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They carry genetic information as RNA (Figures 1.3b and 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). Oncogenes have 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.
| Oncogene | Proto‐oncogenic function | Host organism | Virus‐induced tumor |
|---|---|---|---|
| Abl | Tyrosine kinase | Mouse, cat | Pre‐B cell leukemia |
| Erb‐B | Epidermal growth factor | Chicken | Sarcoma |
| Fes | Tyrosine kinase | Cat, chicken | Fibrosarcoma |
| Fms | Receptor of macrophage colony‐stimulating factor (M‐CSF) | Cat | Sarcoma |
| Fos, jun | Join to produce gene regulatory protein | Cat, chicken | Osteosarcoma, fibrosarcoma |
| Myc | Gene regulatory protein | Chicken | Sarcoma |
| Raf | Serine/threonine kinase | Chicken, mouse | Sarcoma |
| H‐ras | GTP‐binding protein | Rat | Sarcoma |
| Rel | Gene regulatory protein | Turkey | Reticuloendotheliosis |
| Sis | Growth factor from platelets | Monkey | Sarcoma |
| Src | Tyrosine kinase | Chicken | Sarcoma |
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.
| Cell and tissue type | Function |
|---|---|
| A. Plant cells and tissues | |
| All plant organs consist of three basic kinds of tissue (epidermal, ground, and vascular tissue) | |
| Epidermal tissue | |
| Epidermis | Epidermal cells form one or two layers of epidermis with a thick cuticle |
| Guard cells | Gas exchange |
| Trichomes | Epidermal hair cells: storage of terpenes; protection against evaporation |
| Root hairs | Uptake of water and ions |
| Endodermis | Innermost layer of the cortex |
| Protoderm | Primary meristem (growth of the epidermal tissues) |
| Ground tissue | |
| Parenchyma | Not very specialized, flexible primary walls |
| Mesophyll cells | Photosynthesis |
| Storage parenchyma | Storage tissue |
| Xylem parenchyma | Exchange of substances with xylem vessel elements |
| Collenchyma cells | Living cells with thick primary walls (support), no secondary walls and no lignin |
| Sclerenchyma | Dead cells with a support function |
| Fiber cells | Long extended lignified sclerenchyma cells |
| Sclereids (stone cells) |
Irregularly shaped sclerenchyma |