Playing Card Meiosis

Meiosis is a strong driver of diversity.  Since speciation relies on biodiversity, the array of organisms and their ability to adapt to change would be reduced in the absence of new genome compositions created by meiosis.  Meiosis and sex go hand-in-hand.  Sexual reproduction relies on the combination of genes from two donors (“mother and father”) to create a new individual (the zygote).  To properly introduce the important biological concepts, you need to get everyone to understand what haploid and diploid means. 

Image by Ehamberg from Wikimedia Commons. 

http://upload.wikimedia.org/wikipedia/

commons/9/99/Haploid_vs_diploid.svg

Haploid: means having one of each kind of chromosome.  Abbreviated “N” or “n”.

Diploid: means having duplicates of each kind of chromosome (with the exception of sex chromosomes).  Abbreviated “2N” or “2n”.

Many animals are diploid.  They are multicellular, and each of the cells contains an identical diploid genome:  two of each chromosome.  One set of chromosomes came from the individual’s mother; the other from its father.

If the mother and father simply fused their genomes, you’d end up with FOUR of each chromosome … too many!  You probably already know that Down Syndrome is the result of having too many chromosome 21s (three of them = trisomy).  Too much (genetic) information is often as bad as too little because of the balance of proteins in the cell.  Trisomy of chromosome 1, for example, is lethal very early on – usually before the mother knows she is pregnant.  A tetraploid person would not be viable.  So … how do Mom and Dad create offspring with sex?  Meiosis! 

But before we get to meiosis, we should look at what happens in normal cell division.  For cells with multiple chromosomes, the genetic material moves through a series of events that make up mitosis.  The job of mitosis is to organize the genetic material so that the situation of too few or too many chromosomes is avoided.  Chromosomes are tracked and relocated to separate positions prior to cell division.  Sister chromatids separate in mitosis in order to get that job done correctly.

Meiosis has another job on top of that:  separate the chromosome pairs.  The job of meiosis is to take a diploid cell and make it haploid.  In humans, this creates a gamete, but that’s not how all organisms do it!  Fungi, for example, are often quite healthy (and presumably happy … as happy as a mushroom can get, anyway) in the haploid state.  They still have sex, though.  Sperm and egg – haploid cells each – fuse to make a diploid zygote.  However, the zygote in some creatures does not divide through mitosis.  It instead undergoes meiosis to generate haploid cells.  This creates four biologically diverse cells (rearrangement of alleles on chromosomes = diversity!)  Once haploid, these cells are separate individuals that divide mitotically.  This leads to an important correction that educators should heed:

MISCONCEPTION: MEIOSIS RESULTS IN GAMETES.Corrected Statement: Meiosis results in haploid cells.Explanation: In humans and most animals, these haploid cells will always form gametes. However, in plants and many protists (single-celled eukaryotes), meiosis creates haploid cells that divide mitotically. Only a few of those these haploid cells will be gametes, and only if the organism is pursuing a sexual life cycle. Some organisms don’t, by the way!  (From Science³; Rawle et al. 2014)

The point is that there are two events that change the chromosome numbers during a normal lifecycle.  Fertilization is the fusion of cells.  These cells must therefore be haploid, and they are called gametes.  The result of gamete fusion is a new diploid cell:  the zygote. At some point we need to reduce the number of chromosomes before sex happens again.  That’s the job of meiosis.

To imagine this as a steady-state, one just has to remember to alternate fertilization with meiosis. 

The demonstration you will see at the Symposium shows how you distribute cards (chromosomes) so that you’re dealt a “full hand”.  We’ll follow a diplontic cycle.  This is the one humans exhibit.  You must have one of each type of chromosome for a gamete (10, J, Q, K, A), and two of each for a somatic cell.  Red can represent maternally-derived chromosomes, and black can represent the paternal ones.

With large cards, you can show the class how you partition the chromosomes into distinct piles:  Each pile gets ONE 10, ONE Jack, and so on.  You go from diploid to haploid deliberately.  Combining two haploid piles give you a zygote. 

The message here is that there’s a continuity of information.  Each parent passes HALF his or her genetic material to offspring:  the combination is a new diploid individual.  Surviving to maturity suggests that the parents survived “a test”.  They were healthy and vital enough to attract a mate and breed.  By combining two “winning” genomes, a child who likely will at least preserve the best traits – or perhaps might even have a selective advantage – will be tested to see if it survives to maturity.