It is known that the length of telomeres can provide some indication of the life span of a cell. As such, telomeres and aging are related to each other.
Have you ever wondered why humans get old? We have plenty of stories and evidence in our mythological and folk knowledge about aging and mortality, but these are rarely confined to scientific findings. Immediately after a baby is born, there are specific changes in that human’s body that consistently push it towards the next phase of life. Several studies have found a relationship between telomeres and aging, but what connection do they have? Let’s find out!
Living on a knife’s edge
A Nobel Prize-winning molecular biologist, Elizabeth Blackburn, once said that life is all about living on a knife’s edge. She discovered some teeny tiny parts of a human chromosome back in 1984. She described the significance of these segments of the chromosome by explaining that if they present too much, then they may indicate the presence of disease. On the other hand, if these segments are too small, then signs of old age will be apparent. Therefore, it is precisely like ‘living on a knife’s edge.’
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This biologist was primarily talking about telomeres and the activity of the enzyme telomerase, the subjects for which she won the Nobel Prize in 2009. The wellbeing of a human body depends on the balance that this enzyme is able to maintain. If the amount of this enzyme increases beyond a particular limit, it results in the formation of an abnormal cell type.
What are telomeres?
Think back to your high school days. When they made you observe onion cells under the microscope, you probably noticed a dot in the middle of each cell. This dot is called a nucleus, which is essentially the boss, if your cell was considered as a company. The cells have other “employees” too, which are called organelles. One such organelle that resides in the nucleus is the chromosome.
That word might sound familiar, or maybe “familial”, since the chromosomes are so closely associated with heredity. At some point, you probably received compliments because you looked like one or both of your parents. Your dad may have said something like, “It’s in the genes.” Well, these genes are residents of the chromosome. Not only this, but chromosomes can also be understood as a tight cord of DNA, which instructs the cell what to do.
However, where does the telomere come into this picture? Well, the telomere can be compared to the cap of this DNA. It is referred to as non-essential because it doesn’t have any genes on it, and therefore doesn’t form any proteins. It prevents the chromosomal ends from sticking to each other. However, a point to note is that every time a cell divides, a small part of the telomere’s sequence breaks off.
What is Hayflick’s Limit?
To understand this segment, let’s consider the entire process of cell division. When a cell divides, the daughter cells are provided with a set of genetic material. In this case, after the replication of DNA is processed, a little part of this telomere gets depleted. A more straightforward way to understand this concept is by considering the example of cutting a cake. Every time you try to divide it further amongst a group, some part gets wasted in the form of crumbs.
Now, when further cell divisions take place, even more telomeric sequences will get snipped off the end. The very reason why these sequences are present at the end of the DNA is that they provide protection. After each cycle of replication, it is the telomeric sequences that get chopped off. In this way, the DNA is kept safe from damage. As the length of the telomere reaches its shortest limit, a DNA damage response is initiated. This response tends to protect the DNA from the damage, now that the telomere has become so short.
After the initiation of the DNA damage response, the cell loses its ability to divide further. In scientific terms, this phase is called senescence or, more commonly, aging. Hayflick’s Limit is the number of division cycles that a cell can undergo before it loses the ability to divide. Generally, cells have a Hayflick’s limit of about 50-52.
What is telomerase?
The limited replicative capability of a cell pushes it towards cellular aging, which consequently corresponds to human aging. However, there’s one cell type that doesn’t typically follow this phenomenon. The stem cells continue to have a relatively high Hayflick’s limit due to an enzyme called telomerase. This enzyme continually adds the lost parts of telomeric sequences after every replication.
If we were to plot a graph between the cell division capacity of a regular body cell to its number of divisions, then the curve would be downwards. However, if the same graph is plotted for a stem cell, it comes out to be a straight line parallel to the x-axis, thanks to the telomerase enzyme.
The Nobel Prize-winning molecular biologist Elizabeth Blackburn might have tried to make her discovery a little more interesting by describing life as being balanced on the edge of a knife, but she is not wrong. The functioning of telomeres might sound subtle, but they hold extreme importance in the day-to-day functioning of the cell. The length of the telomeric segment in the DNA not only gives us an idea about the life span of an individual, but also some additional clues about the disease.