Lab Mitosis

Name _____Katherine Nowlin_________________________________ Date __October 28, 2009_ AP Biology Core Lab: Mitosis Observations Stage of Cell Cycle Labeled Drawing Major Events Occuring

Interphase

• • •

Nucleus distinct Nuclear envelope distinct Chromosomes are threadlike chromatin

• •

Chromosomes are condensed Nuclear envelope is not apparent

• •

Chromosomes are thick and coiled Chromosomes are lined up in the middle on the metaphase plate Spindle fibers are now attached to the chromosomes

Mitosis

Prophase

Metaphase

•

• •

The chromosomes are now separated The chromosomes are now moving towards the poles

•

The chromosomes are becoming more diffuse The chromosomes are at the poles Nuclear envelope is reforming Cytoplasm may be dividing now

Anaphase

Telophase

• • •

- division is now complete into two daughter cells Cytokinesis

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Results Table I Phase Interphase Prophase Metaphase Anaphase Telophase Total

Number of Cells 11 3 1 1 3 19

Time in Minutes 835 230 72 72 230 1440

Table II

212

Number of Cells Field 2 Field 3 Card # Card # _11___ __21___ 43 70

15

12

3

Percent of total cells counted

Time in minutes of each stage

325

82.7

1169.3

23

50

12.7

179.6

3

2

8

2.0

28.3

2

2

1

5

1.3

18.4

2

1

2

5

1.3

18.4

Field 1 Interphase Prophase Metaphase Anaphase Telophase

Total Cells Counted

Total

393

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Analysis 1. Contrast animal and plant cell division. In animal cells, a cleavage furrow forms during telophase whereas in plants, a cell plate begins to form, which becomes the cell wall of the new cells. In animal cells, there are centrioles that create spindle fibers that pull the sister chromatids apart, on the other hand, in plants, the fibers grow on their own without centrioles.

2. What is the restriction point in mammalian cells? The restriction point is a G1 phase checkpoint (control mechanism during the G1 phase) during the cell cycle. Once a cell progresses through this point, it is committed to enter the S phase (where DNA synthesis and replication will occur). Should the cell not be ready (for reasons such as lack of growth factors) or the conditions are not suitable for the cell to continue into the S phase, then the cell may enter the G0 phase (dormancy). Mutations in the factors contributing to cell cycle arrest at the restriction point are thought to be one of the main contributors to cancer.

3. What is the G0 phase of the cell cycle? How and why do cells enter this phase? The G0 Phase is a period in the cell cycle where cells exist in an inactive state. This phase is viewed as either an “extended” G1 phase (where the cell is neither dividing nor preparing to divide) or as a distinctive stage that occurs outside of the cell cycle. During this phase the cell cycle “machinery” is taken apart and the cyclins and cyclin-dependent kinases vanish and cells then remain in the G0 phase until there is a reason for them to divide. Cells enter the G0 phase from a cell cycle checkpoint in the G1 phase (restriction point in animal cells or the start point in yeast cells) because of either a lack of growth factors or nutrients.

4. What are MPFs, Cdks and cyclins? What is the relationship among these molecules in controlling the cell cycle? MPF stands for mitosis-promoting factor, which is a protein complex that is responsible for triggering mitosis in somatic (body) cells as well as for maturation of oocytes into egg cells. It works by catalyzing the phosphorylation of proteins that in turn bring about the events of mitosis such as the formation of the mitotic spindle, the breakdown of the nuclear envelope, and the condensation of chromosomes. It consists of cyclin B (bound to a cyclin-dependent kinase). Levels of MPFs rise as the cell enters mitosis, reaching a peak during mitosis, and then falling during anaphase. Cyclin-dependent kinases (CDK) are in a group of protein kinases that are involved in the regulation of the cell cycle. It is also involved in the regulation of mRNA processing and transcription. CDKs phosphorylate proteins and are activated by connection with a cyclin. Cyclins are the family of proteins that control the progresson of cell through the cell cycle which work by activating the CDK enzymes. Thus, the MPFs are attached to the CDKs, which are activated by cyclins.

5. Can a cell that is normally non-dividing be stimulated to divide? When does this happen and how is it accomplished in an organism? Cells can by stimulated to divide because of a demand for cells. For example, although liver does not usually divide, when part of one’s liver is taken out, it will go through cell division to replace the missing liver. This is stimulated by extracellular signals from cytokines, which are substances that are secreted by specific cells of the immune system that carry signals between cells. They are signaling molecules, or growth factors, (proteins, peptides, or glycoproteins) that are used in cellular communication.

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6. Review the data below for time spent in stages of the cell cycle in normal and cancerous chicken stomach cells. Interphase Prophase Metaphase Anaphase Telophase Total time Normal 540 60 10 Chicken 3 mins 12 mins 625 mins mins mins mins Stomach Cells Cancerous 75 15 Chicken 2 mins 1 minute 3 mins 96 mins mins mins Stomach Cells Determine the time of the cell cycle for both normal and cancerous cells. How does it differ? The normal chicken stomach cell cycle, which took 625 minutes, is much longer (529 minutes longer) than the cancerous chicken’s stomach cell cycle, which took 96 minutes.

7. What percent of time does a normal cell spend in interphase? A normal cell spends 86.4% of the cell cycle in interphase.

8. What percent of time does a cancerous cell spend in interphase? About 78.1% of a cell’s life is spent in interphase when the cell is cancerous.

9. In two sentences, how does Neil Campbell (god of AP Biology) describe the abnormalities of a cancer cell? Cancer cells do not respond normally to the body’s control mechanisms, divide excessively, and invade other tissues. Cancer cells do not heed the normal signals that regulate the cell cycle, thus they do not stop dividing when enough cells are present and/or when growth factors are depleted.

10. Cancer cells do not respect their neighbors. Describe what this means in more technical terminology. How is this different than normal cell functioning? Cancer cells do not respond to signals to stop division process, but just continue on and on. In the body, cancer cells aggressively invade and kill their neighbor cells and often outgrow their own blood supply, thus killing even themselves. Some cancers produce abnormal or excessive signaling chemicals such as hormones or cytokines that upset the chemical balance of the body and they have altered metabolic pathways which makes them ravenous and inefficient users of nutrients that cause them to act as competitors with normal cells. In an artificial cell culture the cells will grow so much that they will pile up on each other and eventually suffocate. On the other hand, normal cells will grow until they form a continuous layer, and then they will chemically signal each other to stop replication.

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