What evidence shows that mitosis is a continuous process rather than a series of separate events?

MITOSIS AND MEIOSIS

One of the tenets of cell theory is that all cells come from pre-existing cells. All individual

organisms begins with one cell, and yet in multicellular organisms the number of cells in the

adult may be in the trillions. This requires cells to repeatedly divide during the life of an

organism.

The average adult human body is made up of about 37 trillion cells. Of these, approximately 50

billion are fat cells and 2 billion are heart muscle cells. By the time you finish reading this

sentence, 50 million of your cells will have died and been replaced by others. Human cells are

estimated to divide nearly 2 trillion times every day. Amazingly, humans contain at least 10

times as many bacteria cells as human cells. The 100 trillion bacterial cells are much smaller

than human cells and have a faster generation time.

Mitosis and meiosis are two processes that produce new cells through cell division, which occurs

as a part of the cell cycle. The new “daughter” cells produced by these processes are quite

different because they have different purposes. These differences occur because the processes

have several key differences as outlined in the video lecture. You will be doing several lab

activities examining mitosis and meiosis and what can happen if problems occur during these

cell division processes.

Why are we doing this lab?

1. To gain a better understanding of the mitotic and meiotic processes of cell division that occur in humans and all other animals.

2. To examine how issues in mitosis and meiosis can lead to diseases and disorders in humans.

CBIO Lab: Mitosis and Meiosis p. 2/10

Background: Phases of mitosis

For each phase, draw and label:

a. Chromatin or chromosomes b. Centrosomes c. Microtubules/spindle d. Cell membrane

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Activity 1: Mitosis under the microscope

1. Use Google images of mitosis (Google “mitosis of onion root tip”) to identify cells in interphase and all phases of mitosis.

Cells in…Interphase will have chromatin, not distinct chromosomes

Prophase will have distinctly visible chromosomes

Metaphase will have chromosomes lined up along the equator of the cell

Anaphase will have chromosomes separating at the centromeres

Telophase will have chromosomes decondensing into chromatin and a cell wall

(plant cell) or cleavage furrow (animal cell) forming

1. Observe the box-like cells that are arranged in rows. The cells have been stained to make the chromosomes of the cells clearly visible. Make sure to focus on the larger cells just

above the root cap of many new small cells.

2. Select a cell whose chromosomes are clearly visible and sketch it in the first box below.

3. Look around at the cells again. As you look around, you may notice that some cells appear to be empty (no dark nucleus or visible chromosomes). Ignore these – cells are

three dimensional and we are only looking at thin slices that may not include genetic

material. Select four other cells whose internal appearances are different from each other

and the first one that you sketched. Sketch them in the boxes below.

4. Using your notes, identify the stage of each of your 5 cells in Fig. 1 and write it below the cell.

Cells in…Interphase will have chromatin, not distinct chromosomes

Prophase will have distinctly visible chromosomes

Metaphase will have chromosomes lined up along the equator of the cell

Anaphase will have chromosomes separating at the centromeres

Telophase will have chromosomes decondensing into chromatin and a cell wall

(plant cell) or cleavage furrow (animal cell) forming

Fig. 1. Your drawings 5 different onion root cells with different internal appearances

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Analysis (answer each question in the space provided)

1. What is the data of mitosis? Explain how you know.

2. What is the theory of mitosis? Explain how you know.

3. What evidence shows that mitosis is a continuous process rather than a series of separate events?

4. The onion plant began as a single cell, as do humans. If the cell had X number of chromosomes (the exact number doesn’t matter), how many chromosomes are in each of

the cells that you observed? Give your answer in terms of X. How do you know?

5. If the onion plant then went on to reproduce sexually, it would need to produce gametes by the process of meiosis. After meiosis, how many chromosomes would be in each cell?

Give your answer in terms of X. How do you know?

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6. If the gametes of this onion plant then completed the process of fertilization, how many chromosomes would be in the resulting zygote? Give your answer in terms of X. How do

you know?

7. What would happen if the process of mitosis skipped anaphase? How many daughter cells would result? How many chromosomes would be in the resulting daughter cells?

Give your answer in terms of X.

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