Genetics 20
HC 15
20.2
Antibodies/Immunoglobins are proteins produced by the B-cells of the immune system,
whose function is to recognize foreign material (bacteria and viruses) and target it for
destruction. Antigens are such a foreign substance that is recognized by antibodies and thus
elicits an immune response. The recognition between an antibody and antigen is very
specific. A B-cell only makes one type of antibody, but an organism has millions of B-cell
types, so there are millions of different antibodies. Moreover, because gene encoding
antibodies allow variation in genetic change – giving variation in amino acid sequences of
antibodies – one antibody can recognize multiple antigens.
There exist millions more of antibodies then there are genes coding for them; this so,
because the DNA/antibody genes are cut and reconnected through site-specific
recombination. Therefore, only a few antibody precursor genes are needed that are spliced
in many different ways to give a great variation in polypeptides with differing amino acid
sequences, aka antibodies. They are tetrameric proteins (four subunits) with a light and a
heavy chain – connected by disulfide bridges – and a variable region in which the amino acid
sequence varies a lot, which causes that one antibody can recognize multiple antigens.
Splicing happens in pre-B-cells, once it has spliced the first time, the mature B-cell will
remember this specific splicing and make the same antibodies for life.
One type of light chain is the kappa light chain, which formation starts with its precursor
gene. Its structure is altered by site-specific recombination. Initially, it exists of 300 variable
V domains – four joining J domains – one constant C domain. All the V and J domains code
for another amino acid. During maturation of the B cell, the kappa light chain is formed
through splicing out random V and J domains, to end up with one V domain adjacent to one
of the four J domains. It can do so through recombination signal sequences (RSS) at the
beginning and end of each V and J domain. RAG 1 and RAG 2 recognize RSS generate two
dsDNA breaks, one at the end of a V and one at the beginning of a J the intervening
region is lost and the two ends are ligated, catalyzed by NHEJ proteins transcription
the fused V-J region is connected to the C domain (so intervening J domains are lost) via
splicing of the pre-mRNA final mRNA contains one variable, one joining and one constant
domain after this has occurred once, only this specific set of splice-site recombination
and splicing occurs so that only this mRNA/protein/antibody is formed.
All the V domains before the ‘chosen one’ are not lost nor degraded, they are just not
expressed. Every V domain has its own promoter and due to the fusion of a J domain to a
specific V domain, only that V domain is expressed – because of the enhancer the J domain
carries. So, the other V domains are not expressed.
Alternatively, the heavy chain of antibodies is produced similarly. It only has more V
domains and an extra 12 diversity D domains; in between V – J. Therefore, recombination
first involves D-J, followed by recombination of V to the DJ-region. In sum, this is V(D)J
recombination which only occurs in heavy chain genes.
In numbers:
- light chain 300 (V) x 4 (J) x 1 (C) = 1 200 variables
- heavy chain 500 (V) x 12 (D) x 4(J) x 1 (C) = 24 000 variables
o Together: 1 200 x 24 000 = 28 800 000 variable antibodies
HC 15
20.2
Antibodies/Immunoglobins are proteins produced by the B-cells of the immune system,
whose function is to recognize foreign material (bacteria and viruses) and target it for
destruction. Antigens are such a foreign substance that is recognized by antibodies and thus
elicits an immune response. The recognition between an antibody and antigen is very
specific. A B-cell only makes one type of antibody, but an organism has millions of B-cell
types, so there are millions of different antibodies. Moreover, because gene encoding
antibodies allow variation in genetic change – giving variation in amino acid sequences of
antibodies – one antibody can recognize multiple antigens.
There exist millions more of antibodies then there are genes coding for them; this so,
because the DNA/antibody genes are cut and reconnected through site-specific
recombination. Therefore, only a few antibody precursor genes are needed that are spliced
in many different ways to give a great variation in polypeptides with differing amino acid
sequences, aka antibodies. They are tetrameric proteins (four subunits) with a light and a
heavy chain – connected by disulfide bridges – and a variable region in which the amino acid
sequence varies a lot, which causes that one antibody can recognize multiple antigens.
Splicing happens in pre-B-cells, once it has spliced the first time, the mature B-cell will
remember this specific splicing and make the same antibodies for life.
One type of light chain is the kappa light chain, which formation starts with its precursor
gene. Its structure is altered by site-specific recombination. Initially, it exists of 300 variable
V domains – four joining J domains – one constant C domain. All the V and J domains code
for another amino acid. During maturation of the B cell, the kappa light chain is formed
through splicing out random V and J domains, to end up with one V domain adjacent to one
of the four J domains. It can do so through recombination signal sequences (RSS) at the
beginning and end of each V and J domain. RAG 1 and RAG 2 recognize RSS generate two
dsDNA breaks, one at the end of a V and one at the beginning of a J the intervening
region is lost and the two ends are ligated, catalyzed by NHEJ proteins transcription
the fused V-J region is connected to the C domain (so intervening J domains are lost) via
splicing of the pre-mRNA final mRNA contains one variable, one joining and one constant
domain after this has occurred once, only this specific set of splice-site recombination
and splicing occurs so that only this mRNA/protein/antibody is formed.
All the V domains before the ‘chosen one’ are not lost nor degraded, they are just not
expressed. Every V domain has its own promoter and due to the fusion of a J domain to a
specific V domain, only that V domain is expressed – because of the enhancer the J domain
carries. So, the other V domains are not expressed.
Alternatively, the heavy chain of antibodies is produced similarly. It only has more V
domains and an extra 12 diversity D domains; in between V – J. Therefore, recombination
first involves D-J, followed by recombination of V to the DJ-region. In sum, this is V(D)J
recombination which only occurs in heavy chain genes.
In numbers:
- light chain 300 (V) x 4 (J) x 1 (C) = 1 200 variables
- heavy chain 500 (V) x 12 (D) x 4(J) x 1 (C) = 24 000 variables
o Together: 1 200 x 24 000 = 28 800 000 variable antibodies
