Final answer:
MHC I and MHC II differ in structure; MHC I has an alpha chain associated with β₂ microglobulin, while MHC II has two similar-length protein chains. The antigen-binding cleft for MHC I involves α₁ and α₂ domains, while for MHC II, it involves the α₁ and β₁ domains. β₂ microglobulin associated with MHC I is not polymorphic.
Step-by-step explanation:
The differences between the dimers of MHC I and MHC II molecules revolve around their structure and antigen presentation roles. MHC I molecules consist of a larger alpha (α) chain that is non-covalently associated with a smaller protein called β₂ microglobulin. Only the alpha chain spans the cytoplasmic membrane, with this chain folding into three domains: α₁, α₂, and α₃. The antigen-binding cleft for MHC I is formed by the α₁ and α₂ domains. On the other hand, MHC II molecules are comprised of two protein chains of approximately similar lengths, the alpha and beta chains, both spanning the plasma membrane and each folding into two domains: α₁ and α₂, and β₁ and β₂, respectively. The antigen-binding cleft of MHC II is formed by the α₁ and β₁ domains. Regarding polymorphism, MHC I and MHC II molecules show polymorphism within their alpha and beta chains, respectively, though the degree and pattern of polymorphism vary between the two. However, β₂ microglobulin is not polymorphic. It does not have multiple alleles and is the same in all MHC I molecules.
MHC I and MHC II are transmembrane glycoproteins that assemble as dimers in the cytoplasmic membrane of cells, but their structures are different. MHC I consists of a longer α protein chain coupled with a smaller ß₂ microglobulin protein, while MHC II consists of two protein chains (an α and a ß chain) that are approximately similar in length.
The α chain of MHC I folds into three domains (α₁, α2, and α3). MHC II chains fold into two domains (α₁ and α2, and ß₁ and ß₂). In MHC I, the antigen-binding cleft is formed by the α₁ and α2 domains, while in MHC II, the cleft is formed by the α₁ and ß₁ domains.
B2 microglobulin is a small protein that associates with the α chain of MHC I. It does not contribute to the antigen-binding cleft, but is important for the stabilization and proper folding of MHC I molecules. B2 microglobulin is not polymorphic and humans carry a single allele for it.