Lesson 6 - Cell Cycle
Student Performance Objectives
1. Diagram the cell cycle in a typical reproducing cell of the adult human body.
2. Explain the significance of the Go stage in cells like neurons and muscle fibers.
3. Explain the significance of checkpoints within the cell cycle.
A. The Cell Cycle - this is a cell's life cycle. Few if any cells in your body live as long as "you" do. So in each organ, populations of cells that do similar things are maintained by mitosis, as described in a previous lesson. Cells that undergo mitosis go through a life cycle that is typically a day or so although it can be much longer - days, weeks or months. For a cell dividing each 24 hours, like a stem cell, the cell may spend 1-3 hours dividing, and the remaining 21-23 hours doing other things it is programmed to do, which may be just preparing for the next division.
1. Some cells will never divide. All cells not dividing are said to be in a G0 phase. Examples of non-dividing cells follow. Skin cells that formed from the lower layers of the epidermis will not divide - they have desiccation and death as their ultimate fate and function - they will flake off your surface and protect and water-proof you in their final days. Cells compacted into your body hairs will not divide as they are dead and highly condensed. Cells lining your digestive, respiratory, and genitourinary tracts will not divide as they have a limited life span as they perform various protective, lining, secretory and/or absorptive functions prior to sloughing off from the surface as dead cells. A full 25% of your living cells are erythrocytes and they will not divide as they have no nucleus - they are simply carriers of respiratory pigments and gases. Another 25% of your cells, the leucocytes, will not divide as they only live for a few hours to a few days and are programmed to seek out and destroy invaders from without (microorganisms) and within (cancers). Most neurons of the brain and spinal cord and skeletal muscle fibers do not divide in an adult either.
2. Dividing cells are said to enter the G1 phase right after division. Dividing cells can be the stem cells of each tissue that give rise to the cells of that tissue. There are stem cells in the lower levels of the skin's epidermis and in all epithelial linings, in the bone marrow, and in the central nervous system.
a. During G1 the cell produces a adequate amounts of all organelles (note that the mitochondria have their own DNA and replicate themselves) and the cell does whatever it is genetically programmed to do. such as to be a liver or a kidney cell. Some stem cells have division (mitosis) as their main function so they just prepare the raw materials required for the next division. In this case of stem cells, the G1 phase lasts for 8-12 hours. In cells that have other functions than just division, G1 may last longer, even for weeks or months.
b. S phase occurs after the cell has completed its G1 activities. The cell duplicates its DNA and histone protein content during this phase. The cell now has two complete sets of its genetic heritage that can be distributed to daughter cells in the next mitosis. The actual replication of the cells nuclear DNA content involves at least three enzymes, two of which are now briefly described: a helicase that unwinds the double helix so that the next enzyme can get next to each DNA strand. DNA polymerase positions the nucleotides so that they can be correctly lined up and bonded to the growing, new DNA strand.
c. G2 phase occurs after the S phase and only lasts 2-5 hours. The cell makes last minute preparations for mitosis including the replication of the centriole. The cell then enters the mitotic or M phase.
B. Observe animations of the cell cycle on the web:
C. Draw cell cycle on the board and discuss meaning of G1, G2, S, M, and G0.
D. Discussion of the Cell Cycle's checkpoints