Aging is part of life but is getting older a requirement? Understanding why we age or what causes aging is a puzzle scientists are still attempting to solve. Hypotheses abound: is oxidative stress damaging our DNA? Is glucose to blame? Are our cells just following a pre-programmed biological timetable regardless of any other factors? Most likely, it’s a mixture of all of these and other causes we haven’t yet found.
One theory suggests that the aging process is partly linked to our telomeres. These are the little protective caps at the ends of the DNA molecules that make up the chromosomes. The telomere’s job is to stop the ends of chromosomes from fraying or sticking to each other, like the plastic aglets at the ends of shoelaces.
Telomeres are crucial in ensuring our DNA gets adequately copied when cells divide. Thanks to a quirk of the DNA replication machinery, a few nucleotides (the building blocks of DNA) at the very end of the strand don’t make it into the new DNA copy, leading to the loss of some genetic information like a photocopier blurring and cutting off the last line of a page. DNA strands get shorter and shorter with every cell division.
So, how do cells have any DNA left at all? Enter telomeres. Telomeres consist of the same sequence of six nucleotides repeated over and over again, and all this repetition makes them somewhat disposable. They protect the rest of the DNA, as they can gradually get shorter over time without losing any crucial genetic information.
In egg and sperm cells, an enzyme called telomerase keeps adding more of the repeating sequence onto the end of DNA strands so that the telomeres in these cells don’t shorten. In other cells, telomerase is less active, leading to the gradual shortening of telomeres over time.
Leonard Hayflick After research found that normal human cells divide roughly 50 times before stopping. The Hayflick Limit is the cell division limit. A cell can no longer divide when telomeres are too short; once they reach a critical point, the cell becomes inactive (or ‘senescent’), slowly accumulating damage that it can’t repair or dies. This shortening process is believed to be one of several factors that causes cells to age.
A too-short telomere signals to the rest of the cell that there is a problem with the DNA. Problematic DNA needs to be repaired rather than continuously copied, so it’s crucial for broken DNA to be flagged and dealt with quickly by cellular repair mechanisms. In this sense, telomeres also play a critical role in preventing cancer, which is uncontrolled cell division. Uncontrolled cell division is why we can’t just keep extending the lives of our cells by boosting telomere regeneration with more telomere-producing telomerase: our inbuilt cancer prevention systems would be compromised.
It’s not just telomere length that matters but also telomere shape and structure. Healthy telomeres form neat little paperclip-shaped loops at the ends of chromosomes, so the end is tucked away and safely protected. If you have ever tried forming a loop or a knot with a piece of string, you will know that as the part of the string gets shorter and shorter, creating it into a loop gets more and more difficult. It’s the same with telomeres.
When this loop unfolds, the end is exposed, which sounds the alarm to show the presence of broken DNA. Cell division stops, and the cell may become senescent, at which point it can no longer respond appropriately to damage from other sources. The slow deterioration of function that follows is part of the aging process.
Does this mean lengthy, healthy telomeres are the key to a longer life? We still have a lot to learn before we can answer this question. We can’t yet say whether telomere shortening is a direct cause of aging or just an outward indicator that we’re getting old. However, we know that some lifestyle choices, like smoking or UV light exposure, can speed up the degradation of our telomeres, so it’s wise to take action to slow that process down.
Our cells are highly complex machines; we must take care of them.
The original article found at https://www.science.org.au/curious/people-medicine/what-are-telomeres written by Emma Berthold, Content Producer, Australian Academy of Science and reviewed by the following expert: Professor Jenny Graves AO FAA School of Life Sciences, La Trobe University.
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