Lecture 6 Flashcards
Whats in a kappa light chain
A single constant region gene next to 5 J segments and downstream there are a larger number of 38 V genes
Whats in a heavy chain
There’s a single constant region (codes for IgM). Next to it are 6 J segments, 23 different D segments and downstream, a larger number of V segments
What needs to happen during B cell differentiation
Rearrangement of light and heavy chain genes (somatic recombination)
What happens during somatic recombination
A V gene is spliced next to a J gene and all of the intervening DNA is deleted so you get gene rearrangement occurring on the chromosome.
The VJ segments are now spliced together and they lie close to the kappa constant region gene. Each V gene has its own promoter so once its spliced, there’s an enhancer element that can cause that gene to be transcribed
The intervening sequences are removed by RNA processing and you end up with mRNA
What are CDR1 and CDR2 encoded by
V segments
The mechanism of recombination- what does it involve
Involves lymphocyte specific recombinases and conserved recognition signal sequences
Where are RSS’s found and what do they do
They’re found directly adjacent to the coding sequence of V, D and J gene segments. This guide rearrangement of the V, D and J segments
What does each V gene component contain
Contains its own promoter, a leader exon, an intervening intron and an exon that encodes the first 3 framework regions
What does initiation of VDJ recombination require
Recombination activating genes one and two (only expressed in developing lymphocytes).
They introduce double stranded breaks between the terminus or the rearranging segments and its adjacent recombination signal sequence.
What are the enzymes responsible in VDJ recombination
Recombinases: recognise signal sequences that lie adjacent to the genes that need to be joined together
What are RSSs comprised of
Conserved hepatmer (7 base pairs) and a nonamer (9 base pairs) that are separated either by 12 or 23 random nucleotides
What is the V(d)J recombinase and what do they include
Its a complex of several enzymes required for somatic V-region gene recombination.
-Normal DNA cleavage/repair enzymes
-RAG1-RAG2 protein complex encoded by recombination activation genes
-Terminal deoxynucleotide transferase (TdT)
What do mutations in V(D)J recombinases lead to
Severe immunodeficiency (Scid), no adaptive immunity. There aren’t any B or T cells
What happens during somatic recombination (part 2)
The RAG1-RAG2 complex recognises the RSS signal sequences and aligns the RSSs adjacent to the gene segments to be joined
The RAG1-RAG2 complex has endonuclease activity and cleaves the DNA adjacent to the RSS sequences so that peices can be joined together
The cleaved DNA is repaired to form the coding joint and the signal joint
What’s RAG1-RAG2’s structure and function
The complex is made up of a dimer of RAG-1 which is the bit that recognises the DNA and does the cleavage
RAG1-RAG2 acts as a transposase.
RAG2 acts as a cofactors when it binds DNA it closes (undergoes a confirmational change)
