Before mitosis begins, the chromosomes and other cell materials are copied. [are copied? Who or what does the copying???] The pairs of centrioles, which are two cylindrical structures, are also copied. [Besides being cylindrical, what is a centriole, and what is its significance for mitosis???] Each chromosome now consists of two chromatids. [Remind us what a chromatid is!!!]
Mitosis Phase 1
Mitosis begins. The nuclear membrane brakes apart. [Why?] Chromosomes condense into rodlike structures. [Why is the new, rodlike structure important and significant?] The two pairs of centrioles move to opposite sides of the cell. [Significance?] Fibers form between the two pairs of centrioles and attach to the centromeres. [Remind us what a centromere is and why it is significant!]
Mitosis Phase 2
The chromosomes line up along the equator of the cell. [How??? and Why???]
Mitosis Phase 3
The chromatids separate [How?] and are pulled to opposite sides of the cell by the fibers attached to the centrioles. [This crucial event should be the centerpiece of the whole discussion of mitosis].
Mitosis Phase 4
The nuclear membrane forms around the two sets of chromosomes, and they unwind. The fibers disappear. Mitosis is complete.
From
Cells, Heredity, and Classification (Holt, Rinehart and Winston), with my queries in brackets.
With all the questions it begs and explanations it lacks, this is little more than a list of terms and series of steps to memorize, with no obvious general concepts to guide or interest you. This approach seems to have a long history. It includes my own biology book of a generation ago, which is why I never pursued biology after 9th grade.
But now that my autistic son is studying it in middle school, I need to understand it better.
Only after multiple readings of the passage above did I sort of figure out what the underlying concepts were. (Perhaps if I were a more visual thinker, it wouldn't have taken me so long.)
Assuming that I'm more or less on target, it strikes that a more engaging introduction to mitosis might go somewhere along these lines (ideally generated by some sort of Socratic dialog, with accompanying illustrations):
We already know that cells consist of crucial elements, for example, the mitochondria and the chromosomes. We also know that for organisms to grow, their cells must divide. But is cell division as simple as a cell dividing itself into two? Bear in mind that each "half" of the cell must have all the crucial elements. This means that each element must be copied, and each half must end up with one copy of the element.
Making sure that each "cell half" has exactly one copy of a given element is particularly complicated when it comes to the chromosomes. Is it enough for each chromosome to make a copy of itself? Imagine what would happen if the chromosome copies simply swam around in the cytoplasm while the cell divides. Then what's to stop one half from ending up with two copies of chromosome 1 and no copies of chromosome 2, or vice versa? We already know how each chromosome contains different sets of crucial instructions for the cell, so the results of this kind of lopsided split would be disastrous.
So how can a cell make sure that exactly one copy of each of its dozen or more chromosomes ends up in each "cell half" before the division? Since the cell has no "brain" or other centralized information processor, as soon as the chromosome copy separates from its original, there's no way for the cell to "know" which copy goes with which original, and therefore no way to guarantee that each cell half gets exactly the right number of copies.
Well, suppose each chromosome copy remains attached to the original up until right before the cell divides. This preserves the information about which copy matches up with which original. Then suppose the chromosome pairs (original plus copy) all line up in such away that a simultaneous, symmetrical force emanating from each cell half can pull them apart, so that each original copy ends up in one half while its copy ends up in the other half.
Let's picture how this could happen. Imagine if the chromosome pairs line up along the equator of the cell, with one pair member on each side of the equator. Now imagine tentacles reaching out from the middle of the edge of each cell half and pulling at each chromosome pair from each side. If these tentacles are equally strong, and strong enough to separate the chromosome pairs, the result is just what we want: exactly one copy of each chromosome pulled into each cell half.
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