Introduction to Genetic Inheritance
A one-day workshop with lecture and lab for middle school students
Adapted by Andreas Madlung
This document is a step-by-step lesson plan for a one-day workshop for introducing middle
school students to DNA, inheritance, and ploidy. It describes hands-on-activities and concludes
with a short quiz (plus key).
Learning Goals: a) DNA is the genetic material, b) every cell in an organism contains the same
DNA complement (a genome), c) cells may contain one or several copies of its genome (haploid,
diploid, polyploid), d) DNA is tightly wound to make it fit into a cell (wrapped around histones),
e) DNA is made of two complementary strands, f) DNA can be purified from the cell and
manipulated.
Morning lecture/activity (1.5 hours)
Questions to start out with:
1. How come we look like our parents? (Heritable traits (genes), DNA transmits this
information)
2. Why don’t we look exactly like our parents? (Our genes are reshuffled)
3. Where is DNA stored? Which part of the cell? (Nucleus)
4. What is it made of? (sugar, phosphate and a “base” that we give the letters ATC and G)
5. How long do you think DNA is? (If you blow up a cell to the size of a gel capsule or a
vitamin pill, it would be 10 m (30 feet) long.
6. How can you stuff so much DNA into a small place like a cell? (The DNA has to be
tightly wound around something (histone proteins), like yarn around a spool).
DNA is the genetic material that is found in every cell. All heritable traits are passed
down from parents to their children in the form of DNA molecules that encode the
information that each cell needs to produce enzymes that perform all necessary function
in the body. Features, such as hair or eye color are also transmitted from parents to
children in the same way.
What does DNA look like?
On a molecular scale, DNA is made of three subunits: a phosphate group, a sugar
molecule and a base, which specifies the function of the strand of DNA. One such unit is
called a “nucleotide”.
-Draw on the board P-sugar-P-sugar chain with branches coming off from the sugar.
Then show overhead Figure 11.7
Question: If you have 4 different bases at your disposal and want to make a strand with
just two units (nucleotides), how many different combinations are possible?
(allow time for them to figure out)
AA, AT, AC, AG (times 4) or 24 = 16
Question: If you have 5 nucleotides? Answer: 54 = 625
In other words: Combinations are basically unlimited!
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How much DNA is there in each cell? If you were to magnify an average body cell so
that it was the size of a gel capsule (like the kind for vitamins or drugs), and you were to
magnify the DNA molecules in these cells by the same amount, then each cell would
have about 10 meters of DNA. How is it possible to fit so much DNA into such a small
cell? The answer is that cells wrap up DNA around little pieces of proteins, called
histones, that serve the same function that the spool of the yarn serves in organizing and
tightening yarn that is wrapped around it.
(Showing a figure for “DNA/histones” from any Introductory textbook would work here)
Activity: Let’s give it a try!
Obtain a gel capsule and a spool of yarn. Measure out 20 meters of the yarn. Why 20
meters? Because DNA actually consists of 2 strands tightly bound together.
Can you fit all the DNA into the cell?
DNA is made up of two complementary strands
We said DNA molecules have two strands (20m of yarn instead of 10m). Let’s look why:
DNA looks a lot like a zipper: for each base called A, it can stick together with a base
called T on the other strand. And for each base G there is a base called C that it can pair
with. (adenine, thymidine, guanine, cytidine)
(Showing a figure of the two strands of DNA from any Introductory textbook would
work here)
Show the first animation (Note: some terms will be unfamiliar) (Make sure computer has
Flash add-in)
Task during the animation: find the answer to these questions: (write them on board
before starting it)
1.
2.
3.
4.
5.
6.
Where in the DNA do you find “hydrogen bonds”? What do they connect?
What is meant by “DNA backbone”?
DNA is charged. Why?
Assume DNA were a circular staircase. What part of the DNA would the steps of this staircase
correspond in this analogy?
Which bases pair with each other?
Which pair of bases do you think is more strongly held together? Why? (CG has 3 hydrogen
bonds, AT only 2).
http://www.sumanasinc.com/webcontent/anisamples/molecularbiology/DNA_structure.ht
ml
After the animation ask the questions again. Collect answers.
Now ask: Ok, apart from all the structural information I have told you about, what do
you think DNA is used for or might be useful for? (If no one answers ask this question
and let them discuss it in groups of 2-3 for a few minutes: DNA is unique to each person,
like a fingerprint. It also hides a lot of information about your physical traits. Assume
someone could get a hold of your DNA, what could they use it for? How might that
benefit or harm you?)
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Possible answers: crime scene DNA match, paternity testing, medical history or future,
life or medical insurance policy sale, employer preference for certain DNA encoded
traits.
