Lecture 1: The Cell Cycle
- All living organisms comprised of cells (approximately 2 x 10^14 cells in the human body = over
200 trillion cells)
- All cells derive from pre-existing cells
- Organisms may be single celled (such as amoeba or bacterium) or organisms may be multicellular.
1) Structure (a liver cell will have a different structure from a nerve cell)
2) Function (cells carry out different functions)
3) Reproduction (cell division)
4) Cell Cycle (regulation of division and function).
*Refer to Figure 6.8.
The purpose of cell division is for:
1) The reproduction of new cells
2) Growth and development of organisms
3) Repair of damage of tissues.
Take note that cell division must give rise to genetically identical “daughter” cells through division of the
nucleus in the process of mitosis. Similarly, the process of cytokinesis is the division of the cytoplasm.
- The simplest form of cell division is found in bacteria, where binary fission takes place. This can occur
between minutes to hours.
* Refer to Figure 12.12.
- The cell cycle occurs in multicellular organisms. Mitosis is merely a small portion of this.
- There are four important phases in the cell cycle. These are as follows:
1) G1; or the first gap phase. Cell growth occurs here.
2) The „S‟ phase. DNA synthesis (DNA replication) occurs here.
3) G2; or the second gap phase. Cell division occurs here.
4) The „M‟ phase. Mitosis and cytokinesis occur here.
- G1 + S + G2 combined create interphase, while the „M‟ phase represents mitosis and cytokinesis. - In a 24 hour span, the G1 phase will take approximately 5-6 to complete. The S phase will take
approximately 10-12 hours, the G2 phase will take 4-6 hours, and the M phase will take about 1
hour to finish.
*Refer to Figure 12.6.
1) Prophase – Chromosomes appear with sister chromatids (Remember that the DNA has already
been copied at this point). The mitotic spindle, which is merely microtubules, begins to form in the
2) Pro-metaphase – The nuclear envelope breaks down, and the mitotic spindle begins to organize at