A Cell's Life
All living organisms, from the simplest bacteria to complex multi-cellular organisms like humans, are the result of cell division events stretching back to the beginning of life on earth (talk about a complicated family tree). Like almost every process in biology, there are both common fundamental principles of the cell cycle that all organisms share and other aspects that are specialized for certain organisms, or even cells within organisms. At the fundamental level though, all organisms need to copy their DNA with high fidelity and then segregate these replicated copies of DNA into two identical cells. The cell also needs to replicate its organelles and grow.
When scientists talk about a cell's life, they often use familial terms. For example, a chromosome that has undergone DNA replication has an exact copy of itself. We call this exact copy a sister chromatid. When a cell undergoes division to create two new identical cells, we call them daughter cells. Cell division is also called cellular reproduction. Find it a bit strange that these 'big' life terms are being used for events taking place on such a microscopic scale? They're simple reminders of how cell division connects us humans to our parents, our ancestors, and even the earliest, simplest life forms on earth.
Under the light microscope, the eukaryotic cell cycle can be visually characterized into two phases, called M phase and interphase. If you looked at rapidly dividing cells from your body under the microscope, few cells would actually be physically separating their genetic material. That's because the actual division phase (M phase) takes only about 30-60 minutes. In M phase, in a series of dramatic events, the duplicated chromatids condense and become visible under a light microscope, are segregated to opposite poles of the cell, and the cell is pinched into two separate cells. We refer to mitosis as the division of the nuclear DNA, while cytokinesis refers to the process by which the cytoplasm is divided into two daughter cells. The rest of the time the cell is in interphase, and interphase can last for days, weeks, or even longer depending on the situation. Weirdly, interphase is often called a resting phase which, as we will see, is anything but the truth. M phase may get all the attention because it is dramatic (remind you of anyone you know?), but most of the preparations for division occur during interphase. And, in cell division, as in life, preparation is everything…
What is interphase really? Interphase can actually be broken into three phases: G1, S, and G2. In G1 phase, or gap 1, cells grow, do their job, replicate their organelles, and produce the proteins needed for DNA replication and chromosome segregation. S phase, or synthesis phase, is when the cell replicates its DNA. In a typical human cell, S phase takes up almost half of the time of the cell cycle, and therefore, S phase often takes about 10-12 hours. G2 phase, or gap 2, follows S phase. The cell uses this gap period to continue to grow and carry out its normal functions, replicate its organelles, and produce the proteins required for cell division. Sounds like a lot of work happens in interphase, huh? No wonder poor old interphase feels a bit slighted by all the attention given to M phase. See what we meant about it not being much of a rest?
What controls the cell going from one phase to another? Eukaryotic cells have evolved a complex regulatory system that involves a large network of proteins. These proteins control the main events of the cell cycle, namely DNA replication and the segregation of the identical chromatids to opposite sides of the cell. It is helpful to think of these proteins as a sort of biochemical traffic-light system, giving the cell either the red or green light to continue along its way.
The gap phases are actually important for coordinating this regulation, and it is during this time that the cell senses its environment and can make decisions about whether or not to proceed. This system is crucial because it helps make sure that all the events happen when they should, for example by preventing the segregation of the chromatids until after DNA replication has finished. That'd be like baking chocolate chip cookies before adding the chocolate chips and sugar—yuck. What happens if the regulation fails? Such a failure can lead to cell death, or even cancer. We will talk more about that, though, when we come on to regulation.
Sometimes the cell exits from the cell cycle into special phases. One such phase is G, sometimes called the quiescent phase, which its considered a resting phase the cells go into once they have stopped dividing, but again, it is not exactly sitting around with its feet up, doing nothing—it is still carrying out its normal functions. Other times, the cell enters into a specialized phase of division called meiosis so that it can produce gametes for sexual reproduction. In meiosis the number of chromosomes in a parent cell is reduced by half. Meiosis occurs in two successive division stages, meiosis I and meiosis II, which we will cover in more detail later. Exactly when cells go into meiosis will differ, depending on whether you are male or female.
- Soda and the cell cycle? Sure we all consume drinks with caffeine. However, did you know that caffeine is a commonly used tool by scientists studying the cell cycle? When caffeine is introduced to a cell stalled in its cell cycle because its DNA is damaged, the cell ignores its regulatory system and continues dividing. You thought that caffeine was only good for keeping you awake.
- Don't eat the daisies: Another chemical used to study the cell cycle (and diagnostically in the field of cytogenetics) is colchicine. It stops the mitotic spindle from forming properly, arresting the cell cycle in metaphase. Where does colchicine come from? The autumn crocus. It is not so much don't eat the daisies, just don't eat any crocus bulbs if you want your cells to keep dividing.