Generation of antibody diversity Flashcards
(32 cards)
immunoglobulin variation types
isotopic -> variations in heavy chain types + either kappa or lambda chain
allotypic -> smal variation in the constant region
idiotypic -> variability in the variable chain of an antibody = the variation that generates antibody specificity
what is germline DNA
information acquired from sex cells (egg and sperm)
somatic recombination?
somatic = alterations in DNA after conception –> somatic mutations can occur in any of the cells of the body except germ cells (sperm and egg)
“structure” of the immunoglobulin genes
germline genes are a large cluster of gene segment -> which are non functional (until somatic recombination)
what are the gene segments that code for immunoglobulins
variable region (V) segments
diversity (D) segments -> only Ig heavy and TCR beta chains
Joining (J) segments
constant (C) segments
RSS? where are they located
recombination signal sequence, and either right before or after a gene segment in the loci
where are the RSSs located (with specific segments)
each V segment has an RSS right after it
each J segment has an RSS immediately preceeding it
D segment has an RSS on each side
what are the two different types RSSs
RSS with a 23-base-pair-spacer and RSS with a 12-base-pair-spacer
structure of a RSS
an RSS contains a heptamer and a nonamer of a conserved sequence (a base sequence in a DNA molecule that has remained relatively unchange throughout evolution) + either the 23 or 12 bp spacer
RSS function
critical in somatic recombination -> bringing different segments of DNA together while losing the loop in between them (changing the DNA sequence of the Ig)
what is the “12-23 rule”
RSS with a 23-base-pair spacer can be only joined an RSS with a 12-base-pair sequence
RAG?
recombination activating gene enzymes
function of RAG enzymes
RAG-1/2 bind to the RSSs and bring them together = forming a synapsis. then RAG induces DNA cleavage exactly at the junction of the gene segment and the RSS. this leaves a hairpin of DNA at the end of the gene segments
Ku70:Ku80 ?
protein complex which holds the two hairpin DNAs together after RSSs are cleaved
the formation of the signal joint
after RSS cleavage, Ku70:Ku80 protein complex holds together the DNA ends. DNA ligase then binds the DNA ends together which generates a precise signal joint (RSSs are bound together)
how is the coding joint produced?
the hairpin DNA needs to be cut open so the two strands can be joined together -> facilitated by a comnination of proteins. DNA protein kinase and artemis open the haipin. then a bunch of enzymes process the DNA ends and facilitate their ligation
TdT ? function?
terminal deoxynucleotide transferase -> one of the enzymes that facilitate coding joint ligation
SCID?
severe combined immunodeficiency
gene rearrangement can result into additional nucleotides added to the joint, what are different types of additions?
- non-templated addition -> when the TdT adds extra random bases
- palindromic additions -> by DNA polymerase (dont know the mechanism)
what is the importance of nucleotide additions in the coding joint?
they alter the potential peptide reading frame –> basically, it is essential for producing the diversity of antibodies
allelic exclusion ?
a single T or B cell will only express the product of a single allele for each of its relevant antigen receptor genes = aka a single BCR will express only one heavy chain even though it has two receptors for it)
why is monospecificity important?
it ensures that a cell will only be able to interact with one antigen –> important because during clonal expansion, the clones produced will be identical as the primary B cell (meaning they will al have the same specificity)
location of somatic hypermutation
germinal centres of secondary lymphoid organs (where B cells are interacting with antigens)
what is the process of somatic hypermutation
the accummulation of mutations in the V region of immunoglobulins -> both heavy and light chains
