Cell division requires the active remodelling of the cell’s cytoskeleton. The cytoskeleton is made of multiple kinds of protein that offer it the right structure and aid during the process of division.
The centriole is a tubey spaghetti thing that aids in cell division when the duplicated chromosomes need to move into their subsequent new offspring cells from the parent cell (during mitosis).
They’re made of a special protein called tubulin because they’re tubeeeeeeeeeeeees. Why didn’t they call it spaghettulin? I guess spaghetti aren’t hollow but…
Microtubules also play a key part in how cell organelles are moved and placed within the cell. Two associated centrioles form the centrosome. The microtubules are also where spindle fibres extend from when they separate chromosomes during cell division.
Cell cycle and mitosis
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.
A decrease in the cell cycle beyond appropriate levels would lead to degenerative disease, while too high an increase would cause tumours. Therefore, a balanced cell cycle is key to the good functioning of the body.
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. For unicellular organisms, cell division is actually their reproduction itself (asexual reproduction). 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.
1. 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?
1. 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.
Cell division regulation and apoptosis
Cells have checkpoints throughout the cell cycle that regulate if, and when they divide. These checkpoints are at G1, G2 and metaphase.
G1 is about cell growth prior to initiating DNA duplication. Unless the right level of growth has taken place, DNA duplication will not go ahead. Cells that do not proceed through G1 remain in the aforementioned G0 state.
If the cell does grow adequately, a number of proteins called cyclins are produced, which bind and activate their respective cyclin-dependent kinase proteins (Cdks). These go on to phosphorylate multiple protein factors that take part in cell division. The amount of phosphorylated proteins determines whether the threshold for sending the cell into the next stage has been met.
If so, one of the phosphorylated proteins responsible for DNA replication (retinoblastoma/Rb, a transcription factor inhibitor) enables DNA replication to start. This has taken the cell into the S phase.
DNA replication is an imperfect process, so DNA damage occurring from it causes certain proteins to be released, such as p53. This can aid towards DNA repair, make the cell cycle come to a halt, or cause cell death.
Apoptosis (cell death) signals can come from both outside the cell and inside the cell. They activate previously inactive DNAses and proteinases (a.k.a. caspases) that break down DNA and proteins.
Outside signals e.g. from lymphocytes, bind cell surface receptors and initiate a protein cascade that results in the production of active caspases. As mentioned before, the p53 DNA damage protein can trigger cell death – in this case, it would be a signal from within the cell.
Another scenario where apoptosis would be initiated is in the absence of cell growth factors.