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
- 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
- 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
- photomicrograph of a spread (Figure 2, see references for other sources)
- metric ruler
- Karyotype card (Figure 3)
- 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
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
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?
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.
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
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
Science: Find all the information known about any one chromosome and draw
a map of that chromosome,Web page
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
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
Chromosomal Information on the Internet:
For a nice spread to cut out:
For general chromosome information on the Internet:
For answers to questions:
For FAQ s
Master Teacher: Peter Faletra
Lin-Wood School, Lincoln, NH
Lesson Plan Database
Thirteen Ed Online