How do new genes arise? The standard way is via gene duplication. As discussed in chapter 13, mutations may cause the duplication of a segment of DNA that carries a whole gene or several genes. The original copy must be kept for its original function but the extra copy is free to mutate and may be extensively altered. In most cases the mutations that accumulate will inactivate the duplicate copy. Less often, the extra copy will remain active and be altered so as to perform a related but different function from the original copy.
Multiple duplication followed by sequence divergence may result in a family of related genes that carry out related functions. One of the best examples is the globin family of genes. Hemoglobin carries oxygen in the blood, whereas myoglobin carries it in muscle. These two proteins have much the same function, have similar 3-D shapes, and their sequences are related. After the ancestral globin gene duplicated, the two genes for hemoglobin and myoglobin slowly diverged as they specialized to operate in different tissues (Fig. 20.11).
The actual hemoglobin of mammalian blood has two alpha (a)-globin and two beta (b)-globin chains forming an a2/p2 tetramer, unlike myoglobin, which is a monomer of a single polypeptide chain. The a-globin and b-globin were derived by further duplication of the ancestral hemoglobin gene. In addition, the ancestral a-globin gene split again, to give modern a-globin and zeta (Z)-globin. The ancestral b-globin gene split again, twice, to give modern b-globin and the gamma (g)-, delta (S)-, and epsilon (e)-globins (Fig. 20.12).
These globin variants are used during different stages of development. At each stage, the hemoglobin tetramer consists of two a-type and two b-type chains. The Z-globin and e-globin chains appear in early embryos, which possess Z2/e2 hemoglobin. In the fetus, the e-chain is replaced by the g-chain and the Z-chain is replaced by the a-chain, so giving a2/g2 hemoglobin. A fetus needs to attract oxygen away from the mother's blood, so the a2/g2 hemoglobin binds oxygen better than the adult a2/b2 hemoglobin (Fig. 20.12).
globins Family of related proteins, including hemoglobin and myoglobin, that carry oxygen in the blood and tissues of animals
(A) Over the course of evolution, a variety of gene duplication and divergence events gave rise to a family of closely related genes. The first ancestral globin gene was duplicated giving hemoglobin and myoglobin. After another duplication, the hemoglobin gene diverged into the ancestral a-globin and ancestral p-globin genes. Continued duplication and divergence created the entire family of globin genes. (B) The different members of the hemoglobin family are adapted for specific functions during development. Thus the fetus uses two a chains and two g chains to form its hemoglobin tetramer. This form is able to extract the oxygen from the mother's blood because it has a higher affinity for oxygen than the adult form of hemoglobin.
The globin genes are an example of a gene family, a group of closely related genes that arose by successive duplication. The individual members are obviously related in their sequences and carry out similar roles. During evolution, continued gene duplication may give rise to multiple new genes whose functions steadily diverge until their ancestry may be difficult to recognize; this gives a gene superfamily. The genes of the immune system provide good examples of gene families and superfamilies.
In eukaryotes, retro-elements that encode reverse transcriptase are relatively common (see Ch. 15). Consequently, occasional reverse transcription of cellular mRNA molecules may occur. This gives a complementary DNA copy that may be integrated into the genome. This results in a duplicate copy of the gene, although this lacks the introns and promoter of the original gene. Such inactive copies are known as pseudogenes and usually accumulate mutations that inactivate the coding sequence. Rarely, a pseudogene may end up next to a functional promoter and be expressed. This gives a duplicate functional copy of the original gene that may be altered by mutation as already discussed.
Rare mistakes during cell division may result in the whole genome being duplicated. In particular, errors in meiosis may give diploid gametes. Fusion of two diploid gametes would give a tetraploid zygote and hence a tetraploid individual. More often, a triploid individual forms by fusion of one mutant diploid gamete plus one normal haploid gamete. Most triploids are sterile, as they give gametes with incorrect numbers of chromosomes. But occasionally triploids will be able to generate tetraploid progeny. Aberrant ploidy levels are fairly common in plants. Around 5 in 1000 plant gametes gene family Group of closely related genes that arose by successive duplication and perform similar roles gene superfamily Group of related genes that arose by several stages of successive duplication. Members of a superfamily have often diverged so far that their ancestry may be difficult to recognize
A. Globin family tree a z g e s b Myoglobin vvy
B. Fetal hemoglobin is better
Adult hemoglobin a2p2
Adult hemoglobin a2p2
Fetal hemoglobin a2y2
Fetal hemoglobin a2y2
Reverse transcriptase may generate duplicate genes that lack introns and promoters and are located far away from the original copy.
Occasionally whole genomes may be duplicated.
Paralogous and Orthologous Sequences 549
In this example, an ancestral gene duplicated and diverged into genes A and B, which by definition are paralogs. These two genes were both present when the ancestral species diverged into species 1 and species 2. Thus both species 1 and species 2 have genes for A and B (referred to as A1 & B1 and A2 & B2 respectively). Each such pair are still paralogs. However, since species 1 and 2 are now two separate species, the A1 and A2 genes are orthologs, and the B1 and B2 genes are also orthologs.
are diploid. Therefore, in a cross between two different parents, approximately 2.5 in 10-5 zygotes will be tetraploid. Over time, the duplicate copies of genes in a tetraploid organism will gradually diverge. Eventually, once the duplicate copies diverged far enough to be distinct and have assumed new functions, the organism will effectively become "diploid" again.
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