Download Mitosis

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Mitosis and Meiosis
Mitosis is the process of cell division that results in each daughter cell having an
exact copy of the DNA found in the mother cell. Cell division can actually be divided into
two stages, Interphase and Mitosis. Interphase is usually the stage that most cells are in.
During interphase, cells do the tasks that they are designed to do. For example, pancreas islet
cells produce and release insulin into the blood stream during interphase. Interphase can be
divided into three stages: 1) Gap 1, usually the longest stage during which proteins are made
and the cytoplasm typically doubles in size; 2) Synthesis, during which an identical copy of
the DNA found in the nucleus of the cell is made; and 3) Gap 2, a usually brief stage before
cell division actually starts. Without the production of an exact copy of the DNA found in
the nucleus of the cell during interphase, mitosis would not result in two cells with identical
DNA.
Mitosis is typically divided into 4 stages or phases: 1) Prophase, the stage when DNA
is condensed and packaged; 2) Metaphase, when condensed chromosomes line up along the
equator of the cell; 3) Anaphase, when one copy of each chromosome goes to each pole of
the cell; and 4) Telophase, when new nuclear membranes are formed around the
chromosomes and cytokinesis occurs resulting in two daughter cells.
Meiosis resembles mitosis, but results in daughter cells with half the genetic
information of the mother cell. This process occurs only in the gonads and is how the
gametes, sperm and eggs, are made. Meiosis is actually two divisions, the second of which is
identical to mitosis. The net product of these two divisions is 4 cells. In sperm, the
cytoplasm is divided evenly resulting in 4 small haploid cells while in egg production
cytokinesis is uneven resulting in one big haploid cell and 3 very small haploid cells. The
critical difference between meiosis and mitosis occurs during the first division of meiosis,
called meiosis I. During prophase of meiosis I homologous chromosomes, chromosomes
having the same genes, come together and exchange genetic information. Remember that
while homologous chromosomes may have the same genes, the one you got from your
mother may have different alleles of some genes than the one from your father. Thus, slight
differences exist between homologous chromosomes. These homologous chromosomes line
up side by side along the equator of the cell during metaphase I and one homologous
chromosome travels to each pole during anaphase I. The result at the end of meiosis I is that
the two daughter cells are haploid, having half the genetic information of the mother cell. As
was previously mentioned, meiosis II is essentially mitosis on the haploid daughter cells
produced by meiosis I.
Exercise 1: Onion root tip
The best places to look for cells undergoing mitosis are areas of rapid growth. Root
tips are consequently ideal places to look. Your instructors have started onions roots
growing by placing them in water four days before the start of this laboratory.
1.
Examine and make detailed drawings of the prepared slides of onion root tips that are
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available on the front bench. This will help you know what you are looking for. Try
to find cells in each stage of mitosis.
WARNING: Hydrochloric acid (HCl) is very dangerous and will cause severe burns if
it gets on your skin or clothing. If HCl gets on your skin, flush with lots of water
IMMEDIATELY and inform your instructor. If it gets on your clothing, remove the
clothing IMMEDIATELY and flush the skin underneath with water.
2.
Cut off the last 1 cm of a growing onion root tip, place it in a watch glass and cover it
with 1 M HCl. Leave it in the HCl for 5 min. or until it is soft.
WARNING: Aceto-carmine is a dangerous poison that may cause mutations resulting
in cancer or birth defects. Wear gloves when handling it and wash your hands after
finishing with it. If it gets on clothing or skin, wash immediately with lots of soap and
water.
3.
Transfer the softened root tip to a slid and cover it with 1 drop of aceto-carmine stain,
then cut it up into small pieces with a razor blade. The iron in the razor blade helps
the stain work so chop away with gusto.
4.
Place a cover slip over the stained root tip then heat, but do not boil, the slide over a
flame.
5.
Invert the slide onto a paper towel so that the cover slip is down and press firmly on
the slide so that the root tip cells are crushed and spread out on the slide under the
cover slip.
6.
Examine the root tip cells under the microscope. You will want to note the relative
number of cells undergoing mitosis versus those in interphase and also which stages
of mitosis are most commonly found.
Exercise 2: Mitosis in animal cells
Prepared slides of whitefish blastula cells are available at the front of the laboratory
room. Examine these using your microscope, find and do detailed drawings of each stage of
mitosis, and note differences between mitosis in animal and plant cells.
Exercise 3: Meiosis in Ascaris
Ascaris is a type of parasitic nematode worm commonly found in the guts of several
vertebrates including humans. Because of its haploid chromosome number of 2, it is
relatively easy to study the behavior of chromosomes. Use the prepared slides provided to
identify the various stages of meiosis in the eggs of Ascaris. The eggs should be easy to
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identify as they should be the largest cells present.
Materials
Equipment
Cover slips
Gloves, latex disposable
Microscopes
Paper towels
Prepared slides of:
Ascaris genitals
Onion root tip
Whitefish blastula
Razor blades
Slides
Watch glasses
Chemicals
Aceto-carmine stain
Hydrochloric acid, 1 M HCl
Supplies
Onions that have been rooted in water starting four days before lab. Do not get onions from
the grocery store, they will have been treated to prevent rooting. Get onion sets from a
garden supply store.