The Cell Theory, initially proposed by Schleiden and Schwann, puts forth one of the fundamental tenets of biology, which is that all life is cellular, and that these cells propagate their lineages through the process of cell division. Now while there is a process called meiosis that is involved in sexually reproducing organisms, and this is a different form of cell division, we may safely ignore it for purposes of this blog post, which is on the Eukaryotic Cell Cycle.
The Cell Cycle describes the life and times of your typical cell. It consists of several phases which undergraduate students are usually required to acquaint themselves with.
Phases of the Cell Cycle.
Once a cell takes birth from its parent cells by division, it enters a Gap phase, in case the cell is committed not to divide further it enters a resting state, marked by G0 , if it does have a fate that entails division however, it stays in the G1 phase, where it tends to perform its normal cellular role and also undergoes cellular growth, including production of organelles and duplication of materials with the exception of chromosomes.
This is then followed by the S phase, where the DNA in a cell is duplicated, this is then followed by the G2 phase where the duplicated DNA is checked for errors and suitably repaired by DNA repair pathways. This is then followed by the Mitotic or the M phase where actual cell division takes place, the daughter cell then enters the G0 or G1 phase and the cycle continues and so on and so forth. G0,G1, S and G2 can all be collectively called the Interphase.
The M Phase (Mitosis)
Now this is something that gets repeated and repeated ad nauseam through the course of biological education. I wish to reiterate the process one more time. 😛
The M phase itself is divided into four phases on the basis of definite cytological characters that may be observed during the process.
In this phase the nucleus disappears, chromatin begins to condense into chromosomes and spindle fibres begin to appear.
Sister chromatids, joined at the centromere, line up on the Metaphase plate.
Sister chromatids separate and one of each moves towards the poles, driven by the contraction of spindle fibres.
The Nucleus reappears, the mitotic spindle apparatus disappears and a cleavage furrow begins to form. This is more or less the reverse of Prophase. This is followed by the division of the cytoplasm to form two daughter cells.
The division of the nucleus is called Karyokinesis and the division of the cytoplasm is called Cytokinesis.
You can see a video of mitosis occurring, visualized by fluorescent microscopy, below.
How is Mitosis Regulated.
The regulation of mitosis happens through the interaction of proteins called Cyclins and Cyclin Dependent Kinases which are only active in the presence of Cyclins. These proteins act in concert at specific points in the cell cycle, called Cell Cycle Checkpoints.
This will however be described in a future blog post. This concludes a very short (by my usual standards) post for laymen introducing them to fundamentals of basic cytology. You may want to read about topics such as Cell Cycle Checkpoints, how Cyclins operate and how the process is regulated by looking around on the web until I do come up with a post on this topic.
That is all from me as far as this post is concerned.