Telomeres: A Root Cause of Aging
Why Do We Age?
There’s the obvious factors, such as: our environment, sleep, diet, and exercise. We also have a biological clock, meaning that as humans, we are genetically programmed to die. Our biological age refers to how old our own cells really are and therefore, our real age. Biological age varies based on our lifestyles. This is different from our chronological age (5 years old, 6 years old, 7, 8, etc…), which is irreversible and does not depend on our habits.
So what causes biological aging? There are many factors, but one root cause is the shortening of telomeres.
Telomeres:
The cells in our bodies get beaten down throughout our lifetimes. Environmental factors, such as ultraviolet rays, unhealthy diet, and alcohol, as well as stress, put our cells at risk for significant damage. These factors cause damage to the DNA in our cells, predisposing us to cancer and other aging-related diseases.
However, we do have biological systems to fight this damage. One of these mechanisms which ensure that cells can live for a certain amount of time before dying are telomeres. Cell replication occurs in a cell every 300 to 500 days when the cell dies and is then replaced by a new cell. Cell division is needed to grow new skin, blood, bone and other cells when necessary. There are protective caps on the end of our DNA sequences called telomeres, which aid in cell replication to ensure that no genes are lost in the replication process. Telomeres allow cells to divide without losing genes. Without telomeres, the main part of the chromosome (the part containing genes essential for life) would get shorter with each cell division.
Like the rest of a chromosome and its genes, telomeres are sequences of DNA. And like all other DNA, they are made up of four nucleic acid bases: G (guanine), A (adenine), T (thymine), and C (cytosine).
We can compare these telomeres to the small plastic cylinders on the tips of shoelaces. Eventually, when those tips fall off, the shoelace starts to fall apart too. This is what happens in our cells. As we get older, the telomeres shorten with each cell division, until they reach their end and can no longer divide anymore. This removes the protection from our cells’ DNA and enables disease and aging.
Senescent Cells
But, there’s more. Cells normally can divide only about 50 to 70 times, with telomeres getting progressively shorter each time. Once telomere length reaches the end of its length, the cell becomes senescent, meaning that it can no longer divide and will die. Senescent cells, aka “zombie cells” are cells that were once healthy but have been damaged or their DNA has been mutated. This is attributed to be an effect of the shortening of telomeres. To avoid passing that damage onto their daughter cells, they stop replicating. The immune system cleans them up at first, but over time the system stops responding, so these senescent cells accumulate in our bodies as we age. The accumulation of senescent cells can lead to a major increase in our biological age, causing problems such as cataracts.
Why do telomeres get shorter each time a cell divides?
Prior to cell division, the chromosomes within a cell are duplicated so that each of the two new cells contains identical genetic material. To do so, a chromosome’s two strands of DNA must unwind and separate and DNA polymerase then begins making the two new strands of DNA to match each of the two unwound strands. It does this with the help of short pieces of RNA. So, when each new matching strand is completed, it is a little bit shorter than the original strand it came from because of the room needed at the end by a small piece of RNA.
Simulating Telomere Shortening:
Take a look at the telomere quiz I created to model telomere shortening by relative age.
