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.
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Image by Ehamberg from Wikimedia Commons.
http://upload.wikimedia.org/wikipedia/
commons/9/99/Haploid_vs_diploid.svg
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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.
