Modeling the Cell Cycle: Chromosome Construction and Mitosis

Question 1

How to build a homologous pair of single chromosomes threading 20 beads/beans/counters of one color for one member of the pair, and 20 beads/beans/counters of another color for the other member of the pair.

1. Place the cotton ball centromere at any position in the chromosome, just make sure they are in the same position for both chromosomes.

2. Build a second homologous pair of single chromosomes, but thread 12 beads/beans/counters instead of 20 this time.  Centromeres must again be in the same place. To model G1 – interphase, place all the chromosomes piled in the center of the work area.

How technically the chromosomes are decondensed at this time, but our beads do not give us that capability. Place the centrosome (pair of centrioles) just outside of the nucleus.

This cell would be referred to as a diploid cell that is 2n = 4. There are two versions of each chromosome (20 beads/beans/counters of one color and another color, 12 beads/beans/counters of one color and another). There are two homologous chromosomes present: 2 of 20 pop beads, and 2 of 12 beads/beans/counters.To model S – interphase, duplicate your chromosomes and centrioles.

For the first pair of homologous chromosomes this will include the addition of 20 beads/beans/counters and a centromere. The beads/beans/counters should have the same color and distribution, and the centromere should have the same position as the parent chromosome. Each copy of the replicated chromosome is called a sister chromatid, so there are two sister chromatids attached at the centromere, for each color. In actuality, the centromere is a single unit. For this model, the two combined cotton balls will serve as a single centromere.

At this point of the cell cycle, the chromosomes should have been successfully replicated. The cell will be preparing for mitosis: enzyme and protein synthesis, general cell activities.

1. To model Prophase – Mitosis, leave the chromatid pairs piled in the middle of the work area. Duplicate your centrioles, now there are two centrosomes.

2. At this time the chromatids would begin to condense, we don’t have this capability with our pop beads.

3. Centrosomes will begin to move to opposite poles of the cell, and the mitotic spindle will begin to form.  You cannot represent this either with our materials.

4. To model Prometaphase – Mitosis, move the two centrosomes to either pole of the cell.  Move some of the sister chromatid pairs to the midpoint between the centromeres.

5. During prometaphase, the nuclear envelope disintegrates, and the mitotic spindle forms by attaching microtubules (spindle fibers) to the kinetochore at each homologous pair’s centromere. This process begins a tug-of-war that forces each homologous pair towards the middle plane of the cell. Build a homologous pair of single chromosomes threading 20 beads/beans/counters of one color for one member of the pair, and 20 beads/beans/counters of another color for the other member of the pair.