Читать книгу Cell Biology - Stephen R. Bolsover - Страница 105

DNA helicases are essential proteins required to open up the DNA helix during replication. In Bloom's syndrome, mutations give rise to a defective helicase. The result is excessive chromosome breakage, and affected people are predisposed to many different types of cancers when they are young.

People who suffer from the genetic disorder xeroderma pigmentosum are deficient in one of the enzymes for excision repair. As a result, they are very sensitive to ultraviolet light. They contract skin cancer even when they have been exposed to sunlight for very short periods because thymine dimers produced by ultraviolet light are not excised from their genomes.

Figure 4.7. The human α‐and β‐globin gene family clusters. ψ indicates a pseudogene. Adults only express α, β, and δ, and of these the expression of δ is very low. The exon/intron boundaries of the β‐globin gene are indicated at the bottom.

In fact, there is an evolutionary rationale to this apparently perverse arrangement. As we will see (page 114), a single protein is often composed of a series of domains, with each domain performing a different role. The breaks between exons often correspond to domain boundaries. During evolution, reordering of exons has created new genes that have some of the exons of one gene, and some of the exons of another, and hence generates novel proteins composed of new arrangements of domains, each of which still does its job.