What is the cell cycle?

"MITOSIS, CYTOKINESIS, AND THE CELL CYCLE by Neural Academy (Aug, 2019)"

Image: Simple cell cycle image, Image by Simon Caulton, Sourced Under a Creative Commons 4.0 License from Wiki Commons The cell cycle can be described as the ordered sequence of events in the life of a cell, beginning from when the cell is formed from its parent cell and is then completed with its own division. The cell cycle takes place between cell divisions, and the whole process collectively comes under interphase.

What are the phases of the cell cycle?

1. G1 phase: This first growth phase is a period of metabolic activity and growth, in which new proteins are made. This phase ends when DNA begins to replicate. 2. S phase: This phase, otherwise known as the 'synthesis' phase, is when DNA replication takes place. The cell synthesises a complete copy of DNA in the nucleus, and centrosomes are duplicated. 3. G2 phase: In this second growth phase, the cell grows larger, creating more proteins and organelles. The cell reorganises its contents in preparation for mitosis, the 'M' phase to occur. 4. M phase: This phase is only a small part of the whole cell cycle, and is where cell division actually occurs, which forms new daughter cells. This phase also contains the process of cytokinesis. Extra: G0 phase: This phase includes cells which are in a non-proliferating state. The cell is undergoing an extended G1 phase, but is not preparing to replicate its DNA for cell division to occur. There are some important things to note regarding the cell cycle. In reality, the cell cycle is a continuous process which is divided into phases for simplicity. The length of the cell cycle is also varied depending on the cell type. During development and in area of high wear-and-tear, such as the skin, cells divide at a faster rate. It's also important to note that some cells do not divide at all, therefore not undergoing the cell cycle, for example nerve and retinal cells.

What is mitosis?

Image: Mitosis cells sequence image, Image by LadyofHats, Sourced Under a Creative Commons 4.0 License from Wiki Commons

For many students, mitosis is one of those topics you've done to death. However, it is essential to know for your exam, as it could come up in an extended response or as a short answer question. Mitosis is what takes place in the 'M' phase of the cell cycle, where cell division of somatic cells occurs. Cells in the body undergo mitotic divisions for two primary purposes; growth and repair. The result of mitosis is two genetically identical daughter cells, which will go on to perform mitosis over and over again.

There are four phases of mitosis.

Prophase
Metaphase
Anaphase
Telophase

These can be remembered as 'PMAT' or 'IPMAT' if you include interphase (which isn't a part of mitosis!).

Prophase

In prophase, chromatin threads in the nucleus condense and coil up tightly to become chromosomes, which are two sister chromatids held together by a centromere.
The nuclear membrane disintegrates, and the nucleolus disappears.
Mitotic spindle forms from the centrioles and spindle fibres attach to each chromosome at its centromere.
Two centrosomes that contain two centrioles move to opposite poles of the cell.

Metaphase

In metaphase, chromosomes move to the centre of the cell and line up on the cell's equator (note: the equator may also be called the metaphase plate).
The centromeres of the chromosomes are aligned together on the equator.
The centrioles are located at opposite poles of the cell.

Anaphase

During anaphase, the spindle fibres shorten and pull on the centromeres.
The chromosomes separate into sister chromatids and are pulled to opposite poles.
At the end of anaphase, each pole has an identical set of maternal and paternal chromosomes as DNA replication occurred during interphase.

Telophase

In the last stage of mitosis (telophase), chromosomes decondense (uncoil) to form chromatin.
Two new nuclear membranes form, one for each new daughter cell.
The nucleoli reappear, and the spindle apparatus disappears.
The cell elongates, and a cleavage furrow (the dip between the (almost) 2 new cells) to become ready for cytokinesis.

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