What Is a Virus ?

Viruses have always been difficult to define. Recall in chapter 1 that Beijerinck thought the disease agent he called a virus was a contagious living fluid. Pasteur had originated the term "virus" in 1885. He took the name from the Latin for poison. By 1908 we knew that viruses could cause diseases of plants, animals and even humans. And by 1940 we had seen pictures of viruses taken through the transmission electron microscope. But what is a virus? Is it like a cell?

To be considered a cell, three criteria must be met. First, there needs to be a membrane serving as a boundary for the structure. Second, a fluid environment must exist, in which biochemical reactions occur, surrounded by the membrane. And third, there must be genetic information in the form of DNA. The DNA is arranged into one or more chromosomes and contains the information codes for the cell. Eukaryotic cells, like plants, animals, fungi and protists may have additional organelles inside the fluid environment. Prokaryotic cells, such as the bacteria, do not have these additional organelles. Does a virus meet the criteria for being a cell? If a virus does not qualify as a cell, it cannot be considered to be alive. The Cell Theory tells us that all living things are composed of one or more cells. The cell is the basic unit of life. Therefore, if a virus does not qualify as a cell, can we consider it a living thing? Can we call it an organism? Before we can answer these last few questions, we need to look at the structure of a typical virus and then the structure of the influenza virus.

A. Viral Basics

The structural parts which comprise a virus have been know since the 1930’s. Viruses continue to surprise us with their diversity and their unique solutions to the problems of survival. A virus contains a single type of nucleic acid, either DNA or RNA, never both. The DNA or RNA may be double or single-stranded. This core of nucleic acid is known as the viral genome and is covered by a protein coat called a capsid. The genetic information in the DNA or RNA has the codes for producing and assembling more viruses. The capsid is composed of protein subunits called capsomeres. Those viruses that consist only of a capsid and a nucleic acid are called nucleocapsids or naked viruses.

Some viruses have an additional outer covering called an envelope. The envelope is composed of phospholipids and glycoproteins in most viruses. You may remember that the cell membrane consists of phospholipids and glycoproteins. As new virus particles are being assembled and finally leave their host cell, they take some of the membrane materials from the host cell. The virus may also add some of its own glycoproteins to the envelope. Some of these may appear as spikes. A virus with an envelope, with or without spikes, is called an enveloped virus.

As you can see, viruses lack the structures we associate with cells. In addition, viruses have no metabolic machinery of their own. They cannot carry out any of the functions we associate with living things unless they are inside a host cell. Viruses use the raw materials and the metabolic machinery of their host cells to direct the production and assembly of new viruses. You could consider a virus an intracellular (within a cell) genetic parasite. Since viruses depend on their host cells for replication (making exact copies), viruses can be difficult to grow in the laboratory. In order to do research and testing on viruses, the viruses must be grown in animal cells, such as chicken eggs.

B. Structure of the Influenza Virus

The influenza virus is an enveloped virus. This envelope is composed mainly of a lipid bilayer and is lined with a type of protein known as matrix protein. This combination of lipids and protein is sometimes called the matrix protein membrane. The outer surface is covered with two types of spikes made of glycoproteins and embedded in the envelope. The first type is known as hemagglutinin or HA. The name refers to the fact that the influenza virus can attach to red blood cells and cause them to clump or agglutinate. This same HA protein is responsible for the attachment of the virus to the host cell and beginning the infection of the cell. The second type of glycoprotein spike is called neuraminidase or NA. As you can see by the -ase ending on its name, it's an enzyme. Its major job seems to be allowing the newly formed viruses to leave the host cell without sticking to each other or the host cell. There are about 4-5 times more HA proteins than NA proteins in the lipid envelope.

Electron Micrographs of Influenza Virus

There are three types of influenza viruses. Type A contains lots of subtypes and has been the major culprit in epidemics and pandemics in the last 100 years. Type B has been responsible for some regional level epidemics. Type C seldom creates major problems and is known only in humans. Neither Type B or Type C have any known subtypes. Differences in the three types of viruses are due to differences in the HA and NA proteins, the viral genetic information and the matrix protein.

C. Influenza Genomes

The influenza genomes for Types A and B consist of 8 separate single-stranded RNA segments containing 10 genes. Type C contains only 7 RNA segments. These RNA segments are coated by helical nucleo-proteins creating segments sometimes known as ribonucleoproteins(RNP. Recall that this combination of genome and protein covering is also known as the nucleocapsid. The nucleocapsid of influenza viruses is surrounded by an envelope. Each of the RNA segments has the code for one or more of the viral proteins. The table below provides our current understanding of the genes and their functions . The influenza virus is one of very few to have its genome in separate segments. This segmenting of the genome increases the likelihood that new genetic sequences will develop if two different strains of virus infect a cell at the same time. Gene segments from each of the strains may produce new combinations leading to a new strain of flu. On the positive side, laboratory duplication of the genome segments may lead to new vaccine strains.

Genes of Influenza A and Presumed Functions

Diagram of Genome Structure of Influenza Viruses

D. Naming Viral Strains