18.1 Classification

Part of the genome of many viruses is a gene for the enzyme reverse transcriptase. As its name suggests, this enzyme can synthesise DNA from an RNA template, which is the reverse reaction to transcription within most other organisms' cells. 

Suggest one advantage to the virus of producing the enzyme reverse transcriptase. 

Give four reasons why viruses are not classified within the traditional three-domain system of Archaea, Prokaryotes and Eukaryotes. 

The number of fully assembled virus particles (virions) released from an individual host cell after an infection can vary from tens of virions to tens of thousands. For example, the HIV virus can burst out around 5 000 virions from a single infected host T-cell.

(i) Calculate the maximum theoretical number of T-cells that can be destroyed by a single host cell infection followed by one further cycle of infection.

(1)

(ii) Suggest the implications to a human of your answer to part (i) in the light of the fact that a healthy adult human possesses around 50 million T-cells.

(2)

Viruses are not included in the three-domain classification system.

Explain why.

Discuss whether viruses should or should not be classed as living organisms.

State the two main features that are used to classify viruses.

Archaea and Eukarya are two domains of life that exhibit distinct characteristics and evolutionary relationships.

Contrast the cellular structures of Archaea and Eukarya.

Scientists compared four species from the kingdom Achaea. They compared the base sequence of their DNA to determine the evolutionary relationships between the species.

Their results are shown in Table 1. 

Table 1

Use the information in Table 1 to explain which two species are the closest evolutionary relatives.

Archaea were previously classified as bacteria due to similarities in their features. 

Describe three features of archaea that led to their classification as a separate domain.