Читать книгу An Introduction to Molecular Biotechnology - Группа авторов - Страница 32

Viruses (or phages when found in bacteria) are not autonomous organisms. Although they have some cell elements in common with bacteria (DNA or RNA as genetic information) (Table 3.7), they depend on host cells for their propagation. They invade bacterial, plant, or animal host cells to live as parasites. Excessive viral multiplication causes the death of host cells and thus disease in the host. The way how a virus enters the body and how it establishes itself in cells is very intricate and differs between virus and cell type.

Table 3.7 Classification of major animal and human pathogenic viruses.

Class	Example/disease
I. dsDNA (double‐stranded DNA)
Papilloma virus	Papilloma warts, cervical cancer
Adeno virus	Infections of the respiratory tract, tumors in animals
Herpes simplex virus 1	HV I (blisters on skin), HV II (blisters on genitals)
Varicella zoster virus	Chicken pox, shingles
Epstein–Barr virus (EPV)	Mononucleosis, Burkitt lymphoma
Smallpox virus (variola)	Smallpox
II. ssDNA (single‐stranded DNA)
Hepatitis B virus (HPV)	Hepatitis B
Parvovirus	“slapped‐cheek disease”
III. dsRNA (double‐stranded RNA)
Reovirus	Diarrhea viruses, diseases of the respiratory tract
IV. ssRNA (working as mRNA)
Poliovirus	Poliomyelitis
Rhinovirus	Common cold
Coronavirus	Common cold, respiratory disease, SARS, MERS, Covid‐19
Hepatitis A virus	Hepatitis A
Hepatitis C virus	Hepatitis C
Yellow fever virus	Yellow fever
Togavirus	Rubella
West Nile virus	Flu‐like symptoms
Zika virus	Flu‐like symptoms
Dengue virus	Dengue fever
V. ssRNA (used as matrix for mRNA synthesis)
Rhabdovirus	Rabies
Paramyxovirus	Measles, mumps
Influenza virus	Influenza viruses (H1N1, H5N1)
VI. ssRNA (used as matrix for DNA synthesis)
Retrovirus	RNA tumor viruses, HIV (AIDS)

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 1.3b 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 cells with thick lignified secondary walls
Ground meristem	Primary meristem (growth of ground tissue)
Vascular tissue
Phloem	Transport of synthesized nutrients (sucrose, amino acids) to roots, stems, and fruits
Sieve tube element	Living cell without nucleus and ribosomes, sieve plates between neighboring sieve cells
Companion cell	Exchange of substances with sieve tube elements
Xylem	Transport of water and inorganic ions
Tracheids	Long tubular system consisting of dead cells (sclerenchyma)
Vessel elements	Lignified secondary walls with pits, surrounded by living xylem parenchyma
Procambium	Primary meristem (growth of vascular tissue)
B. Animal cells
The human body contains more than 200 cell types and 4 types of tissues (epithelia, connective tissue, nerves, and muscles)
Embryonic stem cell	Omnipotent cell that can differentiate into all other cell types
Epithelia
Intestinal cells	Prismatic epithelial cells, secretion of digestive juices, and absorption of nutrients
Ciliated epithelium	Prismatic epithelial cells, secretion and absorption, transport of mucus (bronchial epithelium)
Glandular cells	Cubic epithelial cells in glands and kidney tubules, main function secretion
Endothelial cells	Simple squamous epithelium inside blood vessels
Connective tissue
Fibroblast	Production of proteins for the extracellular network, including collagen and elastin and many other proteins
Osteoblast	Bone‐producing cell
Chondrocyte	Cartilage production, secretion of collagen and chondroitin sulfate
Adipocyte	Production and storage of fat in fat tissue
Mast cells	Storage and release of histamine
Blood
Hematopoietic stem cell	Precursor cell of all other blood cells
Erythrocyte	Oxygen and CO2 transport through hemoglobin
Platelet	Blood coagulation
Lymphocyte	Specificity and diversity of immune response
T cells	T helper cells (Th) recognize antigens and activate B cells
Cytotoxic T cells	Tc cells recognize antigens and attack infected cells
B cells	Form antibody‐secreting plasma cells
Monocytes	Migrate to infection foci and mature into macrophages, devouring bacteria and debris
Granulocytes (leucocytes)
Neutrophilic granulocytes	Phagocytize bacteria
Eosinophilic granulocytes	Destroy parasites, important in allergies
Basophilic granulocytes	Release histamines in some immune reactions
Natural killer cells	Destroy infected body cells and tumor cells
Nerve tissue
Neuron	Reception, storage, and transport of information
Glial cell	Supporting the structure and metabolism of neurons
Schwann cell	Forming a myelin sheath around the axons of the peripheral nervous system
Oligodendrocyte	Forming a myelin sheath around the axons of the central nervous system (CNS)
Astrocyte	Large glial cells that give structural and metabolic support to neurons are crucial for the blood–brain barrier
Sensory cells
Mechanoreceptor cells	Cells containing mechanoreceptors that are sensitive to pressure, touch, stretching, movement, and sound
Hair cells	Cells (in the ear of vertebrates, in the side lines of fish) with mechanoreceptors that pick up movement in relation to their surroundings and sounds
Pain receptor cells	Cells containing nociceptors; free nerve endings (dendrites), e.g. in the epidermis. Nociceptors react to heat, pressure, and irritants and are sensitized by prostaglandins
Temperature receptor cells	Cells containing thermoreceptors that gauge the temperature
Taste receptor cells	Cells containing chemical and taste receptors. They can distinguish the categories sweet, sour, salty, and bitter
Smell receptor cells	Cells containing smell receptors
Light receptor cells	In the retina of vertebrae, cones and rods serve as photoreceptors
Muscles
Striated muscle cell	Rapid and forceful contractions (skeletal muscle), controlled via the somatic nervous system
Smooth muscle cell	Slow and sustained contractions (in the intestine tract, in bladder, in arteries, and in veins); no striation, controlled via the autonomous nervous system
Heart muscle cell	Striated, heart contraction
Gametes
Sperm cells	Male gamete (haploid)
Egg cells	Female gamete (haploid)