Stages of the Cell Cycle - Mitosis (Interphase and Prophase)
The Mitotic Cell CycleClick thumbnail to view full-size
Why Do Cells Divide?
There are two methods of cell division: mitosis and meiosis. In brief, mitosis is the dividing of one cell into two, genetically identical daughter cells; meiosis is the dividing of one cell into four genetically different daughter cells.
All organisms need to produce genetically identical daughter cells. Single celled organisms use this method to reproduce - each of the produced cells is a separate organism. For multicellular organisms, there are three main reasons why cells divide:
- Growth - multicellular organisms can grow in two ways, increasing the size of their cells, or increasing the number of cells - achieved through mitosis.
- Repair - when cells are damaged, they need to be replaced with identical cells capable of doing exactly the same job.
- Replacement - no cell lasts forever. Even the most long lived of cells will need to be replaced at some point. Red blood cells only last three months, skin cells even less. Identical cells are needed to carry on the functions of the cells they are replacing.
This hub will focus on the stages of mitotic cell division. This is divided into four major sections separated by a fifth: Interphase, Prophase, Metaphase, Anaphase and Telophase. Just remember: I Pee on the MAT
Cell Cycle and Mitosis
The terms 'Mitosis' and 'Cell Cycle' are not synonymous. The somatic cell cycle is the name given to the series of events that occur as one cell divides into two cells that are genetically identical both to each other and to the parent cell , which then grow to full size. Even rapidly dividing cells spend only a small percentage of their existence dividing. The cell cycle proper is split into:
- Growth Phase, where normal cellular processes take place and the cell grows to full size.
- Interphase, where the DNA is replicated.
- Mitosis, where the nucleus divides and sister chromatids are separated
- Cytokinesis, where the cytoplasm divides.
There is a very good reason why mitosis occupies such a small proportion of the cell cycle. Copying the information carried by the DNA in a human cell is 'roughly equivalent to copying out, in full, the unabridged Encyclopaedia Britannica (that's 30 volumes by the way)...20 times...making no mistakes*. The rest of the cell cycle is devoted to copying the DNA, checking this process, and growth.
Ok, that last bit is an over-simplification, but you would at least rectify these mistakes, or the mistakes would not effect the meaning of any word. Well sometimes it would, but only once every few tends of thousands of copies.
Simply put, the time between mitoses (sing. mitosis) is known as interphase. This is further broken down into G1, S and G2.
During G1 (Gap 1), the cellular organelles and cytoplasm, including important proteins and other biomolecules, are duplicated. S (Synthesis) Phase is the point at which DNA is replicated. G2 (Gap 2) is spent double checking that no errors have been made during DNA replication.
Checkpoints exist between each of these phases, ensuring that the cell cycle does not progress from one phase to the next until the cell is ready to. If too many errors have been made (during DNA replication for example) then 'guardian' proteins such as p53 are responsible for preventing the cell cycle from moving forward until the error is corrected. In extreme cases, the cell is written off and cycling is halted (G0) or the cell self destructs (Apoptosis). Where these guardians malfunction, cancer often ensues.
Most of the time, DNA is tightly coiled and structured around proteins called histones. This packaged form is known as chromatin. The first stage of mitosis sees this chromatin supercoiling from their operational width of 30nm, to the 500nm thickness associated with chromosomes. (Chromatin cannot perform its normal function in the cell, so it cannot stay supercoiled for long - another reason why mitosis is a short series of events.)
In brief, the events of prophase are as follows. Please note that this is not a sequential list, as the order depends on the cell, species and ambient conditions:
- Replicated chromosomes supercoil - can be seen as consisting of a pair of sister chromatids
- Nuclear envelope breaks down and Nucleolus disappears.
- Centriole (animal cells only) divides and each copy migrates to the poles of the cell.
- Fibres start to move out from the polar centrioles forming a structure called the spindle.
Events of Mitosis
The DNA has been replicated, the chromosomes are now visible, the towing machinery has been deployed. The next section looks at the nitty-gritty details of mitosis, metaphase, anaphase and telophase.
Where Next? Cell Cycles
- Checkpoints and Cell Cycle Control
A wonderful animation from Harvard looking at the control of the cell cycle. Highly recommended.
- A-level Biology The Cell Cycle
A basic, but thorough, look at the cell cycle. Aimed at A-level students and provides a strong foundation. An excellent revision resource too!
- Cell Cycle: An Interactive Animation
An interactive animation illustrates activity as cells grow and divide.