We’ve come an extraordinarily long way, from being nought more than a single fertilised egg - a single cell in our mothers fallopian tubes or uterus - to the complex organism built up of trillions upon trillions of individual cells reading this post. Cells divide. And they divide prolifically. Some cells divide more than others, and many cells lose their ability to divide at some point but a cells potential to autonomously divide is one of the criteria which allow it to be considered a living thing. Division is reproduction.
The process of reproduction is integral to the very nature of life, without the existence of which there is no life. Reproduction makes life persistent. It keeps life going. It allows life to evolve and change. Cells divide, and this reproduction allows living organisms to perpetuate their existence either as a single multi-cellular organism repairing damaged muscles, or a bacterium dividing to form a colony, or rabbits copulating to produce offspring. Whatever the context, reproduction results in the production of new genetic material, and new cells.
A cell needs to prepare to divide. This preparation in the cells which construct humans and every other animal, plant or fungus is even more complex than the rest of life, as we’re all multicellular organisms. Complex life needs to keep a check on how its cells divide. If our cells don’t divide enough, we don’t build organs properly. If they divide too much, they use up too much energy, and can also be cancerous.
The Cell Cycle
Before a cell can divide it needs to double in mass. It needs to create a copy of its own DNA, like copying a CD, and it needs to create a whole new set of proteins for the new cell. The cell undergoes an initial growth phase called ‘G1’. In this, the cell makes some new protein, and determines whether the local conditions are going to allow it to divide. This is often done by sensing the presence of chemical growth signals in the local environment of the cell. These signals are usually released from some cells in the body such as the follicular cells in the thyroid gland. If the local environment is permissible, then it will go ahead with making the new proteins and DNA. Certain cells in the body often release anti-growth signals to stop cell growth. This also keeps cell division in check.
The cell adds up all the factors which have been sensed in this G1 phase and makes a decision whether to go ahead with cell division or not. It is at this point that the cell commits to its fate of cell division, stasis (where it doesn’t divide, also known as senesence) or in hostile environments, apoptosis. Apoptosis is programmed cell death. The cell senses that the local environment is unfavourable to support life and kills itself. This cell suicide is an evolutionary mechanism for multicellular organisms where individual cells sacrifice themselves for the greater good. A cell may sense that the local tissue has been invaded by toxins, so rather than die a gooey, explosive death where it damages local healthy cells, it quietly and cleanly kills itself in a manner which isnt damaging to local cells. This is easily cleaned up by the body’s immune system.
The next phase after G1 is called ‘S phase’, or the ‘Synthesis Phase’. The point just before the S phase is a regulatory point in the cell cycle. During S Phase, DNA is replicated and new proteins are formed. Once the cell has produced a complete copy of its own genome, and produced all the relevant proteins, it can enter the actual process of cell division - what you may have heard of as ‘mitosis’. There are 4 phases of mitosis, which follow a strict order.
The 4 main phases of mitosis are Prophase, Metaphase, Anaphase and Telophase. There are other sub-phases, such as prometaphase mentioned in the diagram.
Most cells in the human body can’t divide indefinitely. This is because DNA replication is flawed in most human cells. Every time DNA is replicated, some DNA is lost from the ends of the chromosomes. Cells work around this by sticking junk DNA on the end of their chromosomes. These ends of chromosomes are called telomeres. These are designed to be lost so no actual genes are damaged in the process of replication. Unfortunately, after a certain number of replications, these telomeres become completely chopped down. Once the cell reaches this point it halts division.
The limit to which cells can divide is known as the ‘Hayflick limit’. If cells continued dividing past the hayflick limit, then genes would start to get damaged and chromosomes would also start sticking together and the cell would enter what is termed ‘crisis’ and then die. Once again, at that regulatory point, the cell senses whether or not its chromosomes and DNA are healthy enough to continue dividing. Sometimes, if DNA is damaged from other means, such as radiation or harmful chemicals, the cell will arrest the cell cycle - preventing it from dividing any longer until the damage in the DNA is fixed. This is because damaged DNA is prone to producing mutations when it is replicated by the cells DNA-copying machinery (enzymes called DNA polymerases). In fact there’s a rule called ‘The A rule' in polymerases where DNA polymerases just put a random 'A' base where they cant tell what base should be inserted. Sort of 'When in doubt, just add adenine’. Of course, this would only work ~25% of the time, considering there’s a choice if 4 bases which could be used.
This is one factor which contributes to our ageing. When we age, more and more cells reach their hayflick limit, and we are less adept at producing more cells. This makes it harder to repair tissues, and makes us more prone to diseases and injury.
The biggest evolutionary kick in the teeth here is that our bodies can get round the hayflick limit, by producing an enzyme called telomerase which continually adds new telomeres when new cells are formed. This would help a cell divide for longer, and produce more progeny if needed. Indeed the cells which produce sperm contain telomerases, so males can produce as much viable sperm as possible and father as many offspring as possible. Evolution loves sex. Unfortunately, it doesn’t usually care for us living very long, just as long we pass on our genes. So the gene for telomerase is present in every cell in the body, but it remains in an ‘off’ state, even when it would be useful. Scumbag genetics.
Cancer is the unregulated, runaway division of cells. This boils down to a malfunction in the cell cycle. This can happen a number of ways, through different mutations and different types of cell damage, presenting with different pathologies - but the unifying concept of all cancers is that they consist of unregulated, over-division of cells. Other processes can accompany this, and these extra factors determine what type of cancer it is; like if it is benign or malignant. Cancer cells get past the Hayflick limit to continue dividing indefinitely by unlocking the telomerase genes, enabling them to produce telomerases to extend their potential to divide.
The very existence of life depends on the ability of cells to divide. But even this defining ability of life to persist against the elements can turn against us. We can see inherent flaws in our biology which can ultimately lead to our downfall. Figures released from The Global Burden of Disease study this week indicated that approximately 8 million people died of some sort of cancer in 2010. That’s much more than were killed in war, by cars, or by guns in the same year. These are the consequences when a fundamental aspect of the very essence of life goes rogue. The mechanisms of cancer and cell division are horribly complex and intricate. There is still much to learn about how they work, and how they can be controlled. Understanding this in depth has massive potential, particularly in teaching us how organisms evolve, how to combat cancer, and how to comfortably live longer. Something I personally gleaned from all this was just how any aspect of life can go wrong.