Every cell in the human body has two alleles that condense into single chromosomes held together by a centromere. These “sister” chromaatids replicate and pair with the newly made homologous chromosomes. In this exercise we will follow the movement of the chromosomes through meiosis I and II to create haploid (gamete) cells.
Materials
2 sets of different colored snap
beads (32 of each)
8 centromeres (snap beads)
Blue and red markers*
*You must provide

Procedure
Meiosis I
A. As prophase I begins, chromosomes coil and condense in preparation for replication.
1. Using one single color of bead, build a homologous pair of duplicated chromosomes. Each chromosome will have 10 beads with a different colored centromere in it.
For example, if there are 20 red beads, 10 beads would be snapped together to make two different strands. In the middle of each of the 10 bead strands, snap a different colored bead in to act as the centromere.
Now, repeat these steps using the other color of bead.
2. Assemble another homologous pair of chromosomes using only 12 (that’s 6 per strand) of the first color bead. Place another, different colored bead in the middle of each to act is its centromere. Repeat this step (2 strands of 6 beads plus a centromere) with the other color of beads.
B. Bring the centromeres of two units of the same color and length together so they can be held together to appear as a duplicated chromosome.
1. Simulate crossing over. Bring the two homologues pairs together (that’d be the two pairs that both have 10 bead strands) and exchange an equal number of beads between the two.
C. Configure the chromosomes as they would appear in each of the stages of meiosis I.
Meiosis II
A. Configure the chromosomes as they would appear in each stage of meiosis II.
B. Return your beads to their original starting position and simulate crossing over. Track how this
changes the ultimate outcome as you then go through the stages of meiosis I and II.
C. Using the space below, and using blue and red markers, draw a diagram of your beads in each
stage. Beside your picture, write the number of chromosomes present in each cell.
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
70
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II

Questions:
1. What is the state of the DNA at the end of meiosis I? What about at the end of meiosis II?
2. Why are chromosomes important?
3. How are Meiosis I and Meiosis II different?
4. Name two ways meiosis contributes to genetic recombination.
5. Why do you use non?sister chromatids to demonstrate crossing over?
6. How many chromosomes were present when meiosis I started?
7. Why is it necessary to reduce the chromosome number of gametes, but not other cells of an organism?
8. If humans have 46 chromosomes in each of their body cells, determine how many chromosomes you would expect to find in the following:
Sperm ___________________
Egg ___________________
Daughter cell from mitosis ___________________
Daughter cell from Meiosis II ___________________
9. Investigate a disease that is caused by chromosomal mutations. When does the mutation occur? What chromosome is affected? What are the consequences?