Our goal is to help the student to understand and memorize the key aspects of mitosis. Each phase of mitosis is described by a sequence of steps in the lab. The lab begins with one cell on the screen, containing a nucleus, a red chromosome, and a yellow chromosome. The student is taken through various phases of cell division, in which he/she observes an animation of the phase, or is prompted to perform tasks needed for completing the phase. The student is required to complete each phase before advancing to the next phase. The program should be available online so that students can access it from any place at any time. The simulation progresses as follows (see Figure 2). Notice that for every stage there should be suitable textual information provided to orient the user of the current state of simulation the let them know what kind of work the system expects them to do before progressing to the next stag

Step 0a (Build Parent Cell)

Before beginning the simulation of mitosis, the parent cell should be assembled. The user does this with a set of “beads” (small oval shapes), two centromeres, two strings (guidelines) to hold the beads, and the cell plasma and nucleus. The centromere and all beads of one color should be threaded on one string to make one chromosome, and the centromere and beads of the other color should be threaded on the other string. To thread a bead on the string, the user clicks on the bead, drags it to an approximate final location and releases the mouse button. The program should find the closest available “slot” and snap the bead to the slot. The assembling of the chromosomes should be possible to interleave, so the user can partially build one chromosome, then part of the other, then switch back to work on the first one, etc. The centromere must be threaded first, and all beads of its color are disabled before the centromere is in place. After a chromosome is completed, its guideline should automatically disappear.

Assuming that the bead diameter is 10 pixels, the centromere ellipse is 20 × 10 pixels, the nucleus diameter is 200 pixels, and the cell diameter is 400 pixels. Initially, the beads should be scattered around, and they should come in two colors (red and yellow). Select a reasonable threshold distance for snapping the centromeres and the beads.

After building the cell, the user clicks “Next” button to proceed to the next step. The user should not be allowed to advance before the cell is properly built. A message is displayed telling the user he has not completed the requirements of the stage and must complete them before proceeding.

Note: A secret key combination, say Ctrl-b, should be programmed to allow for automatic building of the chromosomes, to shorten the process if needed.

Step 0b (Interphase)

This phase is not part of mitosis—it occurs before mitosis begins. During this phase, the chromosomes duplicate themselves. Thus, the system should automatically duplicate the chromosomes and let the mitosis process start.

The user clicks “Next” button to enter the first phase of mitosis.

Step 1 (Prophase)

In prophase, the chromosomes become increasingly easier to visualize as they become thicker and shorter as prophase progresses. For the simulation however, the chromosomes will remain the same. At the end of prophase the nuclear envelope disappears.

The user clicks “Next” button to remove the nucleus (the state indicator still shows “Prophase”). To remove the nucleus, the system uses animation: the nucleus dissolves into the cell and fades away while becoming larger. This is the end of prophase; it is also known as prometaphase.

The user clicks “Next” button to proceed into metaphase.

Step 2 (Metaphase)

In metaphase, the chromosomes line up in the central region of the cell at the equatorial plane. At the beginning of this phase, a guideline automatically appears to indicate the equatorial plane. The user is asked to click and drag the two chromosomes over to the guideline. The user should click and drag the yellow chromosome to the upper portion of the guideline; then, click and drag the red chromosome to the lower portion of the guideline.

After the chromosomes are positioned at the equatorial plane, the guideline automatically disappears. The system should snap chromosomes the only if they are close to their final position. Otherwise, the chromosomes would be left wherever the mouse button is released. A dialog box should post a warning if the user tries to advance while the chromosomes are mispositioned.

The user clicks “Next” button to proceed (the state indicator still shows “Metaphase”). The centromeres of each sister chromatid become attached by spindle fibers to opposite poles of the cell. The lines representing the spindle fibers should appear now, but they should be disabled, so the user cannot manipulate the chromosomes.

The user clicks “Next” button to proceed into anaphase.

Step 3 (Anaphase)

The sister centromeres move to opposite poles and the attached chromatids are carried along, trailing out behind in a V-shape. The user should carry out the movement of the chromosomes by mouse manipulation. The spindle fibers become enabled to allow the user to manipulate chromosomes on screen. The user should click on a centromere and drag it towards the end of its spindle fiber. Only an outline is shown for the centromere that is interacted with. As a chromosome is pulled away by the user, it bends with the direction of movement. The chromosome opposite to it mirrors this action and moves in synchrony. A series of parabolas were used to model the changing shape of the chromosome as it is being pulled. (See discussion below on how to draw the parabolas.)

The user can release mouse button at any time and the chromosomes would freeze in the current position. The user can resume later from where they stopped, until the chromosomes are brought to the poles of the cell. The system should allow swinging the chromosome back and forth, not only in one direction (towards the pole).

The user clicks “Next” button after the chromosomes have been properly placed.

Step 4 (Telophase)

In telophase, the spindle fibers disappear, the chromosomes start to de- condense (not shown in the simulation), and two nuclear envelopes form.

The user should click “Next” to watch the cell divide into two separate cells. Animation of the cell splitting and forming two nuclei is displayed. This concludes the process of mitosis.