Lab#8 Mitosis and Meiosis Lab

Part 1: Modeling Mitosis and Meiosis with pipe cleaners

Using this website: http://www.indiana.edu/~ensiweb/lessons/gen.mm.html, we will use pipe cleaners and beads to model the process of meiosis and mitosis. The website has links to directions, template and worksheets, but in short the procedure is as follows:

Making your chromosomes:
1. Each single fuzzy piece (pipe cleaner) equals one chromosome
– – a pink piece equals one chromosome inherited from the mother;
– – a blue piece equals one chromosome inherited from the father.
2. Two fuzzy pieces, held together by a bead—the centromere—equals one chromosome duplicated into two new
strands (chromatids), each of which becomes a duplicate chromosome when the centromere splits at the
beginning of anaphase.

Using your pipe chromosomes, you will  model both mitosis and meiosis on your templates, and we will discuss what is happening and any details that might not be obvious from a 2D diagram.  Then you will use your summary sheets to draw out the two processes.

Difference between chromatids and homologous chromosomes

One of the things we spent some time in lab discussing is how confusing some of the terminology can be in describing chromosomes. Here is a karotype from a human’s DNA:

It is important to realize that humans have 46 chromosomes, but 23 pairs of homologous chromosomes. What does this mean, exactly? This means that each genes has TWO copies (each copy is called an allele), and so each pair of homologous chromosomes is two chromosomes with the same genes. Remember that alleles can be different and code for variations on the same gene.

See below for more.

The picture to the left shows one of the pairs of homologous chromosomes being pulled out of the picture. You can’t see it in the karytype, but each chromosome has actually already been duplicated. The reason we only see duplicated chromosomes in karyotypes is because chromosomes only condense when they are going to be duplicated (or vice versa, as the case may be). In other words, when a chromosome is in the form of being one long string of DNA, it looks like a large pile of tangled string and cannot be viewed under the microscope.

 

 

The pictures to the right show the location of the same gene on the leg of the two homologous chromosomes, and thus labeled allele H and h. For example, if this was the gene for hair color, one allele could be the dominant brown color, and allele h could be blond (hair color is more complex than that, but it serves as a simple example). Also note that there are two copies of each allele on each chromosome, so you are seeing 4 copies of this gene on all 4 legs.

Here’s a webpage with more pictures and explanation: https://adapaproject.org/bbk_temp/tiki-index.php?page=Leaf%3A+What+are+homologous+chromosomes%3F

Also note that each duplicated chromosome is held in the middle by the centromere and the duplicated legs are called chromatids (think “kids” to help keep it straight).

 

Introduction to Mitosis and Meiosis

Working through the pipecleaner activity above will help bring the diagrams in your textbook alive, but here are some more resources to help. In short, your homologous chromosomes split up in meiosis and stay together in mitosis.

Narrated animation of mitosis and meiosis:

http://www.sumanasinc.com/webcontent/animations/content/mitosis.html

http://www.sumanasinc.com/webcontent/animations/content/meiosis.html

More animations:

Mitosis and Cytokinesis: http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120073/bio14.swf::Mitosis%20and%20Cytokinesis

– – Unique features of meiosis: http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120074/bio16.swf::Unique%20Features%20of%20Meiosis

– – Mistakes in Meiosis: http://www.sumanasinc.com/webcontent/animations/content/mistakesmeiosis/mistakesmeiosis.html

Part 2: Making Mitosis slides with onion and garlic root tips

Seeing mitosis in real life cells is easily done with the root tips of either onions or garlic. Here’s a nice explanation of the process with pictures:
http://www.funsci.com/fun3_en/mitosis/garlic.htm

You should also download the lab manual for mitosis/meiosis from the College Board: AP Biology Mitosis and Meiosis Lab

The procedure for making and staining the slides vary between textbooks, but the one we will be using is this one:

PROCEDURE
– Grow the roots of onions and/or garlic (various methods for this).
– Cut approximately 5mm off the tips of a couple of roots and put them in a small beaker or glass jar with thin walls.
– Place hydrochloric acid with a concentration of 1M in the beaker to a depth of about 5 mm.
– Put the beaker in a saucepan containing a couple of cm of water at 60°C and incubate for approximately 6-7 minutes, or at room temperature for approximately 20 minutes.
– Remove the beaker from the saucepan and transfer the root tips to a microscope slide and with a pipette and some distilled water, rinse away the acid. Dry the root tips with a paper towel without touching them and repeat the rinsing several times.
– With a blade, shorten the root tips to 2 mm in length, keep the tips and throw away the rest. With two needles or pins and under the stereoscopic microscope, carefully chop the tips and separate the fragments. (The root tips should come undone easily, otherwise repeat the acid treatment).
– Colour the tissues with 0.5% Toluidine blue for 2 minutes; mount a coverslip; with a pipette, place a couple of drops of distilled water one one side of the coverslip and absorb the coloured water from the other so to remove the dye;
– With the biological microscope search for cells undergoing mitosis.

NOTE:
1. Choose the longest roots, where the process of mitosis should be more active.
2. the tissues where cell duplication is most active are those near the tip of the root.

Other lab procedures have you soak the root tip cuttings for 8-12 hours in Carnoy’s fixative (1 part glacial acetic acid mixed with 3 parts 95% ethanol) which dehydrate the cells before using HCl. Using fixative means the slides will last longer, but it is not necessary.

Here’s an alternate procedure:

Harvesting the Onion Root Tips

2. Cut off the roots from each bulblet using fine dissection scissors.
3. Place cut root tips into Carnoy’s fixative for 4–18 hours.
4. Decant off fixative and rinse tips with 25 mL 70% ethanol.
5. Place tips in 70% ethanol and store covered at 4°C.

Preparing Chromosome Squashes
1. Place the onion root tip in 12 M HCl for 4 minutes.
2. Transfer the tip to Carnoy’s fixative for 4 minutes.
3. Remove the slide from the Coplin jar containing 70% ethanol, dry with a scientific cleaning wipe, and label it.
4. Place the onion tip on the slide, and cut off the distal 2 mm portion of the tip; discard the remainder of the tip.
5. Cover the root tip piece with carbol-fuschin stain for 2 minutes.
6. Blot off excess stain and cover tip with 1–2 drops of H2O.
7. Place the cover slip over tip and cover the cover slip with a scientific cleaning wipe.
8. Firmly press down on the cover slip with your thumb or with the eraser end of
a pencil. Do not twist the slide.

Counting Cells and Analyzing Data
1. Observe the cells at high magnification (400–500 X). Count the total number of cells you can see and then how many are in some stage of mitosis. See this link for help with finding the different stages:

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