General introduction – viruses

Definition

Generally speaking, a virus is a biological system that can’t divide by itself, so theoretically it can’t be called a living being. Viruses are reproduced by the cells they infect. Once inside, they force the cell to produce many thousands of identical copies of the original virus, a process named propagation, at an extraordinary rate. The newly assembled virions contain genes (DNA or RNA) surrounded by a protective coat of protein called a capsid. Whilst in the propagation phase, infected cells produce only viral proteins and this prevents the cell to look after itself. Actively propagating viruses thus cause cell death, often through a process referred to as apoptosis (cell “suicide”).

A virus measures between 20 and 100 nm, bacteria size up to 1-5 µm, whereas human cells have the dimensions of 10-100 µm.

Click here for cell size and scale.

Classification

Viruses are classified based on their core genetic material. The type of genetic material, either in the form of DNA or RNA, and whether its structure is single-stranded (ss) or double-stranded (ds) are the main factors. The (+) suggests that viral RNA sequence may be directly translated into the desired viral proteins, whereas the (-) means that the strand has first to be inversed before it can be used in the process of translation. The last two groups are special because they imply an enzyme called Reverse Transcriptase. In the case of HIV this is used to reverse-transcribe the RNA genome into DNA, which is then integrated into the host genome and replicated along with it. For Hepatitis B, the matter is different. The virus carries a DNA-genome but replication occurs through an intermediate of mRNA, which is produced by the cell. The reverse transcriptase then converts the mRNA into DNA which is packed into the new virus particle.

I: dsDNA viruses (herpesvirus)
II: ssDNA viruses (parvoviruses, associated with animal species including mammals and arthropods)
III: dsRNA viruses (the yeast virus)
IV: (+)ssRNA viruses (Rhinovirus – a common cold virus, Hepatitis A virus)
V: (-)ssRNA viruses (Ebola virus)
VI: ssRNA-RT viruses (HIV)
VII: dsDNA-RT viruses (Hepatitis B virus)

In this blog we will focus on the 6th group, Retroviridae or retroviruses

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