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Biochemistry

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Purines, Pyrimidines and Nucleic Acid Metabolism

Question
44 out of 59
 

HLA genes are located on (AIIMS Nov 2011)



A Long arm of chromosome 6

B Short arm of chromosome 6

C Chromosome 7

D Chromosome 9

Ans. B short arm of chromosome 6

a. The major histocompatibility complex (MHC) contains at least 128 functional genes, more than 20% of which have functions in immunity, and is the most gene-dense region of the human genome.

b. It is also the region with the most disease associations: most, if not all, autoimmune diseases are associated with genes in the MHC, reflecting the central role of MHC molecules in focusing immune responses.

c. For historical reasons, human MHC molecules are known as HLA, for human leukocyte antigen, and mouse as H-2, for histocompatibility 2, with the different genes indicated by letters.

d. The MHC genes are traditionally divided into three classes: the MHC class I and class II genes, which encode the antigen-presenting MHC molecules; and the class III genes, a miscellany of genes encoding some molecules with important immune functions and others with no known immune function.

e. The gene encoding b2-microglobulin lies outside the MHC, on a different chromosome. All the other genes encoding chains of the class I and class II MHC molecules are present in several different copies within the MHC, and each cell expressing them displays several different MHC molecules. This is important for ensuring sufficient diversity to bind peptides from a wide range of microorganisms, an issue we discuss in more detail below.

f. The peptide antigen-presenting MHC molecules are known as classical MHC molecules. There are also structurally related molecules of both classes that do not function in the presentation of peptide antigens to T cells: these are known as non-classical MHC molecules. The non classical MHC class II molecules (DM and DO in human) regulate peptide loading onto classical MHC class II molecules.

g. A notable distinction between classical and non-classical MHC molecules that bears on their different functions is the polymorphism of the classical MHC molecules.

Purines, Pyrimidines and Nucleic Acid Metabolism Flashcard List

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