REACHING INTO YOUR GENES
Grades 6 - 12

In this lesson, the students will interact with a video to explore the basis for the science of chromosomes. They then perform a chromosomal analysis called a karyotype and learn how seemingly minute errors in chromosomes can lead to devastating illnesses. They will also discover the fascinating complexity in the structure of chromosomes that allows about two meters of DNA to be packed inside every nucleated cell.

Our Human Body, #4: DNA: Life's Controller

Students will be able to:

• describe the shape, size and number of chromosomes in a human somatic cell
• identify some functions of specific genes
• describe how chromosomes are the functional hereditary unit
• draw a simple diagram of mitosis with special attention to the numbers of chromosomes
• name the sex chromosomes and the autosomes
• explain why sex cells must have 1/2 the normal number of chromosomes
• describe how about an inch of DNA fits inside each chromosome

For the class:

• photomicrograph of a Karyotype (Figure 1, see references for other sources)
• photomicrograph of a spread (Figure 2, see references for other sources)
• metric ruler
• Karyotype card (Figure 3)
• scissors
• removable magic scotch tape
• erasable marker

The students should be aware that there are hundreds of proteins that perform critical functions in the body. All of these proteins are coded for in the DNA of a cell s chromosomes. Unfortunately, if a gene for a certain protein is missing, it will not be evident by simply looking at the chromosomes. Nevertheless, there are some instances when large pieces of chromosome are missing (called a deletion) or switched from chromosome to chromosome (called a translocation), resulting in either death or serious multiple illnesses called syndromes. There are scientists who are called Cytogeneticists who can, by looking at pictures of chromosomes, predict if the person will suffer from a certain syndrome. Or, conversely, based on the presence in a patient of a series of symptoms, predict that a certain chromosomal anomaly is present. Cytogeneticists also have Fluorescent probes which can attach to certain places on chromosomes identifying certain DNA sequences and therefore, certain genes. The students should also be aware of the anatomical names of a chromosome shown in Figure 4.

Sometime before the class begins, the instructor must obtain either a chromosome spread (Figure 2, a photograph of all the chromosomes of one cell not arranged in any specific order) or a karyotype (Figure 1, see references for sources of karyotypes and spreads). At least three photostats of the spreads and one of the karyotypes for each student (or students may work in pairs ) are needed. If there is difficulty in obtaining spreads, the instructor can use just karyotypes and have full sets of chromosomes cut out from the karyotype and placed in individual containers. If this approach is necessary, ask the students to perform this task and do not tell them why, making sure the chromosomes are randomized and placed into individual containers (one cell s set of chromosomes per container).

To give students a specific responsibility while viewing, ask them to:
Listen for any time chromosomes are mentioned.
Look for the shapes of chromosomes.
Listen for the number of chromosomes that are in certain life forms.
Listen and watch for the physical relationships among DNA, chromosomes and the nucleus...are chromosomes inside DNA or is DNA inside chromosomes, are the chromosomes inside the nucleus?
Observe the models for accuracy; i.e., are the shapes of chromosomes accurate?

Start the video at the beginning where the title, Our Human Body: DNA: Life s Controller, appears. Allow the students to watch, uninterrupted until about 1 minute and 45 seconds into the video where an animal cell is filmed in mitosis.

PAUSE video where the chromosomes are lining up on the metaphase plate. Ask the students if they can identify the cell as animal or plant, and why. Ask if they can point out the chromosomes. Is there a visible nuclear membrane? What has happened to the nuclear membrane?

REPLAY the mitotic division sequence three or four times to make sure the movement of the chromosomes is clearly perceived and the formation of the two daughter cells is understood. RESUME the video and pause at the end of the animation of a cell undergoing mitosis, which ends at about 3 minutes into the entire video. REPLAY the sequence. Ask if the students see the similarity between the animation and the actual mitotic division seen earlier. RESUME the video and continue watching until the video animation of how DNA is wound into chromosomes and introduced by the narrator saying, "we now know that chromosomes are made up of long chains of DNA."

PAUSE video at about 4 minutes and 10 seconds, where the model depicts DNA being unwound from a chromosome. Ask the students to explain how they perceive this model as the mechanism of compacting DNA into the chromosomes.

RESUME the video and watch until the double stranded model of DNA is shown at about 5 minutes into the video.

PAUSE the video when the two strands are shown with their bases. With an erasable marker, indicate the distances between the bases of one strand of the helix as being .34 nanometers and then mention that about 2-3 centimeters of DNA are in each chromosome of a human.

