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If you've been on the internet for a while, perhaps you've heard it said that lobsters are biologically immortal. This dubious factoid was popular as a meme in some internet circles a few years ago; however, as with many viral trends, those brief, digestible quips did not capture the whole picture. Today, we will return to the curious case of the lobster: an organism that is theoretically, but not functionally, biologically immortal.
To begin our journey into the fascinating physiology of the lobster, we must first dive into the underlying basis of all life: DNA. Deoxyribonucleic acid, also known as DNA, is solely responsible for the burden of maintaining life. As you likely know, our DNA--and the DNA of all living organisms--essentially functions as a database of instructions for important cellular components known as proteins. These proteins take up a wide range of biological functions, from facilitating chemical reactions within our cells to serving as the functional units of our muscles. Even if all of our cellular machinery were to remain intact, we would have no way to make the proteins we need to function without instructions from our DNA.
Because DNA is crucial to the functioning of every cell, it must be replicated every time a cell divides. Otherwise, one (or both) of the cells that remain after division wouldn't have the complete genetic code they need to survive. In simplified terms, DNA replication proceeds as follows:
First, an enzyme known as helicase "unzips" the two strands that make up the DNA molecule by breaking through the hydrogen bonds that connect the fundamental units (nucleotides) together. This allows both "halves" of the molecule's trademark double-helix shape can be replicated. Certain proteins, called Single-Stranded Binding Proteins (SSBPs) help keep the two strands from re-attaching to one another once they are separated. Another enzyme, topoisomerase, prevents the DNA ahead of helicase from getting too tightly wound.
Next, an enzyme known as primase sets down a small piece of RNA (a "sister molecule" to DNA that also carries messages using nucleotides). This piece of RNA is known as a primer. The primer is what allows the DNA-building enzyme, DNA polymerase, to attach new nucleotides to each strand. DNA polymerase can only add nucleotides in a single direction, and it needs to build off of something; that "something" is the small RNA primer supplied by primase.
DNA polymerase adds nucleotides to both strands of DNA to create new complements for each one. The two strands of DNA wind in opposite directions to one another (antiparallel), and nucleotides can only be added in a particular direction due to the chemical makeup of DNA. One "half" of the original DNA molecule has new bases added constantly as helicase "unzips" the DNA and the other half is built in small fragments from multiple primers laid into the constantly-unwinding DNA.
Finally, an enzyme known as ligase connects any disconnected fragments together to form cohesive replicated strands of DNA.
While this seems all well and good, there is one quirk in the system that results in DNA molecules shortening every time they are replicated. Primers are required to kickstart the replication process; however, because they are made of RNA, they don't get assimilated into the final replicated strands. The nucleotides they bound themselves to are blocked from receiving complement nucleotides from DNA polymerase. This means that the newly-made strands are inevitably a few nucleotides shorter than the ones they were based on. Over many generations of replication, this results in a significant shortening of our DNA.
So how do the instructions carried in our DNA remain intact? If our DNA is shortening every time it replicates, how do we avoid losing crucial instructions? Well, every piece of DNA we have is capped off with a series of "junk" messages called a telomere. Telomeres don't code for anything, so, if they get shortened, it doesn't impact the way our cells function. A pretty elegant solution!
However, these buffers, too, are finite. Outside of certain cells (and certain types of cancer cells), telomeres are never repaired and gradually get shorter with each new generation of cells. Eventually, our cells run out of telomere to shorten, and they stop reproducing. As you might expect, the shortening of these finite buffers has been linked with the phenomenon of senescence--the biological changes related to growing old. While telomeres are likely only one part of aging, shortened telomeres in certain cells have been positively correlated to the incidence of ails and diseases experienced toward the end of life, as well as an overall higher rate of death.
So what does all of this have to do with lobsters? Well, most cells in lobsters have their telomeres repaired all throughout their life. A certain enzyme known as telomerase replaces the bases that are lost each time DNA replicates, preventing DNA from getting shorter through replication. As a result, lobsters never experience classical signs of senescence. They don't experience a decrease in fertility (in fact, their fertility may increase with age) or strength, as humans and other animals do as they get older. Their cells never hit a limit where they cease to divide. If there were no other forces at play in a lobster's lifespan, this would be cause enough to grant them the title of biological immortality. However, unfortunately for the lobster, their cells are only theoretically immortal.
Unlike many animals, lobsters continue to grow throughout their entire lifespan. For these exoskeleton-wearing crustaceans, this means that they must molt their shells regularly to keep up with their increasing size. Molting is a very vulnerable and demanding process, which alone may be responsible for 10-15% of lobster mortality. The larger a lobster grows, the more energy is required to successfully molt. At a certain age and size, it becomes physically impossible for a lobster to supply the energy required to molt further. At that point, the lobster will die, not of classical old age, but of exhaustion. They quite literally get too big for their britches, and fall just short of being biologically immortal because of it.
A natural question you might have after this is: why can't we humans use telomerase to stop aging? After all, we don't have to worry about molting like lobsters do, and our bodies naturally produce some telomerase that could be mimicked as a form of treatment. Well, it turns out there may be a very good reason why we don't produce that much telomerase: it is associated with the incidence of cancer. Never-ending telomeres are a double-edged sword; it means good cells can replicate ad infinitum, but it also means bad cells can do the same. Even without increased levels of of the enzyme, extant cancers seen in animals like humans force the production of telomerase. This prevents these dangerous cells from dying out naturally and may make them resistant to some therapies that would put an end to the ordinary cell. By increasing the levels of telomerase in our bodies to try and combat aging, we might inadvertently make it easier for harmful cancers to develop. This effect has been demonstrated in mice, who share many biological and physiological similarities with us. Some researchers still hold out hope that curated parts of telomerase might be useful as an anti-aging remedy, but, even if this proves true, human aging is caused by a wide variety of other factors--many of which we don't yet understand. As always, there is so much more we need to learn!
References & Further Reading
This informative article by the Natural History Museum debunks the idea that lobsters are immortal, and it explains their unique biological hinderances in an easily-comprehensible way. If you are interested in further background on the theoretical immortality of lobsters, this is a great place to start!
While lobsters narrowly miss the echelon of biological immortality, this article by the Australian Academy of Science details a few organisms that are biologically immortal. If you found the idea of immortal lobsters fascinating, this is a great jumping-off point. See also: the Wikipedia page about biological immortality. While we here at STEMx advise a healthy caution towards wiki-style sources, it may be an interesting place to investigate other, more familiar near-biologically-immortal animals, like certain species of turtle and shark.
This article by the Genetic Science Learning Center offers an extensive, easy-to-digest breakdown of telomeres. Not only does it include helpful descriptions and diagrams, but it also includes links to video interviews with leading researchers. If telomeres piqued your curiosity or confused you, this article is the best place to go to learn more.
This research article offers an exploration of the idea that telomerase might hold some key to human immortality. It probes into the many conflicting factors surrounding the idea that telomeres are the sole key to human aging, which makes it a great source for those of you who want to know more. Like many academic articles, it can be dense at times, but it is still a worthwhile read for those of you who want to dive deep into the specifics.
This video by the Amoeba Sisters on YouTube is a great reference for the basics of DNA replication. While we covered a lot of the key aspects in this article, this video goes further into depth on concepts like the antiparallel nature of DNA. It takes these deep concepts and makes them easy for anyone to process; if you've found yourself intimidated by STEM education in the past, you'll find this video very easy to approach!
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