Viruses.

They have very simple structure consisting of the following.

1. CORE: the genetic material rather DNA or RNA may be single stranded or double stranded.
2. CAPSID: a protective coat of protein surrounding the core.
3. NUCLEOCAPSID: the combined structure formed by the core and capsid.
4. ENVELOPE: a few viruses such as HIV and influenza have an additional lipoprotein layer around the capsid derived from the cell surface membrane of the host cell.
5. CAPSOMERES: capsids are often built up of identical repeating subunits called capsomeres.

1. They possess both living and non-living characteristics therefore they are a borderline case between living and non-living.
2. Viruses are very small organisms ranging in size from 20–300 nm. They cannot be seen by the light microscope and on the average they are about 50 times smaller than bacteria indicating that they pass through filter which retain bacteria.
3. They are cellular i.e. they consist of nucleic acid either DNA or RNA surrounded by a protein coat called capsid. These fragments of genetic material are neither nucleus nor cytoplasm it is only the DNA or RNA which form the central core. The fully assembled infected particle is called a virion.
4. Viruses are obligate parasites / endoparasites and can only reproduce inside the living host.
5. They are highly specific to the host, sometimes to the host tissue, and to the cell. Virus will recognize and infect only a certain type of cells.
6. Viruses do not have a mechanism for producing energy for themselves and therefore all the enzymes for protein synthesis and the ribosome come from the host cells.
7. Viruses can be crystallized i.e. they show no life process when outside the cell; however the crystals retain the effective capacity as if they are in appropriate hosts.
8. Viruses rely on passive dispersal or a vector to move them from one host to another because they don't have any locomotive structures.
9. Viruses are metabolically / biologically inert. They cannot carry out any life process such as respiration, reproduction or protein synthesis.
10. They either contain DNA or RNA but not both.
11. Viruses have two symmetries either helical or isometric.

Viruses are classified as either:

1. DNA viruses
2. RNA viruses (in core)

OR

They can be classified according to the host e.g. bacteriophage only infect bacteria and animal viruses only infect animals. Most of animal viruses are DNA viruses and plant viruses are RNA viruses.

1. They are very simple in structure being compared of nucleic acid (DNA or RNA) and protein.
2. They seem to be more related to their host than one another.
3. They usually undergo mutation.
4. They are a borderline case i.e. they possess both living and non-living characteristics.

Demerits of viruses

1. Causes diseases to animals. Man incurs a great loss due to reproduction or death of animals e.g. foot and mouth disease in cattle, Rift valley in cattle, Newcastle in poultry.
2. Causes diseases to plants leading to great losses for example maize streak, cassava mosaic, tobacco mosaic.
3. Cause disease to humans leading to depopulation and loss of man power e.g. HIV causes AIDS.
4. Increases cost of living when you consider the cost of prevention or treatment of viral diseases.

Merits of viruses

1. Used in research purposes this leads to development of drugs and research of the behavior or characteristics of other organisms.
2. Useful in biological control of other pests like harmful which attack plants and animals.
3. Some are used in developing vaccines which are useful in preventing diseases e.g. polio, measles.

1. AIDS is caused by HIV or Human immunodeficiency viruses. It belongs to a group of RNA viruses known as retroviruses.
2. This name comes from the fact that these viruses can convert their RNA back into a DNA copy using an enzyme known as reverse transcriptase. Normally a section of DNA (gene) is copied to make RNA a process called transcription.
3. Making DNA from RNA is therefore reverse transcription and the enzyme is called reverse transcriptase (this enzyme has proved extremely useful in genetic engineering).