Question

Explain how a gene can be duplicated. Explain how a duplicated gene can result in novel gene function over the course of evolution.

Answer 1

Exons act as modules for building genes that has the role in evolution . At one side, an individual exon from one gene may be copied and used in another gene. At the other side, an entire gene, including both exons and introns, may be duplicated. In such a case, mutations can accumulate in one copy without elimination by natural selection as long as the other copy is under selection to remain functional. The selectively neutral copy may then evolve to a new function, become expressed at a different time or in a different cell type from the first copy, or become a nonfunctional pseudogene.

The rate of gene duplication is a 1 % probability that a particular gene will be included in a duplication in a period of 1 million years. After the gene has duplicated, differences evolve as the result of the occurrence of different mutations in each copy. These accumulate at a rate of 0.1% per million years.

The most common type of gene duplication generates a second copy of the gene close to the first copy. In some cases,the copies remain associated and further duplication may generate a cluster of related genes.The best example of a gene cluster is that of the globin genes, which constitute an ancient gene family and play an important role in animals,the transport of oxygen.The major constituent of the vertebrate red blood cell is the globin tetramer, which is associated with its heme group in the form of hemoglobin. globin genes are divided into three exons.In red blood cells of adult mammals,the globin tetramer consists of two identical alpha chains and two identical beta chains. Embryonic red blood cells contain hemoglobin tetramers that are different from the adult form. The embryonic and fetal forms have a higher affinity for oxygen, which is necessary in order to obtain oxygen from the mother's blood.The division of globin chains into alpha-like and beta-like reflects the organization of the genes.Each type of globin is encoded by genes organized into a single cluster.From the organization of globin genes in a variety of species, we should be able to trace the evolution of present globin gene clusters from a single ancestral globin gene.

The leghemoglobin gene of plants, which is related to the globin genes, may provide some clues about the ancestral form, though of course the modern leghemoglobin gene has evolved for just as long as the animal globin genes.(Leghemoglobin is an oxygen carrier found in the nitrogen-fixing root nodules of legumes.)The furthest back that we can trace a true globin gene is to the sequence of the single chain of mammalian myoglobin, which diverged from the globin lineage -800 million years ago in the ancestors of vertebrates. The myoglobin gene has the same organization as globin genes, may take the three-exon structure to represent that of their common ancestor.The next stage of globin evolution is represented by the state of the globin genes in the amphibian Xenopus laevis, which has two globin clusters. However, each cluster contains both alpha and beta genes, of both larval and adult types.Therefore, the cluster must have evolved by duplication of a linked a alpha-beta pair, followed by divergence between the individual copies.Later, the entire cluster was duplicated.

Since a gene duplication occurs in only one cell ,either in a single-celled organism or in the germ cell of a multi-cellular organism its carrier usually compete against other organisms that do not carry the duplication .If the duplication disrupts  the normal function of an organism has a reduced reproductive success compared to its competitors and in certain cases ,the duplication of a certain gene might be immediately beneficial, providing its carrier with a fitness advantage.