Generation of Diversity- B cells Flashcards Preview

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Flashcards in Generation of Diversity- B cells Deck (12)
1

Regions of a light/ heavy chains

-the constant regions are not next to the variable region in germline
-variable region at the site nearest to the constant consisted of a small gene segment called the J region
-the J region did not change its relative position in the genome in IgM and D producing B cells compared to the rest of the body but it did change in all other Ig producing B-cells- this implied a rearrangement of the DNA
-in heavy chain additional region, D region or diversity gene segment
-the remainder of the Variable region is coded for the V gene segment
-all at different chromosomal locations

2

Reasons where there must be gene rearrangment

-thousands of bases of DNA information to code for those amino acids
-billions of bases of information
-only 3 billion bases in the human genome

3

Gene rearrangement leads to many variable regions

-light chains have V and J regions that are randomly chosen
-heavy chains have V,D, and J regions that are also randomly chosen
-recombination is caused by RAG proteins and recombination signal sequences
-in addition you can combine light and heavy chains
-regions of true randomness exist as well

4

Number of combinations

-by random combination of these elements within each cluster, a large number of different V regions can be created (for example Kappa light chains have 40 V segments x 5 J segments making 200 possible Kappa light chains)
-heavy chains have 51 possible Vs x 27 possible Ds x 6 possible Js or 8262 possible heavy chains)
-as each combination is independent one can have over 200 light chains x 8262 heavy chains making up variable regions with kappa light chains
-over a million possible combinations

5

DNA splicing

-RAG 1 and RAG 2
-each immunoglobin gene segment has signal sequences: signal sequences determine which segments can be joined to each other
-joining is imprecise and also there is a special mechanism to increase diversity at the CDR3 site
-joining of V and J in light chains can involve 4 different nucleotide codons (group of 3 bases) to create an in frame gene sequence coding for variable light chain
-joining of D to J and V-D in Heavy Chain gene formation can involve multiple reading frames, as most Ds can be read in all three reading frames

6

Novel genetic sequences from junctional diversity

- P nucleotides are found in all joining junctions
-the hairpins were opened with RAG and RSSs and the palindromic P nucleotides are generated
-N nucleotides are added by TdT which add random nucleotides to V-D and D-J
-N regions are not found in light chains
-N and P nucleotides add a bit of randomness to splice site

7

Productive rearrangement/ unproductive rearrangement

-in the early pro-B cell there is H-chain gene rearrangement D-J rearrangements on both chromosomes
-in the late pro-B cell there is H chain gene rearrangement- try V-DJ rearrangement on first chromosome if it works move on if it doesn't try second chromosome if it still doesn't apoptosis ( you test with a surrogate light chain)
-in Pre-B cell there is L-chain gene rearrangement
first you rearrange K gene on first chromosome and if that doesnt work move on to the next chromosome if that doesn't work do first lambda gene if that doesn't work second lambda gene

8

Selective splicing of primary RNA transcripts to switch from IgM to IgD

-IgD constant region are expressed in mature/naive B cells but there is no class switching involving gene rearrangements
-IgD is formed by selective splicing of an RNA that is transcribed from both IgM and IgD constant regions

9

Class switching

-when class switching occurs except to IgD, a variable region is joined to a new constant region
-this is DNA splicing
-during class switching, the variable region undergoes an unusually high rate of mutation- those mutations that yield amino acids that improve binding to antigen are selected for
-the enzyme is AID, looping out for switch region recombination
-can't go back to being IgM
-Order: IgM, IgD, IgG3, IgG1, IgA1, IgG2, IgG4, IgE, IgA2

10

Mutations take place during immune response

-somatic hypermutation
-takes place in the whole V domain
-single base changes that are random
-does not take place in the constant region

11

The T-cell Receptor

-the mechanism by which the T cell receptor is generated is similar to the mechanism of generation of immunoglobulin
-most mechanism of formation of the T cell receptor are the same as the B cell except the somatic hypermutation

12

Possible mutations

-Lack Rag: no T cells or B cells- combined immunodeficiency
-lack TdT- less diverse and less able to respond