Viruses are infectious particles made of nucleic acid encased in a protective protein coat and, sometimes, a membranous envelope. The genome of viruses may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, depending on the kind of virus. The viral genome is usually organized as a single linear or circular molecule of nucleic acid. The genome is encased in a protein shell called a capsid which is derived from the host cell. The most complex capsids are found in viruses that infect bacteria, called bacteriophages or phages. A virus has a genome but can reproduce only within a host cell. Dr. Beijerinck used the sap from one generation of infected plants to infect the second generation of plants that could, in turn, infect future generations. Dr. Beijerinck determined that the pathogen could reproduce only in the host, could not be cultivated on nutrient media, and was not killed by alcohol which generally kills bacteria.

An isolated virus is unable to reproduce—or do anything else, except infect an appropriate host. This is because viruses lack the enzymes for metabolism and the ribosomes for protein synthesis. Each type of virus can infect and parasitize only a limited range of host cells, called its host range. Viruses identify host cells by a “lock and key” fit between proteins on the outside of the virus and specific receptor molecules on the host’s surface. Most viruses of eukaryotes attack specific tissues. Most DNA viruses use the DNA polymerases of the host cell to synthesize new genomes along the templates provided by the viral DNA.

Phages reproduce using lytic or lysogenic cycles.  Those that reproduce using lytic cycles are known as virulent phages. Temperate phages, like phage lambda, use both lytic and lysogenic cycles. In the lytic cycle, the phage reproductive cycle culminates in the death of the host, in the last stage, the bacterium breaks open and releases the phages. Bacteria produce restriction endonucleases, or restriction enzymes, that recognize and cut up foreign DNA, including certain phage DNA. Natural selection also favors phage mutants that are resistant to restriction enzymes.

In the lysogenic cycle, the phage genome replicates without destroying the host cell. During a lysogenic cycle, the viral DNA molecule is incorporated by genetic recombination into a specific site on the host cell’s chromosome. In this prophage stage, one of the viral genes codes for a protein that represses most other prophage genes, the phage genome is largely silent. Every time the host divides, it copies the phage DNA and passes the copies to daughter cells and every once in a while these enter the lytic cycle.

Animal viruses are diverse in their modes of infection and replication. Key variables include the type of nucleic acid that serves as a virus’s genetic material and the presence or absence of a membranous envelope. The viral genome doubles and tells the host’s protein machinery to synthesize capsomeres with free ribosomes and glycoproteins with bound ribosomes. In some with single-stranded RNA , the genome acts as mRNA and is translated. Retroviruses carry an enzyme called reverse transcriptase that transcribes DNA from an RNA template which is inserted as a provirus into a chromosome in the animal cell. Human immunodeficiency virus (HIV), the virus that causes AIDS (acquired immunodeficiency syndrome) is a retrovirus.

Viruses may have evolved from other mobile genetic elements. Because viruses depend on cells for their own propagation, it is reasonable to assume that they evolved after the first cells appeared. Most molecular biologists favor the hypothesis that viruses originated from fragments of cellular nucleic acids that could move from one cell to another. The evolution of capsid genes may have aided the infection of undamaged cells. The ongoing evolutionary relationship between viruses and the genomes of their hosts is an association that makes viruses very useful model systems in molecular biology.