A gene duplication

The three molecules of the IL-1 family, interleukin-la (IL-la), interleukin-1 3 (IL-ip), and interleukin-1 receptor antagonist (IL-IRa) map to the long arm of chromosome two in the human genome. It appears that the family arose via a gene duplication event some 350 million years ago, and the molecules possess between 27.5 and 36% sequence identity with each other (Table 2) [1,14,15]. In addition, the genes for the two IL-1 receptors IL-1R1 and IL-1RII [16,17], and an IL-1R accessory protein (IL-lRacP), which binds to the IL-1, IL-1 receptor complex [18], have been identified. Together, these molecules via their differential activity serve primarily to modulate the host defense mechanism. 

Lauster R (1989) Evolution of type II DNA methyltransferases. A gene duplication model. J Mol Biol 206 313-321... 

This reaction takes place in a second ATP-grasp domain within the enzyme. The active sites leading to carbamic acid formation and carbamoyl phosphate formation are very similar, revealing that this enzyme evolved by a gene duplication event. Indeed, duplication of a gene encoding an ATP-grasp domain followed by specialization was central to the evolution of nucleotide biosynthetic processes (Section 25.2.3). 

Present-day structures of proteins provide some hints on the role of gene duplications in evolution. Proteins commonly occur in families. These are structurally related enzymes catalyzing reactions of the same class with different substrate specificities. Examples are families of proteases or dehydrogenases. In addition to this, one observes interesting regularities in the structures of many globular proteins Substructures (so-called motifs) are often repeated exactly or with minor modifications only. Such repetitions were found in the same protein molecule as well as in different protein molecules. Both the modular structure of polymers as well as the existence of protein families can be explained by a gene duplication mechanism. 

A FIGURE 9-32 The generation of diverse tubulin sequences during the evoiution of eukaryotes, (a) Probable mechanism giving rise to the tubuiin genes found in existing species, it is possibie to deduce that a gene duplication event occurred before speciation because the a-tubulin sequences from different species (e.g., humans and yeast) are more alike than are the a-tubulin and p-tubulin sequences within a species,... 

Related genes and their encoded proteins that derive from a gene duplication event are paralogous those that derive from speciation are orthologous. Proteins that are orthologous usually have a similar function. 

Figure 25 The structures of the anaerobic cobaltochelatases CbiK and CbiX. CbiK is a monomeric enzyme derived from a single polypeptide chain (helices in blue, purple sheets). CbiX is a dimeric enzyme, where each monomer (green helices/ orange sheets) corresponds to one of the domains observed in CbiK. It has been suggested therefore that CbiK arose from a gene duplication and fusion event of cbiX.

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