The cell cycle refers to the distinct stages through which a cell goes, from the moment it becomes a cell to the moment it divides to result in 2 separate cells. Bear in mind that some cells cease to divide any longer after a certain period of time, depending on cell type. If that’s the case, they are said to be in resting phase termed G0 .
Within the dividing cell, it starts with gap 1, G1 , continues into the S phase (S is for Synthesis) where DNA replicates , followed by gap 2, G2 , and ending with mitosis.
G1 and G2 may sound like codes for some complex enzymes, but they are mere notations for gaps 1 and 2, which are just that: gaps between mitosis and DNA replication (in the S phase) respectively. G1 through to G2 – that’s G1 , S phase and G2 – are all stages which collectively are known as interphase. Inter = between; phase = …phase, so interphase is just the stage between a cell’s creation and that cell’s division by mitosis.
Interphase is by far the stage in which most cells are in most of the time. The other stage, the small one, is called the mitotic phase and it encompasses mitosis (prophase, metaphase, anaphase and telophase) plus cytokinesis.
Overview of Mitosis
Mitosis is the process by which cells divide to achieve growth and repair by simply increasing cell number . The dividing cell is called the parent cell , and the resulting two cells have inappropriately been called daughter cells by scientists so far. Now because cells don’t have a damn gender and we are better than accepting silly nonsensical received wisdom about what to call these cells, we will call them offspring cells instead. The offspring cells are genetically identical i.e. clones , as they contain copies of the parent cell’s DNA.
Stages of Mitosis
Prophase, metaphase, anaphase, telophase and cytokinesis.
There’s no easy way around these stages, so just bloody learn them. Actually there is an easy way. Awesome video time!
1. Chromosomes begin to appear visible under a microscope due to chromatin (the coiled and yet-again coiled DNA fibre) condensing. Before this the DNA is not specifically distinguishable in the shape of chromosomes. This is a terrible word tangle so this is how it is. From a bowl of spaghetti (the nucleus) put the spaghetti in the shape of several chromosomes. Chromatin is the spaghetti initially, and chromosomes are the spaghetti still, just turned and twisted and distinguishable as individual stick-shaped objects. That is all, that’s all it is. Before this happens though, the DNA must be replicated – that’s the reason behind the X shape of chromosomes; they are two “lines” a.k.a. chromatids joined together at their centres called centromeres.
2. The nuclear envelope breaks down.
3. Organelles known as centrioles migrate towards the poles of the cell. These organelles are involved in the act of pulling the chromosomes apart into the soon-to-be offspring cells. They achieve this by the microtubules that extend out of them and connect to the centromeres. Microtubules are like lassos. Sort of.
Chromosomes are aligned at the cell equator by spindle fibres (made of the aforementioned microtubules) which lengthen and shorten themselves on opposing sides (tug of war) until all chromosomes are lined up about halfway across the cell. This area is called the metaphase plate. It looks like a plate. Who said biology can’t be straightforward?
The chromatids split at their centromeres and are pulled towards opposite poles of the cell by the shortening spindle fibres.
1. Nuclear envelopes reform around the two new nuclei .
2. The chromosomes decondense and become indistinguishable under a microscope yet again, and the spindle fibres spread out.
This is the final step of mitosis when the cytoplasm of the parent cell divides to complete the cell division, resulting in two brand new and individual offspring cells.
Uncontrolled cell division in cancer
Knowledge of the cell cycle comes in very useful in the treatment of cancer. Cancerous cells divide out of control, often due to mutations in DNA which result in improper regulation of cell division. These mutations have been isolated in certain genes known as oncogenes. The best drugs to treat cancer must be efficient at targeting and killing the cancerous cells, without damaging any nearby healthy cells. When cells divide too quickly, tumour suppressor genes regulate them and give DNA an opportunity to be repaired, or for the immune response to kill the cancerous cells. If the tumour suppressor genes mutate themselves, then the probability of cancer developing increases significantly.
As such, cancer is a scenario where cell division occurs out of control, unlike in mitosis which is controlled. Therefore, many treatments targeting cancer are aimed at regulating cell division. Lovely little topic. Remember the stage of the cell cycle where DNA replicates (interphase->S phase). Also, as previously mentioned, some cells never undergo mitosis, and so remain in G1 indefinitely. In this particular scenario the cells are said to be in the G0 phase.
Cell division in prokaryotic organisms such as bacteria is very simple. Termed binary fission (splitting in two), it involves duplication of the cell’s DNA and the even splitting of the copied genetic material into its two offspring cells, which in this unicellular organism effectively becomes two new individuals. The cytoplasm is therefore also evenly divided alongside the respective genetic material. However, you might remember that these organisms have extra genetic information in plasmids alongside their main DNA. The number of copies of plasmid that each new cell receives from the parent cell during binary fission is variable.
In the diagram only the main DNA is pictured (plasmids are much smaller) as well as the cell wall which can be seen pinching from the sides of the new emerging offspring cells, and gradually tearing a path towards the middle to separate the two cells. Very much the same idea as cytokinesis in mitosis, although bear in mind these cells do also have a cell wall to worry about in addition to the membrane!
Viruses cannot divide outside a host cell, or indeed even inside a host cell. They simply do not have any machinery to do so. Their replicative activity lies solely in their genetic material and protein units needed to house the genetic material and target new host cells. That’s it.