Laboratory Exercise 1:

							                           Microbiology Chapter 5 Worksheet:
                                   Mitosis & Meiosis

1. The Cell Cycle. The cell cycle can be divided into two major phases: interphase and the
   mitotic phase. Interphase accounts for about 90% of the cycle, divided into three phases: G1,
   S, and G2 phases. Although most of the time in an adult organism the cell are in G0 which is
   a quiescent phase. In this phase cells are busy doing their specific cellular work. Your adult
   cells require a signal to grow and divide, such as a hormone or a wounding signal. When this
   cell cycle is not regulated properly cancer is the result. Mitosis is relevant to microbiology
   for several reasons. First many protozoan and fungi use mitosis as a means of asexual
   reproduction (protozoans) or as a means of growth (fungi). Secondly, cancer is the second
   leading cause of death in the US and as future health care workers, PA’s, nurses, or doctors
   an understanding of cancer is obligatory.
   a.    Label the cell cycle diagram with these terms: interphase, G0, G1, G2, S, mitosis,
        anaphase, metaphase, prophase, telophase, cytokinesis. Identify the key events take
        place during G0, G1, G2, & S.




                                   Key event:
                                                     Key event:




                                                Key event:
2. Mitosis Models. Try to make your drawing from your knowledge and memory and do not
   use your text book. Mitosis is artificially divided into four phases, however mitotic divisions
   occur gradually, and the phases are not naturally distinct.
   a. Prophase. Prophase begins when chromosomes coil and condense. The centrioles (in
      animal cells) are visible as they move to opposite poles of the cell. Spindle fibers begin
      to form between the centrioles.
      i. What cellular material makes up the spindle fibers?
     ii. Describe the distribution of the chromosomes, which are now visible, during prophase.


    iii. Draw three pairs of duplicated chromosomes. Each chromosome should consist of a
         pair of identical chromatids, joined together at the centromere. Individual
         chromosomes can be different lengths, however sister chromatids must be the same
         length. Make the different pairs different sizes so you can identify between them.
         Draw your results in the “cell” below.




    iv. When did the chromosomes undergo duplication to create the sister chromatids?




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b. Metaphase. In metaphase, the chromosomes move to the “equator” of the cell
   (equatorial plate). The sister chromatids are still held together.
  i. Draw metaphase in the space below. Include the spindle apparatus in your drawings.
     Draw your results below.




c. Anaphase. The centromeres now begin to split, with the identical chromatids separating
   and moving to opposite poles.
  i. What is each of these chromatids now called?
 ii. Draw anaphase below. How does this cell elongate? ________________________
     _____________________________________________________________________.
     Separating the chromatids results in the formation of newly independent chromosomes.
     Draw your results below.




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   d. Telophase. In this phase, the chromosomes have now reached the poles, the nuclear
      envelope begins to reform around each bundle of chromosomes, and the chromosomes
      become indistinct. In animal cells, a cleavage furrow begins to form at the equatorial
      plate, and pinches inward until two separate cells are formed. This final process is
      known as cytokinesis, and overlaps with both anaphase and telophase.
     i. Draw telophase below for an animal cell.




3. Does each new cell formed by this process have the same or different chromosomal material
   as each other?                    Is the chromosomal material the same or different from
   the original cell?




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4. Meiosis. In sexually reproducing organisms, the genetic material is combined in the
   offspring. If gametes contained two sets of chromosomes (diploid or 2n) as most cells do,
   the offspring would end up with twice as many chromosomes as the parents after
   fertilization! To avoid this problem, many organisms have cells that undergo meiosis to
   reduce the chromosome number in half in the gametes (haploid or n). In microbiology two
   types of organism use meiosis as a means of reproduction in their life cycles, helminthes and
   fungi.
   a. What is the diploid number for humans?
   b. What is the haploid number for humans?
   c. To study meiosis, we will model the process using drawings. Start with six chromosomes
      (three homologous pairs). Duplicate these chromosomes. When would these
      chromosomes duplicate? ____________ This exercise has started with a large pair, a
      medium pair and a small pair of chromosomes. Homologous chromosomes have the same
      color, but a different shade of that color. You may also use solid and dashed lines to
      represent homologous chromosomes. Use colored pencils.



                Not Duplicated                                  Duplicated




5. Meiosis I.
   a. Prophase I. In this stage, crossing-over occurs. For simplicity this exercise will not
      include drawing the process of crossing over. However, be aware of its significance to
      genetic diversity. Crossing over is an exchange of genetic material between homologous
      chromosomes which allows for the production of hybrid chromsomes which contain both
      maternal and paternal DNA.




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b. Metaphase I. Late in prophase I, the homologous chromosomes move to the equatorial
   plate, entering metaphase I.
  i. Draw metaphase I, showing their alignment at the equatorial plate.




c. Anaphase I. To simulate anaphase I, separate each chromosome from its homologue,
   and move each toward opposite poles. DO NOT separate sister chromatids yet!
  i. Draw anaphase I.




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     ii. How do the chromosomes in anaphase I in meiosis differ from those of anaphase in
         mitosis?

   d. Telophase I. To simulate telophase I, move the chromosomes that you have separated
      completely to opposite poles. Two nuclei may now form, followed by cytokinesis (cell
      separation).
      i. How many chromosomes are in each cell?
     ii. Each chromosome has how many chromatids?
    iii. Draw the end products of telophase I




6. Meiosis II. These are the products of meiosis I. Now both of these cells will have to go
   through meiosis II, producing a total of four haploid cells. Pick one of these cells and draw
   the stages of meiosis II not including prophase II. Be sure to draw your products of
   telophase II.
   a. Prophase II. Centrioles move to opposite poles, the nuclear membrane breaks down, a
      new spindle forms.
   b. Metaphase II. Align the chromosomes for each of your two “cells” on the equatorial
      plate. Note that no replication has taken place during this process.




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c. Anaphase II. In this stage, the sister chromatids pull apart, becoming single stranded
   chromosomes which migrate toward the poles of the spindle




d. Telophase II. During this stage, the chromosomes arrive at the poles, the spindle breaks
   down, and the nuclear membranes reform around the chromosomes. Cytokinesis follows
   this second meiotic division.


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 i. You began meiosis I with one cell. How many do you now have?

ii. Is each gamete haploid or diploid?

iii. Is each chromosome within each cell duplicated or unduplicated?

iv. How many gametes would it take to form a 2n offspring?

v. Are the gametes alike or varied?




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