caused Mrs. Lacks’s death, in a genetic takeover. Today, HeLa cells are not exactly human. Instead of 46 chromosomes, they can bear 82 chromosomes into their nuclei. Evolution and natural selection have acted upon them, so that they are particularly good at keeping their telomeres intact (if they don’t do so, they die out). If by any odds a HeLa cell produces a gamete, the different number of chromosomes prevents it from fertilizing with a human gamete.
Hence, HeLa are reproductively isolated from human species, that is to say, HeLa has turned into a new species, properly described as Helacyton gartleri. If the world were made of culture medium, HeLa would spawn itself freely and independently from researchers’s aid. Currently, from studies with very few multicellular species which seem to be rid of senility (such as the hydrozoan Turritopsis nutricula), and from telomere research, we may have some perspective of future breakthroughs on how to elongate human life span. If we manage to do to ourselves what has already been done to the worm Caenorhabditis elegans, humans will live up to about 200 years.
ReferencesHug, N., & Lingner, J. (2006). Telomere length homeostasis Chromosoma, 115 (6), 413-425 DOI: 10.1007/s00412-006-0067-3
VALKO, M., RHODES, C., MONCOL, J., IZAKOVIC, M., & MAZUR, M. (2006). Free radicals, metals and antioxidants in oxidative stress-induced cancer Chemico-Biological Interactions, 160 (1), 1-40 DOI: 10.1016/j.cbi.2005.12.009
Kenyon, C., Chang, J., Gensch, E., Rudner, A., & Tabtiang, R. (1993). A C. elegans mutant that lives twice as long as wild type Nature, 366 (6454), 461-464 DOI: 10.1038/366461a0
Lucey BP, Nelson-Rees WA, & Hutchins GM (2009). Henrietta Lacks, HeLa cells, and cell culture contamination. Archives of pathology & laboratory medicine, 133 (9), 1463-7 PMID: 19722756