Download Alternative Splicing : Why it Matters to Cells

Alternative Splicing: Why it Matters to Cells
Goal
Learners will understand why alternative splicing matters to cells.
Objectives
1. Learners will describe the role of alternative splicing in a cell.
2. Learners will review alternate splicing and coding sequences used to make
product.
Introduction
DNA, which is a long string of coding sequences, is divided into parts. Imagine
that DNA is like a long string of beads, where each bead is a gene. Genes do many
things, from deciding your eye color and hair color, to keeping your body systems
running. Genes are responsible for almost everything!
Inside all genes are exons, and many of these genes also have introns. The exons
are copied and used as the blue print to make something, but the introns, when present in
a gene, are cut out because they do not directly make products. Some genes will only
make one product, and when the transcription process happens, all the introns are cut out,
and all the exons are “glued” together.
Other genes function differently. Some genes can make many products! Instead
of “gluing” all the exons together, the gene uses the process of alternative splicing to
“mix and match” the options and selects only some of the exons to make a product with
them. It might only use several exons to make a product, and leave the others unused.
For example, if you had a gene that made sandwiches, its exons might include
bread, peanut butter, tuna fish, mayonnaise, jelly, pickle, turkey, onion, and lettuce. The
introns could be represented by things like pens, pencils, and sticks. No one wants pens,
pencils, or sticks in their sandwich, so these are cut out right away. If a gene didn’t
practice alternative splicing, it would make a sandwich using every single one of these
sandwich-making materials (but not the pens, pencils, or sticks- these are introns). But if
the gene practiced alternative splicing, it might make many types of sandwiches.
For example, the gene might make:
 peanut butter, jelly, and bread
 peanut butter, pickle, and bread
 turkey, onion, pickle, mayonnaise, lettuce, and bread
 tuna fish, mayonnaise, and bread
No matter what combination the gene uses, it still makes some type of sandwich.
If you mix together any combination of sandwich-making materials, you could make lots
of different types of sandwiches. If we think of the products of genes (in this case,
sandwiches) as having different jobs, one single gene could make hundreds or thousands
of different products to do different jobs by mixing and matching exons! This would cut
down on the numbers of genes an organism needs to function, and make them very
versatile. This activity will demonstrate alternative splicing.
Materials needed: The students will need:
 White board or chalkboard
 Pencil and paper
 Bread
 Peanut butter
 Jelly
 Salt
 Pickle
 Onion




Lettuce
Mayonnaise
Turkey
Tuna fish
Procedure
Students will be divided into groups of three and assigned the following
responsibilities:
1. Copy Machine
2. Messenger RNA
3. Factory
1. Student 1 (Copy Machine) will be given a list of ingredients that includes: bread,
salt, pickle, jelly, bread, peanut butter, lettuce, bread, onion, and salt. The student
will consult with their group to decide which ingredients will be needed to make a
peanut butter and jelly sandwich. The student will be instructed to cross off any
item they do not think they need to make a peanut butter and jelly sandwich.
Once the student has selected the ingredients, they will pass the ingredient list on
the student designated as messenger RNA.
Note: This student represents the copy machine that is located in the library. The
student will cross off the “exons” that are not needed to make the final product
(the PB&J). This first activity will show how the copy machine copies only
useful, “exons”, and sends the messenger RNA off to the factory to make the final
product. In cellular terms, the student will go through the process of alternate
splicing by selecting “exons”, segments of the DNA to copy into mRNA.
2. Student 2 (messenger RNA) will carry the list of ingredients to Student 3
(Factory). Because student 2 is the messenger RNA, the list can be taped onto
their front, where the factory can “read” the message.
3. Student 3 (the factory) will make a sandwich that includes ALL the ingredients on
the list. This student will make a peanut butter and jelly sandwich. A PB&J has
been created!
Note: The factory always follows its recipe. Even if the recipe is wrong, the
factory doesn’t know, and makes the finished product incorrectly.
