An Introduction to Molecular Biotechnology

Table 3.7 Classification of major animal and human pathogenic viruses.

Viral nucleic acid (Table 3.7) is enclosed by a protein envelope or capsid. Many viruses carry a biomembrane on the outside, which is derived from the host cell. It contains viral proteins (envelope proteins) that act as antigens. Viral proteins are often very variable. By modifying their surface antigen, whenever they multiply, they are able to outcompete the immune system, which cannot keep up the speed to produce the latest specific antibodies. Viral proteins are tailor‐made for each other. This enables them to spontaneously form supramolecular complexes and infectious viral particles.

Retroviruses, such as the HIV pathogen, are medically very significant (Table 3.7). They carry genetic information as RNA (Figures 1.3band 3.28). The retroviral genome codes for a relatively low number of gene products, among others, for reverse transcriptase, which translates viral RNA into DNA (cDNA). Oncogeneshave the ability to transform cells into tumor cells. The discovery of viral oncogenes was essential for the understanding of regulatory mechanisms that are involved in cell division, cell differentiation, and the development of cancer (Table 3.8).

Figure 3.28 Infection cycle and genome of retroviruses. (a) Genome composition of retroviruses: gag, genes coding for capsid proteins, which will be further processed by a protease; pol, codes for reverse transcriptase; env, codes for envelope proteins, which are also cleaved through proteolysis; and onc, oncogene. (b) Infection cycle of a retrovirus. RVT, reverse transcriptase.

Table 3.8 Viral oncogenes that may play a part in the emergence of tumors.

The cellular equivalent of a viral oncogene is known as proto‐oncogene.

Some viral infections can cause cancer in humans: the best studied are the human papillomavirus ( HPV), which causes 90% of cervical cancers. A recent development of a vaccine against HPV was a milestone in molecular biotechnology.

Phages and viruses are important not only as pathogens but also as vectors for cloning and gene therapy.

Our body can protect itself against infections by mechanical barriers, an innate and an adaptive immune system(Table 3.9). A treatment of this complex topic lays outside the scope of this introductory chapters. The reader is referred to Murphy and Weaver (2017), Alberts et al. (2015), and Parham (2015) for more information. By vaccination, a number of viral diseases can be prevented. Many of the modern vaccines have been produced by genetic engineering. The recombinant vaccines have less side effects than the former ones, produced in eggs or other systems.

Table 3.9 Overview of important cell types in plants and animals.