lecture 2 Flashcards
what segments are the heavy chain and TCR beta encoded by?
V region encoded by the gene segments: V, D and J
what is the light chain and TCR alpha encoded by?
V region encoded by the gene segments: V and J
how and where are Ig genes found?
in the genome as families of gene segments
what occurs during B cell development of light chain?
-DNA containing Ig gene segments is deliberately broken between a V and J segment
-gene segments are rearranged to form functional Ig genes (exon)
-are transcribed and translated to give a new variable region of the antibody
what is this process called?
non-homologous end joining recombination
what two loci contain L chain genes?
kappa and lambda
what happens to encode the heavy chain?
-B cell randomly breaks between a D and J segment bringing them together
-DNA is rebroken again
-brings the joined piece of DNA next to the V region and transcript joins
how is DNA changed into a protein?
-somatic recombination where D-J rearrange
-then V-J or V-DJ join the rearranged DNA
-transcription occurs of primary transcript RNA
-splicing to form mRNA
-translation to reach the final polypeptide chain
what is the order Ig rearrangements occur in B cell development?
- heart chain gene segments rearrange (D-J and then V-DJ)
- light chain gene segments rearrange (kappa segments V-J first)
- if kappa unsuccessful, lambda segments V-J rearrange
where is the H chain locus found?
chromosome 14
where is the kappa chain locus found?
chromosome 2
where is the lambda chain locus found?
chromosome 22
how is the heavy chain organised in the human genome?
-each individual B cell breaks DNA bringing together the D and J region
-then join together the DJ to a V region
what are recombination signal sequences?
-special sequences that flank each of the V, D and J gene segments
-bits of DNA that are recognised to bring the segments together
what enzyme helps in rearrangement?
V(D)J recombinase which cut the DNA at the correct place to bring the segments together
what does RAG genes stand for?
recombination activating gene
what do RAG-1 and RAG-2 encode?
lymphoid specific components of the recombinase
what do mutations in RAG genes lead to?
immunodeficiency
what is allelic exclusion?
-each B cell has to make 1 antibody so is carried out on one chromosome
-process is now turned off so it doesn’t happen on the other chromosome as would produce more than one antibody for the cell
what is light chain isotope exclusion?
each B cell expresses either a rearranged kappa OR lambda light chain - never both
what are the mechanisms for generation off antibody diversity?
- multiple gene segments for each chain
- combinatorial diversity - different V, D and J segments recombine to produce different sequences
- combinations of heavy and light chains
- junctional diversity increases diversity of variable region
- somatic hypermutation
what are the additions in junctional diversity?
-imprecise joining (small differences in sequence where VD and DJ segments join)
-N regions - random addition of nucleotides at junctions od VD and DJ by terminal transferase
what is somatic hypermutation?
-mutation frequency in antibody is orders of magnitude higher than seen in other areas of genome
where does somatic hypermutation occur?
in germinal centres
-as B cells recognise antigen and proliferate/become activated
how does somatic hypermutation (SHM) occur?
-AID acts on DNA to de-aminate cytosine to uracil
-uracil is then recognised by error prone DNA repair pathways leading to more mutations
what is the constant region of each heavy chain encoded by?
C region gene segment
what is thebfirst antibody to be expressed by each developing B cell?
IgM as Cu segment is physically closest to the V, D and J gene segments
what does switching to other classes require?
-further DNA recombination
-guided by switch regions
how does looping out to switch isotope occur?
-loops out the exon and joins the VDJ with new downstream constant region
-cuts the region out rearrange so cant be the same again
