Simulation of the Meiotic Process Using Pop Beads



In lab, pop beads with magnetic centromeres are used to simulate chromosomes as they move through meiosis. The meiotic board used includes six circles: one on top; two in the middle and four at the bottom. These circles represent the chromosomal make up of the seven nuclei involved in one meiotic event. The top circle represents the karyotype of mother nucleus, the center two of the two daughter nuclei, and the bottom four of the granddaughter nuclei. Each of these nuclei are genetically distinct.

With this simulation, we modeled two pairs of homologous chromosomes. Each pair consisted of four strands of beads. The two strands of of a given color represent one chromosome with two chomatids. The magnet in the middle represents a centromere. All the chromosomes of one color represent the genetic contribution of one of the parents that gave rise to the mother nucleus.

Meiosis and syngamy (fertilization) are complementary and together are necessary for sexual reproduction. Syngamy ends with the joining of two nuclei and results in a doubling of the number of chromosomes found in one nucleus. Meiosis, through a process of two nuclear divisions both of which are fundamentally different from mitosis, results in four nuclei. Each of these has one half the number of chromosomes found in the original mother nucleus. Hence, meiosis reverses the effect of syngamy.

Meiosis does not simply regenerate nuclei with the same genetic make up of the original two nuclei that joined in syngamy. There are two reasons for this fact:

1. Homologous pairs of chromosomes sort themselves independently of each other during anaphase I

2. Genetic material in each chromosome is exchanged with its homolog through crossing over during prophase I.

The pop beads, while a simplistic model of chromosomes, illustrate these two processes well.


Variation in Meiotically Derived Nuclei Due to Random Assortment of the Homologous Pairs During Meiosis I

This can be demonstrated using two pairs of homologs

Note that when we model meiosis with two pairs of homologs and, discount crossing over, there is a 50% chance of forming nuclei like the original two that underwent syngamy to create the mother nucleus' karyotype (a 50% chance that all the same colors will be together). If we were dealing with three pairs of homologs there would be a 25 % chance.

In every case this probability is equal to one half raised to the power (n - 1) where "n" is the number of homologous pairs of chromosomes. For humans this would equal 1/2 to the twenty second........a very small number. The random assortment of chromosomes during meiosis I creates new chromosomal combinations and is a major reason why so much variation exists between individuals in a sexually reproducing population.


Variation in Meiotically Derived Nuclei Due to Crossing Over During Prophase I

This can be demonstrated using one pair of homologs:

Note that even with one pair of homologs none of the new nuclei are genetically the same. This fact differentiates mitosis from meiosis II. Mitosis produces two nuclei both of which are genetically the same. Meiosis II results in the production of two genetically distinct nuclei.