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Transcript
Immunology
Chapter 7, Lecture 1
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Richard L. Myers, Ph.D.
Department of Biology
Southwest Missouri State
Temple Hall 227
Telephone: 417-836-5307
Email: [email protected]
Homepage:
http://creative.smsu.edu/biology/
myersr/index.html
• TopClass: http://creative.smsu.edu
Organization and Expression of
Immunoglobulin Genes
• The immune system can respond to an
unlimited number of antigens
• Each antibody has a unique amino acid
sequence in the variable region
– the basis of this unique sequence lies in the
organization of the immunoglobulin genes
– these genes, however, have to be rearranged to
become a functional immunoglobulin gene
Genetic model
• The model of immunoglobulin genes should
address:
– the diversity of antibody specificities
– the variable and constant regions of antibodies
– the existence of isotopes with the same
antigenic specificity
• Two models proposed
– germ-line theory
– somatic-variation therory
• The model should account for the
generation of at least 108 different ab
– should speak to this tremendous diversity
• Germ line theory
– says that the genome contains the necessary
genes to account for 108 different specificities
• Somatic-variation theory
– says that the genome contains a small number
of genes
– but from these a large number of antibody
specificities are generated
– from mutational and recombinational events
Two gene model
• Proposed by Dryer and Bennett (1965)
– suggested that two separate genes encode a
single heavy or light chain
– one gene for the V region and one for the C
• The important aspect is that many, many Vregion genes are in the germ line
– but only single copies of C-region genes
• This allows for great diversity
• Results showed great diversity of antibody
specificity in the variable regions
Required reading
Hozumi, N., and S. Tonegawa. 1976. Evidence for somatic
rearrangement of immunoglobulin genes coding for
variable and constant regions. Proc. Natl. Acad.. Sci. USA
73:3628
• Proof of the two-gene theory was
provided by Tonegawa (1976)
– cleaved DNA from embryonic and
adult myeloma cells (used restriction
enzymes)
– separated fragments with
electrophoresis
– checked for hybridization with k
chain mRNA probes
– mRNA hybridized with two
fragments from the embryo DNA
– only a single fragment from the
myeloma DNA hybridized
• They reasoned that V and C genes undergo
rearrangement as embryo develops
– in embryo the V and C genes are separated by a
large distance
– during rearrangement they are brought together
• The actual research involved digestion and
electrophoresis
– two bands from the embryonic DNA
– one and from the myeloma DNA
• Actually, there are multigene families
Tonegawa’s experimental results
• Myeloma cells are equivalent
to differentiated plasma cells
• mRNA probes were used to
identify the DNA digests
• Found that embryonic DNA
gave two bands
• Only a single band from the
differentiated DNA
Multigene families
• Each multigene family contains “gene
segments”
• For example, the k and l light chain family
contains an L, V, J and C gene segment
• Antibody specificity is accomplished by
bringing together different gene segments
– VJ encodes the variable region of light chains
– VDJ encodes varible region of heavy chains
Example - l multigene family
• Tonegawa cloned the germ-line gene and
determined the nucleotide sequence
– showed that the gene contains two coding
regions called exons
• a V segment and a J segment
• these seperated by noncoding DNA called an intron
• The l multigene family contains two Vl,
four Jl and four Cl gene segments
• The first two code the variable region and
the last codes the constant region
Assignment
• Read Chapter 7, Organization
and Expression of
Immunoglobulin Genes
• Review question 4 (pg 192)