Part 2: Modeling Meiosis with Crossing Over
1. Build a pair of replicated, homologous chromosomes. Use ten beads to create each individual sister chromatid (20 beads per chromosome pair). Two five-holed beads represent the centromere.
- Start with 20 beads of the same color to create a first sister chromatid pair. Five beads must be snapped together for each of the four different strands. Two strands create the first chromatid, and two strands create the second chromatid, with a 5-holed bead at the center of each chromatid. This creates an “I” shape.
- Connect the “I”-shaped sister chromatids using the 5-holed beads to create a “X” shape (Figure 3).
- Repeat this process using 20 new beads (of a different color) to create the second sister chromatid pair.
- Assemble a second pair of replicated sister chromatids, this time using 12 beads per pair (six beads per each complete sister chromatid strand). Snap each of the four pieces into two five-holed beads to complete the set up (Figure 4).
- Pair up the homologous chromosomes created in Steps 1 and 2.
3. SIMULATE CROSSING OVER. Bring the two homologous pairs of sister chromatids together (creating the chiasma) and exchange an equal number of beads between the two. This will result in chromatids of the same original length; however, there will now be new combinations of chromatid colors.
4. Configure the chromosomes as they would appear in each stage of meiotic division (prophase I and II, metaphase I and II, anaphase I and II, telophase I and II, and cytokinesis).
- Diagram the corresponding images for each stage in the section titled “Trial 2 – Meiotic Division Bead Diagrams.” Be sure to indicate the number of chromosomes present in each cell for each phase. Also, indicate how the crossing over affected the genetic content in the gametes from Part 1 versus Part 2.