TEACHER Activity Modeling Antibody Structure Using Chenille Stems Objective

TEACHER Activity Modeling Antibody Structure Using Chenille Stems Objective


Activity: Modeling antibody structure using Chenille Stems
Objective: This activity describes a way to model the basic structure
of antibodies. The model uses chenille stems to show the basic layout
of an antibody: the 2 heavy chains, 2 light chains, and the disulfide
bonds that hold those polypeptides together. The model is then used to
demonstrate the vast diversity in the variable regions of antibodies
using VDJ recombination.
Estimated Time: 20-30 minutes
Extensions: This activity can be extended by completing the Protein
Data Bank activity (http://www.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/alphabetical_list.html).
The Protein Data Bank activity requires online computer access.
Materials needed (32 students):
2 whole pipe cleaners of the same color (heavy chain segments) 64
1 pipe cleaner cut into 2 pieces (light chain segments) 32 total
1 “sparkle” pipe cleaner cut into 4 pieces (disulfide bonds) 32
Container of colored beads to use as VDJ segments 320 total

Important concepts:
Antibodies are made of 2 heavy chain polypeptides (blue in the
diagram) and 2 light chain polypeptides (red in the diagram). Each
chain is a polymer of amino acids.
Disulfide bonds (S-S bonds between cysteine residues) hold the 4
chains together. Two disulfide bonds connect the heavy chains
together, while one disulfide bond connects each light chain to
the nearest heavy chain.
The antibody takes on a Y-shape when all 4 chains interact.
From: http://chemistry.umeche.maine.edu/CHY431/Antibody.jpg
The trunk of the “Y” makes up the constant region of the antibody
and is specific to each species. The arms of the “Y” on the
antibody make up the variable region and form a specific shape
that recognizes and binds to one unique epitope (specific region
of an antigen).
Each mature B cell produces antibodies that recognize only one
target. There are only about 21,000 human genes total, so how are
millions of different antibodies created when there aren’t
millions of different genes?
Our B cells create tremendous variety in antibody structure from
three genes (two that code for light chains, one that codes for
heavy) using a special mechanism called VDJ recombination.
W hen a B cell is immature the heavy chain gene contains
multiple V segments, D segments, and J segments, and the light
chain gene contains multiple V and J segments.
As immature B cells mature, one V segment, one D segment, and one
J segment are selected at random and recombined into a revised,
shorter heavy chain gene. A similar process occurs in the light
chain gene (no D segment is present in this chain).
In mature B cells these shorter genes will provide the
instructions for producing the heavy and light chain polypeptides
that compose the structure of the unique antibody.
Creating the Antibody Model
Part One: Building the heavy and light chains
Select 4 chenille stems:
Two of the same color (heavy chains)
One of a color different from the heavy chains (will become
light chains)
One “sparkle” pipe cleaner (will become disulfide bonds)
Lay the two same-colored pipe cleaners side by side on a desk.
Cut the third pipe cleaner in half and lay alongside the outside
of the two large ones aligned at the top.
Cut the “sparkle” pipe cleaner into 4 even pieces – use two pieces
to join the heavy chains together at the lower half (the “constant
region”) of the pipe cleaners, and one to join each light chain to
a heavy chain.
Bend the heavy chains apart in a “Y” shape.

Part Two: Modeling VDJ recombination
Randomly choose three colored beads from the container, then
select the same three colors again so they have a pair of each–
each colored bead represents one V, one D, and one J segment on
the heavy chain.
Place three of the beads in a row on the variable region of one of
the heavy chains, then create the same pattern of beads on the
other heavy chain variable region.
Randomly choose two more beads from the container (these represent
the V and J segments on the light chain), place them on the end of
the light chain. Repeat with the same two colors for the second
light chain variable region so that both light chains match.

Part Three: The Discussion
Are there any two groups of students who have the exact same
colored beads on their model? (the color of the pipe cleaners does
not matter in this model)
There are only 10 different colors of beads in the container. How
many varieties of antibody might you be able to create?
Heavy chain: (10 V segments) X (10 D segments) X (10 J
segments) = 1000 possible combinations
Light chain: (10 V segments) X (10 J segments) = 100 possible
(1000 possible heavy chains) X (100 possible light chains) =
100,000 possible antibody variations
Now how about the real thing:
Heavy chain: (51 V)(25 D)(6 J) = 7650 possible combinations
Light chains: (40 V)(5 J) + (31V)(4 J) = 324 possible
(7650 possible heavy chains) X (324 possible light) =
2,478,600 possible combinations
BUT WAIT!! Because of additional recombination events and
mutations during mitosis, the actual number of possible
antibody variations is potentially much greater than 1X108
(see the Nature Article: The Double Helix and Immunology).
Figures created by Tracey Kwong, MCB Student Summer 2007
Adapted from Biological Science, 2nd edition, 2004, by Scott Freeman
(Prentice Hall)
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