Cell Life Stages
Timing the Cell CycleThe average human cell that is capable of dividing has a life span of 20 hours. Most of these hours the cell is in interphase, a period of growth, metabolism, and DNA replication. As limits of cell growth are reached (remember volume:surface area ratios?), the cell begins mitosis, where the nucleus and the DNA within are divided up. The end result is two cells, each with a full complement of DNA, but half the size of the original cell. 1. Review proper microscope use! 2.Focus on a prepared slide of onion root tip, Find the area just behind the tip of the root; this is where the cells are dividing most rapidly. 3. Progress to higher power. Look for interphase cells with distinct nuclei and nucleoli. Count the number of cells in interphase in the entire field of view, or if there are too many to keep track of, a few rows/columns. Record. 4. Looking at the same field of view, look for cells in mitosis; they will have lost the nuclear membrane, and have DNA forming or formed into chromosomes. Count the number of cells in mitosis, and record.5. Have your partner move the slide and examine a different field of view. S/he should record, as you did, the number of cells in both interphase and in mitosis. 6. Repeat so you and your partner each have counted 2 fields of view. Sum your data to determine the total number of cells viewed, the total number in interphase, and the total number in mitosis. 7. Convert your data to percent, then degrees (% x 3.6), and graph as a pie chart. Follow-Up1. Assume these onion cells had, like human cells, an average life span of 20 hours. Base don your data how much time would be spent in interphase? ... in mitosis? Show how you know. 2. What key events happens in late interphase as the cell prepares for division? 3. A cell with 16 chromosomes prepares for mitosis. How many cells result at after mitosis? How many chromosomes per cell?4. Summarize the key events in mitosis. 5. Germ cells in ovary and testes produce gametes (sex cells) via meiosis rather than mitosis. Meiosis has two DNA/nuclear divisions. Why is this necessary? 6. The graphs below plot the changes in DNA of cells, and the changes in cells in a dividing population-2724166350 C B D AE n m i jWhich letters indicate a cell in homeostasis? Justify.DNA is increasing at which point on the graph? What process explains this change? Which letter represents mitosis? Justify. Which part of the cell cycle – interphase or mitosis - is longer? Use the graph to support your answer. Quantify the amount of change in cell DNA between A and C, and C and E. What is the pattern of the increase in number of cells over time? Cite data as evidence. Correlate the changes in the population size with the changes in cell DNA. ................
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