These types of issues can be discussed in a separate session. An interesting movie to
watch that gives some food for thought (but is 2 hours long) is GATTACA.
Here is a short trailer (GATTACA) (play animation)
http://academy.asd20.org/kadets/lundberg/movies/gattaca/gattaca.mov
Ask: Why/how do you think they came up with this title (GATTACA, all base letters).
If time permits, show first segment of NOVA movie: (10 minutes)
http://www.pbs.org/wgbh/nova/genome/program.html
(Instructions for a Human Being)
The following website has more animations, in case students ask for more.
http://academy.asd20.org/kadets/lundberg/animations.html
for PCR: http://www.dnalc.org/ddnalc/resources/animations.html
What you heard is that traits are passed on from parents to children. But you never look
exactly like your parents because DNA is reshuffled. How is it reshuffled?
During the production of egg and sperm cells the chromosome sets are halved: a so-called
diploid cell is made into a haploid cell. While deciding which of the chromosomes goes
into each sperm or egg cell, the 2 chromosomes that are almost identical (one from Mom
and one from Dad) are broken and rejoined, making “recombinant” chromosomes.
Essentially, chromosomes are formed that have information that came from Mom AND
Dad. Polyploid cells are cells that arise from eggs and sperm where the chromosome
numbers was not halved before their fusion. Two diploid cells will then make a tetraploid
cell.
Afternoon (1.5 hours):
Now let’s try to see if we can extract DNA out of cells. DNA is essentially identical in
structure in every living organism. We will try to get DNA out of a variety of fruit. The
recipe for the DNA extraction was developed by Julie Townsend in Iowa and posted on
http://www.idtdna.com/support/technical/TechnicalBulletinPDF/At_home_Strawberry_D
NA_Extraction.pdf
We will use this protocol but we will try many fruits. Each of you should do this
individually with at least one type of fruit. Which fruit works best?
NB: This recipe works very well. However, the large amounts of precipitate are a mixture
of DNA and polysaccharides. If you have gel electrophoresis equipment it would be a
nice additional experiment to load some of this precipitate into a gel, stain it and see the
DNA migrate through the gel.
From the rest of the fruit we will make fruit smoothies.
Peel and chop up the remaining fruit. Place it in the blender and add some ice, vanilla
yoghurt and blend for a minute.
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Needed:
Fruit, ingredients for buffer, Falcon tubes with 95% ethanol
For the smoothies: food grade ice cubes, blender, vanilla yoghurt
Gel capsules (drug store pharmacy), yarn
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Quiz: What did you learn today?
1. What are the three components that DNA is made of?
2. What bases combine with each other in the double strands?
3. What are the forces called that connect the two strands of DNA?
4. What kind of information is stored in DNA?
5. If you had 4 different base types (nucleotides) and wanted to make as many
different combinations as possible in a chain of 3 nucleotides (eg. AAT), how
many possibilities are there?
6. Think about this: each cell in the body looks different and has different
functions, yet all have the same exact copy of DNA. How do you think this is
possible?
7. Before egg and sperm cells fuse with each other to form a fertilized egg
(zygote) they reduce their chromosome number by one half. Why?
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Quiz: What did you learn today?
1. What are the three components that DNA is made of?
Sugar, phosphate, base
2. What bases combine with each other in the double strands?
A-T, C-G
3. What are the forces called that connect the two strands of DNA?
Hydrogen bonds
4. What kind of information is stored in DNA?
All gene information (proteins which make enzymes that perform functions), traits
(eye color, hair color, nose shape, etc.), giftedness from certain activities (but not all),
diseases or likeliness of developing certain diseases.
5. If you had 4 different base types (nucleotides) and wanted to make as many
different combinations as possible in a chain of 3 nucleotides (eg. AAT), how
many possibilities are there? How about for 4 nucleotides (eg. TCGG)
For 3: 24 = 16
For 4: 34 = 81
6. Think about this: each cell in the body looks different and has different
functions, yet all have the same exact copy of DNA. How do you think this is
possible?
Each cell only uses a subset of the genes present in the cell. Therefore, each cell must
have a way to “switch on or off” the genes it needs for the functions it has.
7. Before egg and sperm cells fuse with each other to form a fertilized egg
(zygote) they reduce their chromosome number by one half. Why?
If cells with two sets of chromosomes fuse with each other the resulting zygote
will contain four copies of each chromosome. Its offspring would have cells with
eight copies (and so on). In order to remain at the same chromosome number
generation after generation cells go through meiosis to reduce their chromosome
number in half before creating a new zygote. Sometime this does NOT happen. The
result is “polyploidy”.
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