Читать книгу Cell Biology - Stephen R. Bolsover - Страница 106

We do not yet fully understand the construction of our nuclear genome. Only about 1.1% of the human genome codes for exonic sequences (i.e. makes protein) with about 24% coding for introns. Most protein‐coding genes occur only once in the genome and are called single‐copy genes.

Many genes have been duplicated at some time during their evolution. Mutation over the succeeding generations causes the initially identical copies to diverge in sequence and produce what is known as a gene family. Members of a gene family usually have a related function, for example the products of the globin family transport oxygen from our lungs to our tissues. These genes generate related proteins or isoforms, which are often distinguished by placing a Greek letter after the protein name, for example, hemoglobin α and hemoglobin β. Different members of a family sometimes encode proteins that carry out the same specialized function but at different times during development. The α‐ and β‐globin gene families, illustrated in Figure 4.7, are an example. The α‐globin gene cluster is on human chromosome 16 while the β‐globin gene cluster is on human chromosome 11. Hemoglobin is composed of two α globins and two β globins. The different globin proteins are produced at different stages of gestation, embryo to fetus to adult, to cope with the different oxygen transport requirements at each step. The duplication of genes and their subsequent divergence allows the expansion of the gene repertoire, the production of new protein molecules and the elaboration of ever more specialized gene functions during evolution.

Some sections of DNA are very similar in sequence to other members of their gene family but do not produce mRNA. These are known as pseudogenes. There are two in the α‐globin gene cluster and one in the β‐globin gene cluster (Ψ in Figure 4.7). Pseudogenes may be former genes that have mutated to such an extent that they can no longer be transcribed into RNA. Some pseudogenes have arisen because an mRNA molecule has been copied back into DNA by an enzyme called reverse transcriptase found in some viruses (Medical Relevance 3.1 on page 39). Such pseudogenes are immediately recognizable because some or all of their introns were spliced out before the integration occurred. Some have the poly(A) tail characteristic of intact mRNA present in the gene (page 76). These are called processed pseudogenes.