There is a special class of virus that attacks bacteria. These are the bacteriophage . Viruses have a nucleic acid core, either DNA or RNA but not both. Retroviruses are unusual in that the virion carries an RNA copy of the genome but upon infection of a host cell a cDNA copy of the virus genome is made using the enzyme reverse transcriptase . Around the nucleic acid core lies a protein coat , made up of units called capsomeres .
Some viruses also have an envelope derived from the host cell membranes.
The envelope may be either baggy or tight, depending upon the nature of the virus. Return to top of the page Microfungi All fungi are eukaryotic . It is also a major component of the exoskeleton of arthropods such as insects. Fungi are heterotrophic . Saprophytic fungi grow on dead organic matter.