Modeling Chromosomes with Airheads

DO NOT EAT ANY PART OF THIS LAB - ALL MATERIALS WILL BE USED AGAIN.

RETURN THIS PACKET AT THE END OF CLASS.

Introduction:

This activity will model the structure of chromosomes as well as the similarities and differences between homologous chromosomes to further understand the relationship between chromosomes, DNA, genes, and traits.

Materials:

For each group of two students:

  • 2 airheads of different colors
  • 1 Sharpie
  • Scissors
  • 1 plastic bag

Part A: Constructing Chromatids

  1. Gather all materials
  2. Construct 6 chromatids using the figure to the
    right as a guide.
  3. Cut one of your airheads down the middle length-wise then cut each half again length-wise. You will now have four thin strips of airheads.
  4. Take two of the long strips and cut width-wise about
    2/3 of the way from the top.
  5. You now have six chromatids: two long, two medium and two short.
  6. Repeat steps a – d with the other airhead (different color)
  7. You now have six pairs of chromatids: two long pairs (of different colors), two medium, and two short.
  8. SET ½ of your strips aside (1 long, 1 medium, 1 short of EACH color) FOR PART C

Read and Answer Questions

Read the text below

Answer questions 1 – 5 on your student worksheet

In eukaryoticcells, the nucleus divides before the cell itself divides. The process in which the nucleus divides is called mitosis. Before mitosis occurs, a cell’sDNAis replicated. This is necessary so that each daughter cell will have a complete copy of the genetic material from the parent cell. How is the replicated DNA sorted and separated so that each daughter cell gets a complete set of the genetic material? To understand how this happens, you need to know more about chromosomes.

Chromosomesare coiled structures made ofDNAandproteins. Chromosomes are the form of the genetic material of a cell duringcell division. It is this coiled structure that ensures proper segregation of the chromosomes during cell division. During other phases of thecell cycle, DNA is not coiled into chromosomes. Instead, it exists as a grainy material calledchromatin.

DNA condenses and coils into the familiar X-shaped form of a chromosome, shown inFigure below, only after it has replicated. Because DNA has already replicated, each chromosome actually consists of two identical copies. The two copies are called sisterchromatids. They are attached to one another at a region called thecentromere.

Chromosomes and Genes

The DNA of a chromosome is encoded with genetic instructions for makingproteins. These instructions are organized into units calledgenes. Most genes contain the instructions for a single protein. There may be hundreds or even thousands of genes on a single chromosome.

Human Chromosomes

Humancellsnormally have two sets of chromosomes, one set inherited from each parent. There are 23 chromosomes in each set, for a total of 46 chromosomes per cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are actually 23 pairs of chromosomes per cell. Each pair consists of chromosomes of the same size and shape that also contain the same genes. The chromosomes in a pair are known ashomologous chromosomes.

Homologous chromosomes are chromosomes that have genes for the same trait at the same location. You have three maternal chromatids (from mom) that are one color, and three paternal chromatids (from dad) that are the other color. Indicate on your worksheet which color of airhead represents the maternal chromatid and which color is the paternal chromatid.

Part B: Labeling Genes

The labeling of genes in this activity reflects the system used by geneticists. The uppercase letter represents the dominant allele or trait; the lowercase letter represents the recessive allele.

An allele (also called a gene locus) is a different form or variant of a gene for a specific trait. For example, the gene for lactose has two alleles, L and l.

The gene coding “L” codes for the production of functional lactose enzyme

And “l” for non-functional lactose enzyme.

Human Traits found on the “Long, Medium and Short” Chromosomes
Trait / Phenotype
(Variant of Trait) / Symbol for Allele* / Chromosome
Eye Color / Pigmented (Brown/Black)
Non-Pigmented (Blue) / B
b / Long
Ear Lobe / Attached
Free / A
a / Long
Tongue Rolling / Can Role
Cannot Role / R
r / Medium
Color Blind / Normal Vision
Color blind / N
n / Short
  1. Label your two long chromatids (these should be different colors) with gene alleles by doing the following:
  2. You will use the Sharpies to make gene labels.
  3. Each different gene will be labeled with a different letter (either capital or lowercase)
  4. You will need to label both long chromatids (each length that are different colors)
  5. Each gene has two alleles. You will determine which allele (dominant or recessive) is present on the chromosome by flipping a coin.
  6. Toss your coin first for the eye color trait. If your coin is “Heads” then label the gene with the symbol for the dominant allele that is a capital letter – B. If the coin is “Tails” then label the gene with the symbol for the recessive allele that is lower case – b.
  7. Do a coin toss for BOTH maternal and paternal chromatids.
  8. Repeat the coin toss for the ear lobe trait using the letter A now instead of B.
  9. Label your medium and small chromatids using the same method and the information in the table.

Answer questions 6 - 9 on your student worksheet

Part C: Duplicating the Chromatids (Making Chromosomes)

  1. One of the major characteristics of life is the ability to reproduce or replicate. Recall that before a cell divides, it must replicate all of its DNA / chromosomes. You will now model this process in steps 6-7.

Answer questions 10 – 13 on your student worksheet

  1. Take the other ½ of your strips (1 long, 1 medium, 1 short of each color) that you had in Part A and add gene labels to match those of the other six chromatids.
  2. Form a duplicated chromosome by slightly bending two identical chromatids (flat sides facing you) and joining them in the middle to make an “X”. The point of attachment is called the centromere.

Answer the remaining questions on your student worksheet

Put your chromosomes in a Ziploc bag with your group names and period. Make sure that all the chromosomes lay flat and are not stacked on top of each other.