Viruses are not cells. They do not have a cell membrane or other components of living cells.

Living host cells are required for their reproduction. Outside of the host, they act as nonliving chemicals.

They do not metabolize or respond to stimuli.

They have genetic material and can therefore mutate and evolve.

They form parasitic relationships with living organisms; the virus benefits at the expense of the living organism.


Viruses contain an inner nucleic acid core (genetic material) and an outer protein coat (capsid).

The virus may also contain some enzymes.

An outer envelope (membrane) that is derived from the cell membrane of the host cell may surround the capsid.

The envelope contains viral glycoprotein spikes.

They are usually smaller than 200 nm.

The genetic material in some viruses is DNA; in others it is RNA.



Typically, a virus can enter a cell when either a part of the capsid or the spikes in the envelope match receptors in the host cell.

Some viruses have a narrow host range and can infect only one or a few species. For example, the smallpox virus infects only humans. Viruses with a broad host range can infect many different species. For example, the rabies virus is able to infect several different species of mammals.

In multicellular organisms, viruses infect specific host cell types.


Influenza infects cells lining the respiratory tracts.

Poliomyelitis virus infects nerve cells.

Tobacco mosaic virus infects tobacco leaves.

Protein Synthesis

In a normal cell, DNA is copied to make mRNA by a process called transcription. The information stored in mRNA is used by ribosomes to assemble proteins from amino acids (called translation). The normal sequence is:

DNA Viruses

When viral genetic material enters a cell, it is replicated, transcribed (mRNA is produced) and translated (proteins are produced from the mRNA) by the host cell. By this process, the host cell uses the genetic instructions in the virus to make more viruses.


RNA Viruses

In some RNA viruses, the viral RNA serves as mRNA after infection.

The RNA of some viruses serves as a template to synthesize more RNA within the host cell. Some of the replicated RNA serves as mRNA and is used to produce proteins and some of the replicated RNA is packaged in new viral particles.

Retroviruses are RNA viruses that use RNA as their genetic material but the host cell must synthesize a DNA copy of the RNA before it can be transcribed or translated.



are viruses that infect bacteria.

have 2 kinds of life cycles: lytic and lysogenic.

Lytic Cycle

The virus attaches to the bacteria cell, a viral enzyme digests away a part of the wall and its DNA enters the host cell.

The host DNA disintegrates.

The viral DNA is transcribed and translated, producing protein coats and enzymes.

The viral DNA is replicated.

The protein coats and DNA are assembled into new viral particles.

Lysozyme (an enzyme that digests the cell wall) causes the host cell wall to rupture. The bacterium dies in the process.

Lysogenic Cycle

The viral DNA becomes integrated into the bacterial DNA after infection. It is replicated along with the host DNA when the host reproduces. The viral DNA is referred to as a prophage.

Certain environmental factors (example- UV radiation or chemicals) will trigger the prophage to begin a lytic cycle.

Animal Viruses

Most RNA viruses and some DNA viruses that infect animals have a membranous outer envelope.

In some RNA viruses, the genetic material is translated to produce proteins in the host cell. In other kinds of RNA viruses, mRNA is produced using the viral RNA as a template. The enzymes needed for this process are carried in the virus.


In retroviruses, the RNA genetic material is copied to produce DNA by an enzyme called reverse transcriptase.

The new DNA produced from the RNA template is called cDNA.

The cDNA becomes incorporated into the host DNA. It contains the instructions to produce the proteins needed to make virus particles.

The viral DNA incorporated within the host DNA is called a provirus.

RNA produced by transcription can serve as mRNA for the production of viral proteins or it can become viral genome.

New viruses escape the host cell by budding.

Retroviruses and Cancer

When new viral particles are eventually built, some of the host genes may accidentally become incorporated into the viral genetic material. These genes may cause cancer when they are later inserted into a host chromosome.

Genes that cause cancer are called oncogenes.

Some RNA Viruses

Some RNA viruses have no DNA stage. Messenger RNA is synthesized from an RNA template.

Viral Diseases

Some examples of viral diseases are the common cold, influenza (flu), mumps, measles, polio, rabies, infectious hepatitis, and AIDS.

Viruses are not living and therefore viral diseases cannot be treated with antibiotics. Antiviral drugs such as AZT function by interfering with DNA replication.

Viral genes (oncogenes) can cause some kinds of cancer.

Vaccines stimulate the immune system to defend against certain viruses. The use of vaccines has limited or prevented the spread of some viral diseases. For example, smallpox has been eliminated as a result of vaccination programs.


Viroids are naked RNA molecules that are several hundred nucleotides long. They replicate within the host cell.


Prions are proteins that cause diseases. 

Prions are thought to be a form of a normal protein that is not folded correctly. Prions do not reproduce. Contact between a prion and the normal form of the protein can cause the normal protein to become a prion.