An antibody molecule consists of two identical light chains (220 amino acids each) ad two identical heavy chains (about 440-450 amino acids each) held together by disulfide bridges; this constitutes the monomeric form of an antibody. Enzymes papain cleaves each monomeric form into two fragments that bind to the antigen (designated as FAB; fragment with antigen binding) and one fragment which does bid to antigen but forms crystals (hence called Fc, crystal forming fragment).
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About 100 amino acid long amino-terminal ends of both light and heavy chains constitute their variable region denoted VL and VH, respectively; the amino acid sequence of this regions of the heavy and light chains are called constant region (designated as CH and CL, respectively) since there is little variation in the amino acid sequence of this region among the antibodies belonging to the same class.
Each antibody molecule has two antigen-binding sites or domains, each domain being constituted by the variable regions of one light and one heavy chains of an antibody molecule form its effector function domain, which determines its interaction with the other components of the immune system. The light chains are of two sorts as: Kappa (K) and lambda (λ); the type of a light chain is determined by its constant region. Apart genes encode
- the Kappa (located in human chromosome 2)
- lambda (chromosome 22) light chains
- the heavy chain (chromosome 14)
The variable region of each chain, in fact, contains 3 highly variable regions, called hyper variable regions and denoted as CDR1, CDR2 and CDR3 (CDR = complementarity-determining region) separated by 4 invariant regions called framework regions (designated as FR1, FR2, FR3, FR4). The constant region of each heavy chain has 3 homologous regions (CH1, CH2 and CH3) which most likely originated from a common parental gene (3 tandem repeats of the parental gene, followed by mutations).
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