General Characteristics of Viruses

Viruses are non-living agents that can infect all forms of life, including all members of the Bacteria, Archaea, and Eucarya. They commonly are referred to by the organisms they infect. The word bacteriophage or phage that infect bacteria is both singular and plural when referring to one type of virus. The word phages is used when different types of phages are being referenced. This is the same usage as fish and fishes. Animal viruses infect animals and plant viruses infect plants. These organisms are the hosts for the viruses. Since phages have been isolated from extremophiles of the Bacteria and Archaea, these phages must be capable of multiplying and existing under extreme conditions of high temperature, high salt, and low pH.

Virus Architecture

Each virus particle, also called a virion, consists of nucleic acid (DNA or RNA) surrounded by a protective protein coat, termed a capsid. Different viruses have different shapes (figure 13.1). Some are isometric, and are composed of flat surfaces, forming equilateral triangles. They appear spherical when viewed with the

324 Chapter 13 Viruses of Bacteria

(a) Isometric (adenovirus)

Protein coat

Nucleic acid (inside the capsid)

(b) Helical (tobacco mosaic virus)

Nucleic acid Protein coat

(inside the coat)

(c) Complex (T4 bacteriophage)

Nucleocapsid

Nucleic acid (DNA)

Collar

Nucleic acid (DNA)

Protein coat (capsid)

Base plate

Tail spike

Tail fibers (protein)

Protein coat (capsid)

Collar

Base plate

Tail spike

100 nm

Tail fibers (protein)

Figure 13.1 Common Shapes of Viruses (a) Isometric adenovirus virions; spikes protruding from the capsid are not visible. (b) Helical, tobacco mosaic virus. (c) Complex,T4 bacteriophage.

100 nm electron microscope; others are helical, which gives the virion a filamentous or rodlike appearance. Most phages are more complex in shape, having an isometric head with a long helical component, the sheath or tail. It is estimated that more than 1023 phage particles of this shape exist in the world.

The shape of a virus is determined by the shape of the protein capsid, either helical or spherical, that encloses the viral nucleic acid genome. The viral capsid together with the nucleic acid that is tightly packed within the protein coat is called the nucleocapsid (figure 13.2). The nucleic acid in some viruses is so tightly packed that the internal pressure is 10 times higher than in a champagne bottle. Each capsid is composed of many identical protein subunits, called capsomeres. All bacterial and animal viruses, but not plant viruses, must be able to attach (adsorb) to specific receptor sites on host cells. In tailless isometric viruses, attachment proteins or spikes project from the capsid and are involved in attaching the virus to the host cell (see figure 13.2). In isometric viruses with tails (sheaths), tail fibers serve to attach the virus to the host cell (see figure 13.1).

There are two basic types of virions. The outer coat of most phages consists of the protein capsid. This type of virion is called naked. Virtually all phages are naked. Some virions that infect humans and other animals, however, have an additional covering over the capsid protein. This consists of a double layer of lipid similar in structure to the cell membrane of the eukary-otic cell. These are termed enveloped viruses. Just inside the lipid envelope is often a protein, the matrix protein. The matrix protein is not found in naked viruses. The attachment spikes project from the envelope (see figure 13.2).

Viruses are notable for their small size (figure 13.3). They are approximately 100- to 1,000-fold smaller than the cells they infect. The smallest viruses are approximately 10 nm

13.1 General Characteristics of Viruses 325

Capsomere subunits

Nucleocapsid -

Capsid (entire coat)

Nucleic acid

Capsomere subunits

Capsid (entire coat)

Nucleic acid

Spikes

(a) Naked virus

Spikes

Matrix protein

Spikes

Envelope

Nucleocapsid -

Capsid (entire coat)

Nucleic acid

Matrix protein

Spikes

Envelope

Capsid (entire coat)

Nucleic acid

Figure 13.2 Two Different Types of Virions (a) Naked, not containing an envelope around the capsid and (b) enveloped, containing an envelope around the capsid and the matrix protein inside the envelope.

(b) Enveloped virus

Figure 13.2 Two Different Types of Virions (a) Naked, not containing an envelope around the capsid and (b) enveloped, containing an envelope around the capsid and the matrix protein inside the envelope.

¡J T4 bacteriophage (225 nm)

¡J T4 bacteriophage (225 nm)

Adenovirus (90 nm)

Tobacco mosaic virus (250 nm)

Adenovirus (90 nm)

Tobacco mosaic virus (250 nm)

Human red blood cell

Figure 13.3 Virion Size Related sizes of an animal virus (adenovirus), plant virus (tobacco mosaic virus), a bacterial virus, phage T4 and Escherichia coli, the host of T4.The bacterial cell is 2,000 nm in length; the red blood cell is 5,000 nm in diameter.

in diameter, while the largest animal viruses are about 500 nm, the size of the smallest bacterial cells. The smallest viruses contain very little nucleic acid, perhaps as few as 10 genes.

The Viral Genome

The structure of the viral genome is unusual. Viruses contain only a single type of nucleic acid—either RNA or DNA—but never both. Depending on the type of nucleic acid they contain, viruses are frequently referred to as RNA or DNA viruses. The nucleic acids can occur in one of several different forms characteristic of the virus. DNA may be linear or circular, either double-stranded or single-stranded. RNA is usually single-stranded but a few viruses contain double-stranded RNA.

Replication Cycle—Overall Features

Viruses can only multiply within living cells that are actively metabolizing. Viruses lack the cellular components necessary to harvest energy and synthesize protein. They have no mitochondria or ribosomes, or even the enzymes necessary to perform these functions. Viruses must use structures and enzymes of cells they infect to support their own reproduction. In all viruses, the nucleic acid separates from its coat before replication begins, and so protein and nucleic acid are synthesized independently of one another. ■ mitochondria, p. 75 ■ ribo-somes, p. 66

Because viruses contain so little nucleic acid, their few genes can code for only a very limited number of proteins, including enzymes. Every virus, however, must contain a mini mum amount of genetic information. Their genes must be able to encode the proteins required to (1) make the viral protein coat, (2) assure replication of viral nucleic acid, and (3) move the virus into and out of the host cell. Some viruses require enzymes for their replication that are not present in uninfected host cells. These enzymes are either coded by the nucleic acid of the virion, or the enzymes enter the cells with the virion.

Viruses exist in two distinct phases. Outside of living cells, they are metabolically inert. Essentially, they are only macro-molecules. Inside susceptible cells, viruses are in a replication form using the metabolic machinery and pathways of their host cells along with their own genetic information to produce new virions. Viruses and cells are compared in table 13.1.

Relieving Your Life of Acne

Relieving Your Life of Acne

A flawless complexion is just moments away. Are you going to let Acne rule your life? Discover how you can enjoy vibrant, healthy and pimple free skin and it doesn’t involve magical potions, products, lotions or gimmicks.

Get My Free Ebook


Responses

  • angela
    Which characteristics of naked virions?
    2 years ago

Post a comment