It’s Variation Baby: Variation of genes using Meiosis
Offspring do not look like the parents or their brothers or sisters due to a process called
meiosis. A main goal of meiosis is to rearrange DNA so no two offspring look the same. Today
you will be simulating how genes are reorganized during meiosis. Follow the steps below using
the beads. Be sure to answer the analysis questions as you progress. Today you will be
working with human chromosome 7, the chromosome that carries the gene for cystic fibrosis.
You receive one copy of chromosome 7 from your mom, and one copy of chromosome 7 from
your dad. The two copies of chromosome 7 that have information coding for the same trait are
known as HOMOLOGOUS CHROMOSOMES.
DAY 1: Creating Sperm and Egg
Beads will represent genes. The male chromosome will be red and the female chromosome
yellow. The white magnetic connectors will represent the centromere. Each group should
assemble two male chromosomes and two female chromosomes. Each chromosome should
have ten genes on each side of the centromere.
We all have specialized cells for the creation of sperm and egg. These are diploid cells that give
rise to gametes (sex cells – egg and sperm, which are haploid cells). We are going to simulate
how these sex cells are created through the process of meiosis. Meiosis progresses in two
stages: Meiosis 1 and Meiosis 2. While we are halving the number of chromosomes we are
also using every opportunity to mix the genes so no two gametes will be the same.
MEIOSIS I:
INTERPHASE I:
1. Draw a cell nucleus on your desk.
2. Begin by locating one chromosome of each color (these colors represent the 50% of your
genes coming from your mom, and the 50% of your genes coming from your dad. Place
these inside the center of your cell. This is a homologous pair of chromosomes. If we had a
full set of chromosomes how many would be in each cell? _________________
3. Replicate your chromosomes by adding beads of the same color and length to the two
chromatids. You now have fully developed chromosomes (Figure 1) If we had a full set of
chromosomes how many would be in each cell now? __________________________
QUESTION: What cellular process occurred
when you added the beads? What is the
genetic relationship between the two sister
chromatids on each chromosome?
FIGURE 1: Replicated homologous chromosomes
PROPHASE I:
4. Now take your two homologous chromosomes pairs and line them up, attaching them at the
centromere. Remember homologous chromosomes are chromosomes coming from separate
parents but containing information coding for the same trait, in this case, eye color. Now look
for the fifth bead on each chromatid, and pop them off and switch them. What you simulated
is a process known as “Crossing Over”
QUESTION: Take a look at the chromatids on your homologous chromosomes. What have
you just done to the genetic composition of each sister chromatid?
METAPHASE I:
5. Now line up your crossed over chromosomes are the center of your cell.
ANAPHASE I:
6. Separate your homologous chromosomes and pull them to opposite ends of the cell.
QUESTION: You started meiosis I with two pairs of
each chromosome.
With the homologous
chromosomes now separated, what have you done
to the chromosome number? Can you venture a
guess as to why this might be important? (think
about what happens when a sperm and egg come
together)
TELOPHASE I:
7. Now separate your one cell into two. Take a look at the genetic composition of the
chromosomes in each of your two new cells.
QUESTION: What is the genetic relationship between the two chromosomes now? How does
this differ from cellular mitosis?
MEIOSIS II:
PROPHASE II:
8. Position your chromosomes in your cell. No replication occurs now.
METAPHASE II:
9. Line up your chromosomes in the center of your cell.
ANAPHASE II:
10. Now separate your two chromosomes into their chromatids, and pull them away from each
other, forming four new cells.
TELOPHASE II:
11. Place each of the chromosomes into one of the four new cells.
QUESTION: take a look at the chromosomes in each new cell. What do you notice about
the genetic relationship between the four chromosomes? Are they all the same?