cell divisionnoworksheets15

8/19/2019 Cell Divisionnoworksheets15

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ABOUT CELL DIVISION

Chromosomes are thread-like structures that assume a compacted form and become visibleduring cell division. A single chromosome may consist of either one or two chromatids,depending on the stage of the cell cycle. If there are two chromatids, they are held together

at the centromere and called sister chromatids.

Sexually reproducing organisms reproduce via the fusion of two gametes – eggs and sperm -that each contain one set of chromosomes. The resulting fertilized egg has two sets ofchromosomes – one set from each parent. The number of sets of chromosomes in a cell arecalled its ploidy or n-number. A fertilized egg is 2n, a gamete (e.g. sperm) is 1n. Some cellsspontaneously replicate entire sets of chromosomes or form from the fusion of more than twocells. These cells can be 3n, 4n, or even 7n. Chromosomes that are similar (that is, carryinginformation for the same trait) but that are from different sets are called homologouschromosomes.

In the process of mitosis, two daughter cells are produced during cell division and will have

identical sets of chromosomes. During mitotic prophase the chromosomes, which havereplicated during interphase, shorten and become thicker. At metaphase they line up on thespindle equator (=metaphase plate); at anaphase the centromere regions divide and pullapart, and one chromatid from each metaphase chromosome subsequently goes to eachpole.

In the process of meiosis, replicated chromosomes shorten and become thicker andhomologous chromosomes pair up during prophase I. At metaphase I the homologouschromosomes line up on the metaphase plate. During anaphase I the homologous pairs pullapart (no centromere splits) and sets of sister chromatids go to each pole.

After telophase I and cytokinesis (division of the cell itself), the chromosomes in each of the

two subsequent cells line up on the metaphase plate again (metaphase II). Each of theresulting cells now has replicated chromosomes (sister chromosomes), but the chromosomesare not homologous to each other. During anaphase II the centromeres divide and pull apartand one chromatid from each set of sister chromosomes migrates to each pole. Cytokinesisagain takes place in telophase II, resulting in the formation of four daughter cells from theoriginal one. Each daughter cell (gamete) has half the number of chromosomes as theoriginal cell. This ensures that the subsequent fusion of two gametes (egg and sperm) willproduced a fertilized egg with the correct number of chromosomes for that organism. Inmeiosis, daughter cells do not have identical sets of chromosomes.

Note: a cell going through meiosis will potentially produce four daughter cells. However, inspecies with separate sexes, meiosis in females results in only one viable egg cell. The

remaining three cells, called polar bodies, degenerate.


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PROCEDURES: You will work in pairs

1. Animal cell mitosis

Obtain a microscope. Obtain a whitefish blastula slide. The blastula is a very earlyembryonic stage that is composed of a solid ball of rapidly dividing cells. Your slide containsseveral cross-sections of that ball of cells.

Use the high dry objective on your microscope to look at cells in each of the phases ofmitosis. Consult your instructor if you wish to use the oil immersion objective. Just as whenyou randomly slice a hard-boiled egg, you may or may not slice the yolk; some of the blastulacell cross-sections do not show any nuclei or chromosomes. Each slide, however, shouldhave plenty of cells with good views of chromosomes.

! Can you see the spindle fibers in any of the dividing cells? Which stages?

How could you distinguish between cells in interphase and cells in prophase?

How can you distinguish between cells in telophase and cells that have completedcytokinesis?

Use the spaces below to sketch each cell stage.

Interphase Prophase Metaphase

Anaphase Telophase


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2. Plant cell mitosis

Obtain a slide of an onion root tip. In plants, cells at shoot tips and root tips are constantlydividing. Locate this area of rapid cell division just above the root cap.Use the high dry objective to find cells in the process of actively dividing. As with thewhitefish blastula, many cells will be in interphase. The nucleus of root tip cell in interphase

is very large, stained red or purple and usually contains one or two nucleoli. Don’t confusethis with the entire cell! The root tip cell is a greenish color and is rectangular.

Locate cells in each of the stages in mitosis.

! How can you distinguish between interphase and prophase in plant cells?

How can you distinguish between telophase and two adjacent cells that have just gonethrough cytokinesis?

Use the spaces below to sketch each cell stage.

Interphase Prophase Metaphase

Anaphase Telophase


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3. Determining duration of stages of mitosis

Cells in onion root tips take approximately 90 minutes to divide. Although we cannot timeeach stage of mitosis in this lab, we can approximate the duration of each stage bydetermining what percentage of each stage is present in the onion root tip slides. Each slide

is a “snapshot” of cell activity. Stages that take up a greater proportion of the cell cycle willshow up more often in the slide “snapshots.”

• Given what you know about the stages of mitosis, which of the stages of mitosis doyou think takes the longest to complete? Which takes the least time to complete?(Remember, the stages of mitosis do not include interphase.) Write your hypothesesbelow.

Procedure1. Start at one end of your onion root tip slide, and work your way across the slide,

counting the numbers of cells in prophase, metaphase, anaphase, and telophase (not interphase). Keep track and record the numbers of actively dividing cells until you’vecounted a total of 50 cells. The easiest way to do this is for one lab partner to call outthe stages as he or she sees them and have the other lab partner record them ashatch marks in each category below. You will probably have to look at all three slicesof onion root tips on your slide and perhaps even go to another slide to get a total of50 actively dividing cells.

2. Calculate the percentage of each stage by multiplying each number by 2. To

determine the average number of minutes each stage takes – multiply yourpercentage by 90 minutes.

3. Record your results on the board and compare to your hypothesis – how did you do?

Prophase Metaphase Anaphase Telophase

cell divisionnoworksheets15

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