Approximately 40 functional VH segments

Figure 6.11 Immunoglobulin molecules and genes. (A) Schematic representation of an immunoglobulin molecule showing two heavy and two light chains joined by disulphide bonds, the heavy chains are in turn joined to a further pair of heavy chains to form the stalk of a Y configuration. The variable region containing antigen binding sites are shown in black with the constant region in dark grey. (B) Overview of how immunoglobulin gene segments come together to form a complete immunoglobulin gene in B lymphocytes. The variable region is assembled by gene rearrangement from the V and J gene segments (light chains) or V, D, and J gene segments (heavy chains). A leader sequence (L) directs the immunoglobulin polypeptide chains to the secretory pathways and is then cleaved (immunoglobulins being extracellular proteins). (C) Schematic of immunoglobulin gene clusters. Note this is a simplified representation and not to scale. Redrawn with permission from Janeway et al. (2005).

among fish, birds, and mammals suggesting an ancient initial event before the vertebrate radiation. OPN1LW (encoding the red photopigment 'opsin 1 long-wave-sensitive' found in cone photoreceptors) and OPNTMW

(encoding the green photopigment, 'opsin 1 medium-wave-sensitive') are highly homologous and share 96% identity at the amino acid level. They are found in a head-to-tail tandem array on the X chromosome at Xq28 with

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