RESUME the video and continue watching until about 8 minutes into the video where a mother and father are shown in a swimming pool with a baby and a karyotype is overlayed on this scene.

PAUSE the video. Ask the students if they can explain the karyotype and what it represents.

STOP the video. Total viewing time is about 10 minutes

Short Discussion
Encourage the students to share key points of the video. What is the reason for having chromosomes? Why are the chromosomes located in the nucleus? Why should cells go through mitosis or meiosis? What are the key differences in mitosis and meiosis? Was the representation accurate and realistic? Could they see any simple way to improve on the video?
Hands-on Activities
If you wish, you may have the students work in pairs.

If chromosome spreads are used, the students must carefully cut out each chromosome. If overlapping occurs, they will need to use two photostats to obtain one of each of the two overlapping chromosomes. This is why they need more than one photostat ( in addition to the normal accidents of nature which so often plague the classroom environment). After they have cut out the chromosomes (or if spreads were not available and they are given containers of 46 chromosomes), they are then given the task of arranging them in their proper order and orientation, and defining the probable sex. Remember to mention to them the recent discovery of determining the sex from the chromosomes... pulling down the genes.

A number of approaches can be used. A common system is to use an erasable marker and number the chromosome on one of the actual spreads, then use the others to cut out the pairs. Others choose the wild cut and slash approach of simply cutting out all the chromosomes and then trying to arrange them in their proper order. A time limit of about 1/2 hour is reasonable, based on basic frustration levels. The students should realize through this portion of the lesson that science is often truly a solving of puzzles.

The students that most often succeed best with the karyotype, recognize that the autosomes are generally arranged by size from the largest to the smallest and grouped by location of centromeres. The centromere is often difficult to locate but its presence is indicated by a slight indentation in the sister chromatids. As shown in Figure 1, Group A are the largest and metacentric (centromere in the middle of the chromosome); group B are large and submetacentric (the centromere is nearer to the top of the chromosome); group C are medium sized and submetacentric; group D are smaller and acrocentric (the centromere is at the top of the chromosome); group E smaller still and approximately metacentric; group F are again smaller and metacentric; group G are the smallest chromosomes and acrocentric. The sex chromosomes are in a category by themselves.

Additional Discussion
Discuss some of the diseases that are known to be inherited.

Guess how many genes are on any one chromosome.


If a person had no Y chromosome but in all outward appearances seemed male, would they allow them to compete in the Olympics as a male? If the person had a Y chromosome but in all outward appearances was female, would they allow them to compete as a female in the Olympics? What constitutes whether a person is a male or a female?

Visit a Cytogenetics laboratory to see how karyotypes are made.

Contact a local hospital that has a genetic counselor and ask her/him to teach the class about genetic counseling.

Science: Watch programs #1-4 of the video, Our Human Body.

Science: Research the known disorders which are based on known chromosomal anomalies. Web page http://www3.n/.nih,gov/omim/ (This is the National Institutes of Health web site... much too big to download all of its more than 35 megabytes of information!)

Science: Access Web page http://www.intelus.com/chrom/chromes.html and download their abundant information on all the human chromosomes!

Science: Research what the progress is on the Human Genome Project Web page http://www.intelus.com/chrom/chromes.html.

Science: Find all the information known about any one chromosome and draw a map of that chromosome,Web page http://www.intelus.com/chrom/chromes.html.

Social Studies: Research how chromosomal analysis has been used (to the objection of most scientists) to disqualify certain Olympic athletes. (Discover Magazine, sometime in the early 90 s had a nice article on this.)

Science/Art: Make a physical model of a chromosome.

Math: Assuming that all the chromosomes of a human cell stretched out end to end would be two meters long and each base is .34 nanometers from the next, how many bases are in all 46 chromosomes? Remember that the DNA is double stranded.


GENERAL REFERENCE SOURCES

Arms, K., and Camp, P. Biology. New York: Saunders, 1995.
Ayala, F.J., and Kiger, J.A. Modern Genetics. California: The Benjamin/Cummings Publishing,
1980.

Chromosomal Information on the Internet:
For a nice spread to cut out:
www.pathology.washington.edu/Ideogram/Albums/chromosomes_scans/human_g.gif

For general chromosome information on the Internet:
www.pathology.washington.edu/cyto_gallery.html
http://www3.n/.nih,gov/omim/
http://www.intelus.com/chrom/chromes.html.

For answers to questions:
For FAQ s
EFaletra@Hitchcock.org

Master Teacher: Peter Faletra
Lin-Wood School, Lincoln, NH