B-III B- and T-cell genes Flashcards
What is the definition of a gene?
A stretch that makes the whole thing functional, including enhancer/silencer, promoter, UTRs, ORFs, and terminator.
What is the importance of introns in genes?
Regulation of expression and signaling of alternative splicing.
What is a limitation of large genomes?
A big genome incurs a big cost of replication/repair and has a positive correlation with extinction.
What defines the isotype of an antibody?
The structure of the heavy chain.
What constitutes the antigen binding portion of B-cell receptors?
Variable heavy and light chain regions.
What are the hypervariability (HV) regions in B-cell receptors?
Regions within the variable region that varies a lot
What are the complementary determining regions (CDR)?
Regions that are responsible for binding antigens, derived from hypervariability regions.
How are BCR genes organized in germline?
Generated through combination of V, (D), J and C gene segments on different loci.
What does VDJ recombinase do?
It facilitates gene rearrangements in developing lymphocytes. Essential for expression of TCR and BCR.
What is the process of somatic recombination in light chains?
Linking V and J segments together
What segments does the heavy chain involve in somatic recombination?
Diversity and joining segments are joined to fuse V-DJ regions.
What is RSS in the context of gene rearrangement?
Recombination signal sequence that guides the recombination.
What happens during deletional joining?
Creates a hairpin loop with two RSSs next to each other, losing everything in between.
What is inversional joining?
A process that creates a knot to join RSSs without losing any genetic material.
What is combinatorial diversity?
Diversity resulting from recombination of V-(D)J gene segments.
What is junctional diversity?
Diversity from different joining ways of V-J or V-DJ segments, allowing addition or removal of nucleotides.
What is somatic hypermutation?
Mutations in the variable domains to enhance affinity of antibodies.
What is allelic exclusion in B-cells?
Inhibition of further rearrangements once a functional heavy or light chain locus is produced.
What is the structure of T-cell receptors (TCR)?
Heterodimer of two transmembrane glycoproteins joined by disulfide bonds, typically α & β chains.
What is the gene organization of the TCR compared to BCR?
TCR genes are smaller and segments of alpha and delta are in the same locus.
What is the main difference in diversity between TCR and BCR?
TCR has higher diversity but lacks somatic hypermutations.
What triggers the inhibition of beta chain gene recombination in T-cells?
Formation of a recombined functional beta-chain locus.
Fill in the blank: The ________ of an antibody is defined by the structure of its heavy chain.
isotype
What are the different loci for light chains?
Kappa and lambda
How many loci are there for heavy chains?
One
Describe how the heavy chain is assembled
- D-J are rearrenged through somatic recombination
- V-DJ are rearrenged through somatic recombination
- Transcription occurs, generating a primary RNA transcript containing the rearrenged VDJ, as well as everything after the J-gene-region
- Splicing occurs, splicing away intermediate regions, combining VDJ with the correct C region.
- Translation occurs, giving the polypeptide chain
Bonus: if it is functional, it inhibits this from happening a second time through allelic exclusion
Describe how the light chain is assembled
- After a functional heavy chain is generated, V and J are rearrenged through somatic recombination
- Transcription occurs, generating a primary RNA transcript containing the rearrenged VJ, as well as everything after the J-gene-region
- Splicing occurs, splicing away intermediate regions, combining VJ with the correct C region.
- Translation occurs, giving the polypeptide chain
How are the RSS matched?
23-bp RSS needs to be matched with 12-bp RSS because of the conformation of RAG 1&2 enzyme in the VDJ recombinase
When does deletional joining occur?
When the transcriptional direction is the same for the recombined elements
When does inversional joining occur?
When the transcriptional direction is different for the recombined elements
When are RAG 1-2 expressed in the VDJ recombinase?
In immature B/T cells
What causes the diversity of the BCR?
Combinatorial diversity, junctional diversity, somatic hypermutations
What are N-nucleotides and where are they mainly found?
Random addition of up to 20 non-encoded nucleotides by enzymes in VDJ recombinase. A part of what adds junctional diversity. Mainly found in the D segment recombination, added by TdT
What are some differences between TCR and BCR when it comes to somatic recombination?
In TCR, the D-segment in the β-chain has one 12-bp-RSS and one 23-bp-RSS. In BCR, the segments has the same RSS
Why does TCR lack somatic hypermutations?
Because its T-cells that activates this in B-cells. T-cells have nothing that activates this in them, and its less necessary
What does RAG1:RAG2 do?
Bind to 23bp and 12bp RSS regions, and when activated by dual binding cleave so V-(D)J can be recombined
