Organisms that can only exist as parasites in the cells of other living organisms.
Reproductive route for DNA viruses where viral DNA is inserted into host DNA and replicated every time the host cell divides, without causing disease.
Reproductive route for DNA viruses where viral DNA takes over the cell biochemistry and uses it to make new viruses, destroying the cell when the new viruses burst out and causing the symptoms of disease.
The movement of large molecules or particles into cells through vesicle formation
Simple repeating protein units that make up the structure of the capsid (protein coat) of a virus.
A thin, flexible sheet-like structure that acts as a lining or a boundary in an organism.
The protein coat of a virus, made up of repeating units called capsomeres.
Viruses range in size from 0.02µm to 0.3µm across, about 50 times smaller than the average bacterium. They are the smallest microorganisms. Viruses are not cells but they are a combination of genetic material and protein that can invade living cells. The viruses take over the biochemistry of the cells they invade to make more viruses. Since viruses can only reproduce as parasites in the cells of other living organisms, most scientists class them as obligate intracellular parasites.
Viruses are usually geometric shaped and have similar basic structures.
Generalised structure of a virus
Viruses are classified by their genome and their mode of replication.
|Type of virus||Genetic material||Mode of replication||Examples|
|DNA viruses||DNA||Viral DNA acts directly as a template for both new viral DNA and for the mRNAs needed to synthesise viral proteins||Adenoviruses that cause colds, bacteriophage that attack bacteria and varicella-zoster that causes chicken pox|
|RNA viruses||RNA||RNA acts as template for viral proteins either directly or indirectly||Influenza virus, measles virus and tobacco mosaic virus|
|Retroviruses||RNA||Viral RNA controls production of reverse transcriptase. This makes DNA molecules corresponding to the viral RNA. They are transcribed to produce new viral proteins and a new viral genome.||Human immunodeficiency virus (HIV)|
Bacteriophage is an example of a DNA virus
The measles virus is an RNA virus
The human immunodeficiency virus (HIV) is a retrovirus
As naturally occurring viruses invade and take over living cells to reproduce they all cause damage and disease of some sort. They can withstand drying and long periods of storage whilst maintaining their ability to infect cells.
Chickenpox is caused by a virus
(Photo credit: CDC/NCIRD)
How do viruses infect cells? They use several different methods:
The life cycle depends on the type of virus. DNA viruses may go into a lysogenic pathway, when they steadily reproduce with the cells, or a lytic pathway, when they become virulent and cause disease.
The lysogenic and lytic pathways of infection of a DNA virus