Saturday, April 25, 2015

Mitosis Limericks: Anaphase

Download the images for this post.

Here's the third in the series of mitosis limericks (see my previous posts on prophase and metaphase).  I've used these in class to highlight important events during mitosis without showing cell images.  Students will see high quality images of mitosis in in their textbook.

Anaphase is my favorite of these limericks...both touching and accurate from a cell biology standpoint.

In this poem a single piece of DNA is referred to both as a chromosome and a sister chromatid. This is not inconsistent.  Correctly identifying the number of chromosomes in a cell is challenging when considering DNA replication and cell division.  In my experience, a full understanding of chromosome number has to begin with understanding what chromatids are.

Above is a drawing from my lecture when I introduce this idea.  I start drawing a single curly line (left) representing a single double-stranded DNA molecule.  If that single piece were to condense the result would be a single chromatid (upper half of the drawing).  However, in the cell cycle condensation of chromosomes (in M phase) is always preceded by DNA replication (in S phase).  In the lower half of this drawing I'm showing that DNA replication has produced two identical DNA molecules (shown as two curly lines).  When these condense the result is sister chromatids.  I try to make it very clear that the two sisters are identical copies of the same chromosome.

The most accurate definition of sister chromatids is "a single chromosome that has been replicated and is in a condensed state."  So during prophase and metaphase of mitosis each chromosome is represented by a pair of sister chromatids (For example, in humans there are 46 total the start of mitosis there are still 46 chromosomes but each one is in the form of sister chromatids.)

The definitions get a bit tricky during anaphase (and telophase).  During metaphase each chromosome is a pair of sisters held together at the centromere.  So what do we call these structures once the centromere separates at anaphase?  I've seen a few textbooks that say the chromosome number in the cell doubles in anaphase (i.e. a human cell has 46 chromosomes in metaphase and 92 in anaphase)...but I've never felt this is helpful for students trying to understand the central events of mitosis.

I think it's best for students to understand that during DNA replication and mitosis the chromosome number never changes.  A human cell has 46 chromosomes before DNA replication, 46 chromosomes after DNA replication and 46 chromosomes at every stage of mitosis.  The resulting daughter cells also have 46 chromosomes.

If my count is correct there should be one more limericks to go.  I will post that soon.

Friday, April 17, 2015

Mitosis Limericks: Metaphase

Download the images for this post.

Here's the second in the series of mitosis limericks (see my previous post on prophase).  I'll be posting anaphase and telophase soon.  In the past I have used these in class to highlight important events during mitosis without showing cell images.  Students will see high quality images of mitosis in in their textbook.

My students always seem to have trouble understanding exactly what and where the kinetochores are (Background information).  Kinetochores are the place on the chromosome where microtubules (also called 'spindle fibers') attach.  At this site many proteins interact to form a connection between DNA and microtubules.  In most organisms the kinetochore forms in the region around the centromere.

The metaphase plate is an imaginary line near the center of a cell in mitosis.  The pairs of sister chromatids move toward this line because they are being pulled hard in opposite directions by microtubules originating from the poles of the cell.  Each pair of sisters is in the middle of a tug-of-war between two equally strong opponents.

I hope this is useful.  Two more limericks to go!

Saturday, April 11, 2015

Mitosis Limericks: Prophase

Download the images for this post.

I was on a limerick kick a few years ago and I did my best to come up with a poem for each phase of mitosis.  Surprisingly these manage to capture most of the main events that students learn from each phase.  Since every textbook has great pictures and descriptions of the phases of mitosis I like to take an alternative approach and use the limericks in class.

(This is the first in a series...four in all.  Stay tuned!)

After showing this limerick I point out that the main events of prophase (and prometaphase):
  1. Condensation of the chromosomes:  Chromosomes condense from loosely-coiled chromatin to more tightly-coiled chromatids (Background information in SciTable.) Since the chromosomes have been replicated condensation produces sister chromatids.
  2. Nuclear Envelope: Dissolution of the nuclear envelope occurs in most cells during mitosis (Background information on NCBI bookshelf).  Dissolution of the envelope is not fully understood but most of the envelope is fragmented into vesicles.  Some yeast cells keep the nucleus in tact throughout mitosis.
  3. Centrioles: Each centriole is two cylinders of mircotubules (Background information).  The microtubules that eventually connect to the chromosomes are assembled near the centrioles.  Not all cells have centrioles...they are common in animal cells and rare in plant cells.