Page 56 - Genetics_From_Genes_to_Genomes_6th_FULL_Part3

350 Chapter 10 Genome Annotation

Figure 10.10 Exon shuffling. Suppose two genes are broken Figure 10.12 Evolution of the globin gene family.
in introns and joined together as shown. Transcription of the newly Duplication of an ancestral gene, followed by divergence of the
reshuffled gene will produce a primary transcript that can be spliced duplication products, established the α- and β-globin lineages.
into a mature mRNA encoding a novel protein, regardless of where Further rounds of duplication and divergence within the separate
in the introns the breakages occurred. If the different exons encode lineages generated the genes and pseudogenes of the current-day
different protein domains, the domain architecture of proteins can globin gene family.
change over the course of evolution, as seen for the transcription Ancestral globin gene
factors in Fig. 10.8.
Exon
Gene 1 Gene 2 Intron
Duplication and divergence
Ancestral -globin gene Ancestral -globin gene
Exon
shu ing Duplication and divergence Duplication and divergence
New gene -globin gene -globin-like gene -globin gene -globin-like gene

Transcription Further duplications Further duplications
and splicing and divergences and divergences
mRNA for protein
with new domain
architecture -globin, -globin-like genes, -globin, -globin-like genes,
and pseudogenes and pseudogenes
Figure 10.11 The genes for human hemoglobin
polypeptides are located in two genomic clusters.
(a) Schematic representation of the α-globin locus. The five functional receptor family includes about 1000 genes. The duplica-
genes are indicated with purple boxes, the two pseudogenes with tion and divergence process is crucial for the creation of
black boxes. All of these genes are transcribed in the same
direction (left-to-right on the map). The red box is the locus control new raw material for evolution. Once a gene has dupli-
region (LCR) described later in this chapter. (b) Schematic cated, divergence allows either or both of the copies to
representation of the β-globin locus; this cluster has five functional assume new specialized but related functions, as long
genes (green) and one pseudogene (brown). as one or both of the copies still fulfills the role of the
(a) -globin genes on human chromosome 16 original gene.
LCR 1 2 1 The genes in such families may be clustered together
on one chromosome or dispersed on several chromosomes.
kb In the case of the hemoglobin family, the α-globin gene
30 20 10 0 cluster (also called the α-globin locus) on chromosome
16 contains five functional genes, while the β-globin clus-
(b) -globin genes on human chromosome 11 ter (β-globin locus) on chromosome 11 also has five genes
LCR G A 1
(Fig. 10.11). The sequences of all the α-like genes are more
similar to each other than they are to the β-like sequences,
kb and vice versa. The β-like genes are exactly the same
60 50 40 30 20 10 0 length, and the five β-like genes have two introns at exactly
Expressed genes Pseudogenes the same position; in fact, the α-like genes also have two
introns at the same positions.
These comparisons suggest that all of the globin
-like -like -like -like
genes can be traced back to a single ancestral DNA se-
quence (Fig. 10.12). Hundreds of millions of years ago,
this ancestral globin gene duplicated, and one copy
With the use of bioinformatics, researchers can see moved to another chromosome. With time, one of the
that each gene family evolved by a process of duplication two copies gave rise to the α-lineage, the other to the
and divergence from an ancestral gene. The two DNA β-lineage. Each lineage then underwent further duplica-
sequence products of a duplication event, which start out tions to generate the present array of α-like and β-like
identical, eventually diverge as they accumulate different genes in humans. By comparing genomes of different
mutations (Fig. 10.12). Additional rounds of duplication organisms, it is possible to estimate when these various
and divergence can further increase the number of related duplication events occurred. For example, the β-globin
genes. For example, the human genome has ten functional clusters of humans and chimpanzees have the same genes
members of the hemoglobin gene family, while the olfactory in the same order, but some other primates have one