LABORATORY #9: MITOSIS LAB
Required Materials:
1. Mitosis slide of Allium
2. Mitosis slide of a whitefish blastula
3. Plant and animal mitosis models
Introduction
Cell division is the process whereby one cell, the mother cell (or parental cell) divides and produces new
daughter cells. Cell division can be classified as either: mitosis or meiosis. Mitosis occurs in the somatic
cells of the body and results in 2 daughter cells, each with the same number of chromosomes as the
parental cell. So if a parental cell has 46 chromosomes, mitosis will produce 2 daughter cells each with
46 chromosomes. Plants and animals use mitosis for growth and replacement of old cells.
Meiosis, on the other hand, takes place only in germ cells - those cells that give rise to the gametes of
reproduction. In animals, meiosis gives rise to gametes used in sexual reproduction. In plants, meiosis
produces spores that will develop into plants (i.e. gametophytes) that will bear structures responsible
for producing gametes and undergoing sexual reproduction. In meiosis, the parental cell undergoes two
rounds of mitotic-like division to produce 4 gametes, each with half the number of chromosomes as the
parental cell.
Cells do not undergo mitosis whenever they like. Dividing cells adhere to a strict “timetable” known as
the cell cycle. The cell cycle can differ in timespan depending on the cell type, but it is comprised of the
same 4 stages in all dividing cells: G1SG2M (for mitosis). The G1, S and G2 phases exist to help
prepare the cell for mitosis. During G1, the cell prepares to copy all of its genetic information (i.e. DNA).
During the S phase, DNA replication takes place. During the G2 phase, the cell prepares for mitosis,
including duplicating its organelles and centriole and making the proteins critical for mitosis. Together
the G1, S and G2 phases are called Interphase.
Depending on the sample you are looking at, the number of condensed chromatin strands known as
chromosomes can differ. In Ascaris, there are 4 chromosomes, 8 in Drosophila, 16 in Allium, 24 in
Lillium, 46 in humans and 80 in Whitefish. You will be looking at Allium and Whitefish chromosomes
under the microscope and identifying the following mitotic phases: Prophase, Metaphase, Anaphase
and Telophase. In addition, you will also be asked to look for Cytokinesis – the division of the cytoplasm
and, hence, the cell into two.
In prophase, the nucleus and nucleolus disappear and the chromatin supercoils into chromosomes.
Since the DNA has been duplicated in the S phase prior to the onset of mitosis – these “duplicated”
chromosomes are comprised of two sister chromatids joined at a centromere. The spindle also forms
during prophase and becomes visible between the two centrioles, located at opposite ends of the cell.
The chromosomes will attach to the spindle toward the end of prophase
In metaphase, the chromosomes line up at the metaphase plate – basically the ‘equator’ of the cell. The
spindle fibers that attach to the chromosome are called centromeric spindle fibers or kinetochore fibers
(named for the kinetochore which attaches the centromere to the spindle).
In anaphase, the cell elongates and the two sister chromatids are pulled apart and guided by the spindle
fibers toward the opposite ends of the cell.
In telophase, a lot of the events of prophase are reversed. The chromosomes at each pole decondense
back into chromatin. When the cell enters its S phase, these single chromatids will be duplicated to
produce the two chromatid chromosomes we find at prophase. The nucleolus and nuclear envelop
reappear around the decondensing chromatin. Finally the cytoplasm gets divided up between the two
cells through cytokinesis. In animal cells, a cleave furrow forms in the middle of this elongated cells and
pinches off two daughter cells. In plant cells, a cell plate forms down the middle of the telophase cell,
dividing the two daughter cells.
Before you procede with this lab, refer back to you lecture notes so you are familiar with the forms of
chromatin, what a chromosome is, what a chromatid is, what a duplicated chromosome is. Also remind
yourself of the functions of the centrioles, microtubules and spindles.
Procedures:
1. Phases of mitosis in animals and plants: Observe prepared slides of Allium and Whitefish
blastula. Identify the 4 stages of mitosis plus cytokinesis. Draw and label what you see for each
of the phases. Refer to your textbook to determine what structures are important and should be
identified, drawn and labeled.
2. Mitosis models: Observe the commercial models we have representing the stages of mitosis.
Compare what you see in these models with what you have seen under the microscope and
what you have drawn. Are you missing anything in your drawings? If so, go back and look at your
slides again to see if you have just missed it.
3. Relative frequency and time-length for mitotic phases:
a. Observe either Allium or Whitefish blastula under the 100X oil immersion lens. Count
the cells you see in interphase and the number of cells you see in each stage of mitosis.
b. Divide the total number of cells you counted in the field of view into the number of cells
in interphase, prophase, metaphase, anaphase and telophase
c. Assume interphase and mitosis occur over a 20 hour time span. Calculate the relative
frequency of interphase and each phase of mitosis.