Viruses are Infectious Packages of Genetic Information

Viruses are packages of genes inside protective shells of protein. Viruses cannot grow or divide alone. In order to replicate, a virus must first infect a host cell. Only then are the virus genes expressed and the virus components manufactured using the host cell machinery. Viruses are not merely pieces of nucleic acid like plasmids or transposons and neither are they true living cells with the ability to generate energy and make protein. They lie in the gray area between. Viruses cannot make their own proteins or generate their own energy. They can only multiply when they have entered a suitable host cell and taken over the cellular machinery. Despite this a virus is certainly not inert; it does replicate if it can subvert a host cell.

Virus particles contain proteins plus genetic information in the form of DNA or RNA (Fig. 17.01). The virus particle, or virion, consists of a protein shell, known as a capsid, surrounding a length of nucleic acid, either RNA or DNA, which carries the virus genes and is often referred to as the viral genome. Many simple viruses have only these two components.

Trying to define precisely what is living and what is non-living can be quite confusing. Here we will sidestep the issue of defining life by noting that being alive and being a living cell are not necessarily the same. To qualify as a genuine living cell, a structure must send genetic messages (RNA) from its genes (DNA) to its own ribo-somes to make its own proteins. A living cell generates the energy to produce these proteins and maintain cellular integrity (Fig. 17.02).

In contrast to self-sufficient living cells, viruses rely upon their host for many functions. Living cells store information as DNA and make the messages out of RNA, whereas, viruses can store their genetic information as either RNA or DNA and rely upon the host cell to make the messages out of RNA. Genuine cells possess ribosomes that are capable of making proteins. Viruses are parasitic and rely on the host cell to provide the ribosomes for translating virus mRNA into protein. Cells transport nutrients and metabolize them to generate energy and make a variety of metabolic intermediates. Viruses rely on host cell metabolism for energy and precursors.

capsid Shell or protective layer that surrounds the DNA or RNA of a virus particle living cell A unit of life that possesses a genome made of DNA and sends genetic messages (RNA) from its genes (DNA) to its own ribosomes to make its own proteins with energy it generates itself viral genome Molecule of DNA or RNA that carries the genes of a virus virion Virus particle virus Infectious agent, consisting of DNA or RNA inside a protective shell of protein, that must infect a host cell in order to replicate

FIGURE 17.02 Characteristics of a Living Cell

This simplified cell shows all the essential characteristics of a living cell: an energy source to provide ATP (ATP synthetase), genetic information (chromosomal DNA and messenger RNA), ribosomes to convert genetic information into proteins, and a biological membrane that maintains cellular integrity.

FIGURE 17.02 Characteristics of a Living Cell

This simplified cell shows all the essential characteristics of a living cell: an energy source to provide ATP (ATP synthetase), genetic information (chromosomal DNA and messenger RNA), ribosomes to convert genetic information into proteins, and a biological membrane that maintains cellular integrity.

Living cells have membranes whereas most viruses do not.

Living cells are surrounded by metabolically active cell membranes. Most simple viruses have only a protein shell and no true membrane, although, some complex virus particles have a membrane stolen from the previous host. However, the membranes around virus particles are not active metabolically either in energy generation or nutrient transport. Nonetheless, virus particles do have an outer covering and can survive on their own outside their host cells (admittedly without multiplying).

Viruses are all parasites that cannot multiply without a host cell. Furthermore, viruses are intracellular parasites; that is to say that they must actually enter the cells of the host organism to replicate. Note that not all intracellular parasites are viruses. Certain disease-causing bacteria and protozoans may enter the cells of higher organisms and live inside them as parasites. However, these parasites are nonetheless living cells themselves and contain their own ribosomes to make their own proteins. This chapter does not attempt to cover the realm of virology systematically. Rather, examples are given to illustrate novel aspects of molecular biology found among the viruses.

Virus genes subvert the host cell into manufacturing more virus particles.

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