Viruses – A Level Biology B Edexcel Revision

Viruses are submicroscopic particles that can only be seen under an electron microscope.
Unlike cells, viruses are acellular and cannot carry out metabolic processes.
Each virus is composed of a piece of nucleic acid (either DNA or RNA) surrounded by a protein coat, called a capsid.
Some viruses, like the HIV and influenza viruses, also have a lipid envelope derived from the host cell’s membrane.
Viruses are obligate parasites, which means they depend entirely on the cells of a host organism to duplicate themselves.

The replication process of viruses is referred to as the viral replication cycle.
It begins when a virus attaches to a specific receptor site on the host cell’s surface through a process called adsorption.
The virus then penetrates the host cell and uncoats, releasing its genetic material.
This genetic material (viral DNA or RNA) takes command of the host cell’s machinery to replicate the viral nucleic acid and protein.
The newly formed viral particles, called virions, are assembled inside the host cell.
The budding off of new virions from the host cell (tens or even hundreds can be produced by a single infected cell), a process we call release, often leads to the death and lysis of the host cell. At times, viruses can leave the host cell by budding, keeping the host cell alive.

Viruses cause several diseases in animals and plants. Examples are AIDS, influenza, rabies, measles, and plant mosaic diseases.
There is a need for constant surveillance and research to quarantine new viral diseases and to develop new vaccines.
Treatment typically involves relieving symptoms, but some infections like herpes and HIV are chronic and can be controlled but not eradicated.
Vaccination or immunisation is the most common method for controlling viral diseases. It enables the immune system to produce memory cells that recognise and respond rapidly and robustly to infection by the specific virus. This is the basis for herd immunity.
Antiviral drugs are used to interfere with the viral replication cycle. Though effective, the widespread use of antiviral drugs can lead to resistance.

Viruses play an important role in gene transfer, both among similar kinds of organisms and between different species, which contributes to biodiversity.
Viruses are used in biotechnology to clone DNA sequences, in gene therapy, and in the production of vaccines.
Some viruses may have a role in controlling the population of certain species, benefiting the ecosystem.

Remember, an understanding of the complex role of viruses in life and health is fundamental to biology.