describe classification as a work in progress discuss the characteristics of the three domains: Bacteria, Archaea, and Eukarya describe classification by cladistics summarize how molecular evidence reveals species relatedness identify the structures and shapes of viruses describe different types of viral infections

Bacteria, Archaea, and Eukarya represent the three domains of life by which all living organisms are classified. This type of classification was coined by Carl Woese et al. (1990). Archaea and Bacteria represent different domains because Archaea is considered to have the evolutionary oldest phenotypic features, while Bacteria species have diacyl glycerol diester lipids in their membranes. On the other hand, the Eucarya domain is composed of organisms that possess a nuclear envelope (membrane).  

Cladistics is a biological method used to classify organisms into groups named 'clades'. In this type of classification, organisms are categorized according to the most recent common ancestor. In this regard, it is possible to find the most recent common ancestor by using nucleotide differences between taxa.

Viruses can be classified in a similar manner to the cellular organisms above described. Viruses are categorized according to morphological features, type of nucleic acid (DNA or RNA) by which they are composed, mode of replication, host organisms, etc. Viruses may also be classified according to the type of disease that they cause. There are viruses that cause chickenpox, rabies, influenza, old sores, Ebola, AIDS (HIV), Severe acute respiratory syndromes (SARS), etc.

a- multicellular

b-eukaryotic

c-no cell wall

d-heterotrophic

e-autotrophic