Recombination

Question 1

Antibody Structure

Answer 1

• 2 Identical Heavy Chains (Each chain has 4 Ig Domains)
• 2 Identical Light Chains (Each chain has 2 Ig domains)
• Each Ig domain is about 110 amino acids longs
• Each Ig domain and chains are held by a disulfide linkage.

Question 2

3 Hypervariable Regions

Answer 2

Where the greatest diversity in the Heavy and Light variable regions is found.

• CDR 1,2,3
• The greatest diversity is at CDR 3 (most important for antibody binding specificity).

Question 3

What is the main difference between the secreted vs membrane forms of antibodies?

Answer 3

C-terminal tails due to alternative splicing

*antibody is in membrane form in resting B cells and in secreting form in plasma (activated B cells) cells.

Question 4

T-Cell Receptor Structure

Answer 4

• 1 B chain + 1 a chain OR 1 s chain + 1 y chain
• Each Ig domain and B & a chains are held by a disulfide bond.
• V regions have CDRs.
• Ony membrane form.

Question 5

Heavy Chain Gene vs. Light Chain Gene

Answer 5

H: V, D, J, C gene segments
* VH: CDR 1, 2 are derived from V segment and CDR 3 is derived from D & J segments.

L: V, J, C gene segments
* VL: CDR 1,2 are derived from V segment and CDR 3 is derived from J segment.

Question 6

Light Chain Recombination

Answer 6

V and J segments join and then VJ combines with C segment.

Question 7

Heavy Chain Recombination

Answer 7

D and J segments join and then DJ joins with V segment. Finally VDJ combines with C segment.

Question 8

What is RSS and its function?

Answer 8

Recombination Signal Sequence is a 7 nt “heptamer” separated by a 12 or 23 spacer. It mediates recombination.

Question 9

What is a 12-23 rule?

Answer 9

A 12 spacer RSS can only recombine with a 23 spacer RSS.

Question 10

Mechanism of Recombination

Answer 10

1. Chromosome Looping
2. RAG 1, 2 recombination enzymes recognizes RSS sequences.
3. The 12 spacer RSS synapses (recombines) with the 23 spacer RSS which is carried out by the RAG 1, 2 proteins.

Question 11

N/P Nucleotide Addition

Answer 11

• In P nucleotide addition, additional nucleotides are added at the site of hairpin cleavage.
• In N nucleotide addtion, additional nucleotides are added by the terminal transferase.
• N/P Nucleotide addition creates more diversity.

Question 12

Allelic Exclusion

Answer 12

When only one copy of two chromosomes is used.

• Why each B cell only makes one antibody.
• Chromatin on the excluded locus is heterochromatized (methylated).
• The successful chromatin is open (acetylated).

Question 13

Light Chain Isotype Exclusion

Answer 13

Unlike heavy chain, light chain has 2 isotypes, k and λ.

• If k is successful, λ is inhibited.
• λ rearrangement only occurs if k on both chromosome s are nonproductive.

Question 14

Class Switching Mechanism

Answer 14

Class switching is when IgM becomes IgG, IgA, or IgE.

1. CD40 induces AID (Activation-induced cytidine deaminase) enzyme.
2. AID converts C’s to U’s in switch sequences by deamination.
3. Cutting and rejoining of DNA in switch sequences occur.

Question 15

Lymphoid Tumors and Translocations

Answer 15

B and T cell tumors commonly have translocations of oncogenes into antibody loci that results in over-expression of the oncogene leading to uncontrolled growth.

Question 16

What is somatic hypermutation?

Answer 16

Mutations in the v gene promote B cells to produce antibodies that have a higher affinity for antigen over time.

• Immunoglobulin variants are generated by somatic mutation.

Question 17

T Cell Receptors and Somatic Mutation

Answer 17

T cell receptors DO NOT go through somatic mutations.