Name:
Date:
Title: Mitosis and Meiosis.
Introduction.
The movement of chromosomes must be carefully controlled during cell division to ensure that
the resulting cells contain the appropriate genetic information.
In mitotic cell division, which is used to produce somatic cells, chromosome movement is
organized to ensure that each daughter cell receives a complete diploid set of chromosomes. The
daughter cells will be genetically identical to each other and to the parent cell. See figure 1.
A
B
C
D
Figure 1: Mitosis in Lilium regale. (A) Prophase. (B) Metaphase. (C) Anaphase. (D)
Telophase. For details of each stage refer to your notes or textbook.
In meiotic cell division, which is used to produce gametes, chromosome movement is organized
to ensure that each gamete produced contains a haploid set of chromosomes. The fusion of two
gametes will produce a diploid zygote that will be genetically unique. See figures 2 and 3.
A
B
C
D
Figure 2: Meiosis 1 in Lilium regale. (A) Prophase. (B) Metaphase. (C) Anaphase.
(D) Telophase. For details of each stage refer to your notes or textbook.
Genetics Laboratory 03.1
A
B
C
D
Figure 3: Meiosis 2 in Lilium regale. (A) Prophase. (B) Metaphase. (C) Anaphase.
(D) Telophase. For details of each stage refer to your notes or textbook.
Methods.
The laboratory session today will consist of a number of exercises designed to reinforce your
understanding of the events of mitosis and meiosis.
A: Examination of mitotic nuclei.
(1)
Obtain two small cups. Label one "HCl" and pour just enough hydrochloric acid into it to
cover the bottom. Label the second "Carnoy" and pour just enough Carnoy fluid into it to
cover the bottom.
(2)
Use forceps to transfer an onion root tip into the hydrochloric acid. After four minutes,
transfer the tip to the Carnoy fluid.
(3)
After four minutes, place the root tip on a microscope slide. Using a scalpel, cut off all but
the last one to two millimetres of the tip.
(4)
Place a few drops of toluidine blue on the root tip. Leave for two minutes. Blot away the
stain, taking care not to touch the root tip.
(5)
Place one or two drops of water on the root tip, and cover carefully with a coverslip.
(6)
Cover the slide with a paper towel and press firmly on the coverslip. Do not twist or slide
the coverslip.
(7)
Examine the slide under the microscope. If necessary, squash the slide again.
Light microscopes must be used correctly if the clearest possible image is to be obtained
and observer discomfort and damage to the microscope are to be avoided.
Select the lowest power objective, place the slide on the stage, and centre the specimen
under the objective. The stage will normally be operated using the right hand.
Adjust the spacing of the eyepieces to match the distance between your eyes, then focus on
the specimen using the coarse focus knob. The focus knob will normally be operated using
the left hand. Determine which eyepiece is adjustable. This is usually the one on the left.
Close the eye on that side and correct focus using the focus knob. Switch eyes and correct
focus using the control on the adjustable eyepiece.
Genetics Laboratory 03.2
Open the iris diaphragm. Place a pencil on the illuminator and adjust the height of the
condenser until the silhouette is in sharp focus. Slowly close the iris diaphragm, stopping
just after the image begins to dim. Adjust the illuminator intensity for comfortable viewing.
Select a higher power objective if required and adjust the focus if necessary. Normally this
will be done using the fine focus knob. If a higher power objective is used, return to the
lowest power objective before removing the slide from the stage.
B: Simulation of mitosis and meiosis.
(1)
Obtain forty red beads and forty yellow beads, four magnetic centromeres, four centrioles,
and four pieces of string, each about fifty centimetres long.
From these materials, make two chromosome models, identical in size and shape, one red,
and the other yellow.
(2)
Simulation of mitosis.
Prophase
Tape the two centrioles to the bench, about fifty centimetres apart. Place the chromosomes
between the centrioles.
Metaphase
Attach strings to each of the magnetic centromeres. Pass one string from each chromosome
through each centriole. Line the chromosomes up between the centrioles.
Anaphase
Pull gently on the strings until the centromeres separate. Each of the sister chromatids is
now called a chromosome. Pull the chromosomes towards the centrioles.
Telophase
Remove the strings. Pile the chromosomes together near each of the two centrioles.
(3)
Simulation of meiosis.
Prophase 1
Tape the two centrioles to the bench, about fifty centimetres apart. Place the pair of
chromosomes together between the centrioles. The chromosomes should be twisted around
each other, forming a bivalent.
Simulate crossing over by exchanging sections of two non-sister chromatids.
Metaphase 1
Attach strings to the double centromere of each chromosome. Pass the string from one
chromosome through one of the centrioles, and the string from the other through the other
centriole. Line the bivalent up between the centrioles.
Genetics Laboratory 03.3
Anaphase 1
Pull gently on the strings until the bivalents separate. Pull the homologs towards the
centrioles.
Telophase 1
Remove the strings. Pile the chromosomes together near each of the two centrioles.
Prophase 2
Tape another two centrioles to the bench. Place one chromosome between each pair of
centrioles.
Metaphase 2
Attach strings to each of the magnetic centromeres. Pass one string from each chromosome
through each centriole. Line the chromosomes up between the centrioles.
Anaphase 2
Pull gently on the strings until the centromeres separate. Each of the sister chromatids is
now called a chromosome. Pull the chromosomes towards the centrioles.
Telophase 2
Remove the strings. Pile the chromosomes together near each of the two centrioles.
(4)
Simulation of double crossovers.
Often two or more crossovers will form between a homologous pair of chromosomes.
Multiple crossovers may involve two, three, or four chromatids.
Use the model chromosomes to demonstrate these two, three, and four strand double
crossovers.
Report.
No report is required for this laboratory session.
Genetics Laboratory 03.4