The Cell Cycle and Mitosis
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One of the tenets of biology is that living things reproduce like from like, that is bacteria produce more bacteria, frogs produce more frogs, and golden retrievers produce more golden retrievers. Today we will discuss one strategy for carrying out this like-from-like reproduction and then look more closely at the cellular mechanics involved.
Think about Dr. Cullens analogy of the cell as a house. Lets expand that analogy, thinking about a house that is self-replicating - it contains all of the information and construction machinery necessary to build another. That information, encoded in DNA, is analogous to books, with each book representing a chromosome. Some cells have a single book, while others have large collections of books and special libraries (nuclei) to house them.
II. The Cell Cycle
When the cell is not dividing, the DNA in the nucleus (the genome) is in use, being read by the cell's gene expression machinery. This is the interphase part of the cell cycle. Figure 12.4 shows that interphase is the longest part of the cell clycle. Mitosis, or nuclear division occupies a very small part of the cycle. During mitosis, the DNA is packaged into compact units in order to facilitate its transport across the dividing cell.
It is easy to see the large structures (chromosomes) moving during mitosis, while during the rest of the cycle the cell can appear to be quiescent. Appearances are deceiving. Most of the important cell cycle events occur during interphase. DNA is duplicated during the S phase - chromosomes are duplicated to a pair of sister chromatids. Remember that DNA is in the extended conformation at this time.
III. Mitosis - Nuclear division
During mitosis, or nuclear division, the cell gathers its chromosomes and assorts them into the daughter cells. You should be familiar with the stages of mitosis and the events that occur in each stage. Refer to Figure 12.5. Please complete the tutorials at your text website and at The Biology Project.
IV. Involvement of the Cytoskeleton in Cell Division
A. The mitotic spindle is composed of microtubules and associated proteins. This apparatus first aligns the chromosomes and then moves them to the daughter cells. Kinetochore enzymes depolymerize microtubules. (Figure 12.7)
B. Cytokinesis in animal cells involves actin microfilaments, while cell plate formation in plant cells involves the construction of a new cell wall.
V. Cell Division and Cancer
In an adult multicelled organism like a biology 107 student, most cells are in the G-zero stage of the cell cycle. The cell does not divide unless specifically signaled to do so (see Figure 11.11). Once the cell has made the decision to divide, the timing of cell cycle progression, from G1 to mitosis, is regulated by moilecular control systems. One example of such a control system is the G1/S checkpoint - cells will not begin DNA replication without the resources to complete the process. The cell also maintians tight control over the fidelity of replication.
In addition to the G1/S restriction point and the checks on the fidelity of replication, there are other cell cycle regulatory points. One is the assembly of MPF, or mitosis promoting factor. Cyclins come in a number of "flavors" whose presence or absence can denote the phase of the cell cycle or a specific event that has occurred. CDKs are switching molecules that use the current population of cyclins to decide what is next for the cell. See Figure 12.14.
Failure of any of these controls usually leads to a cell that cannot divide, and is dead. Sometimes, though, a mutation in a cdk or cyclin or other cell cycle control factor (there are hundreds of genes and proteins involved in this process) can lead to uncontrolled cell division - cancer.