Application and Discussion
Products of the cell have a job. In this case, there is another cell that actually
serves boxed lunches. If you had a bad cell that didn’t remove the “introns” that kept
sending bad sandwiches to the boxed lunch maker, no one would want to buy the boxed
lunches because the sandwiches were bad. The boxed lunch seller would go out of
business! Thus, cells must cut out the introns before the exons are used for making the
PB&J sandwich. Also, if the boxed lunch seller had orders for tuna fish sandwiches, and
the copy machine (student 1) forgot to cross off “jelly,” the wrong sandwich (product)
would be made. This would also make the boxed lunch seller unhappy! A properly
functioning copy machine and factory are very important to a cell.
A practical application of alternative splicing
Researchers have been studying the proteins (or sandwiches) made by different
neurons in our bodies. Cells known as “neurons” carry important information around our
bodies, including pain signals. Neurons are found all over our bodies; they make up our
brain, and are found in the nerves that run down our spinal column (back bone). These
neurons send signals at a very fast rate. Think about when you have accidentally burned
your finger. It seems to take no time at all for the pain your finger feels to reach your
brain, and for your brain to say “OUCH!” and send a signal back to move your finger.
One protein in particular, called e37b, is produced throughout the nervous system,
and is responsible for being a pain-relaying messenger. If enough e37b messengers are
made, a pain signal is relayed around the body. However, researchers discovered another
form of e37 can be produced through alternative splicing, and this protein is called e37a.
Alternative splicing makes the e37a protein after an injury has happened. The
presence of the new e37a protein changes how easily a person can feel pain, and it
actually makes injuries more painful. Why do you think our bodies make a protein that
makes our injuries hurt more? People are still studying this, but we think that injuries
become more painful so you do not make them any worse.
For example, imagine that while you were out running around and playing a game
of soccer, you trip and fall, and twist your ankle. Pretty soon, your ankle starts to hurt.
The pain is caused by the damage you did to your ankle when you tripped, but it feels
even worse because your body is producing the e37a protein. Since your ankle hurts so
much, you have to sit out the rest of the game. If your ankle did not hurt so much, you
might have been tempted to get up and keep playing, and damage your ankle even more.
So let us take the injury example and the e37a protein a little further. When you
get home from the soccer field, your ankle still hurts. The first thing you do is get out a
bottle of pain killers, like Tylenol®, Advil®, Excedrin®, or whatever will help make the
pain go away. Have you ever wonder how these pain killers work? They block (or
inhibit) the pain messengers. Your ankle is still hurting, but the message is not getting
through, or not as loudly as before.
Scientists are hopeful that their discovery of how the body uses alternative
splicing to make e37a will be useful in managing pain. If scientists could make a drug
that would block e37a, that drug might be highly effective at controlling the pain of
serious injuries. Imagine all the pain being gone! Managing pain is important in the
medical field in particular, because some sicknesses and injuries are so severe, the only
way to stop the pain is to give someone the drug morphine.
Morphine makes the pain stop, but it also affects other neurons, not just the ones
carrying pain. Morphine gets in the way of talking, thinking, and being able to move (to
stand, walk, etc…). If someone is taking morphine, they can’t function normally. If we
could find a drug that would just block the e37a protein, the pain could be stopped
without all the side effects!
If we apply our sandwich analogy to our soccer example above, e37a and e37b
are two different sandwiches. When you are running around playing soccer, your body is
making the e37b sandwich, which is a peanut butter and pickle sandwich. Your body
would use the peanut butter and pickle sandwich to transmit small amounts of pain. On
the other hand, when you fall and sprain your ankle, you body switches modes and starts
to make a peanut butter and lettuce sandwich instead. The peanut butter and lettuce
sandwich is the e37a protein, and it is transmitting a lot of pain! As scientists, if we
could find a way to stop your body from using alternative splicing to make the peanut
butter and lettuce sandwich, or if we could “mess up” the sandwich so it could not send
pain signals, we would have a great pain-stopping drug!
Evaluation
1. What does the sandwich activity demonstrate? Describe the activity using the
following terms: transcription (copy machine), mRNA, translation site (factory),
alternate splicing and coding sequences.
2. Explain (in terms of sandwich making) how scientists are using alternative
splicing to understand pain and to create pain-blocking drugs.