Viruses

Basic Properties and structure

Viruses are obligate pathogens in that they can only replicate inside the cells of the human body. The fact viruses use the host cell machinery for vital functions like replication means that it is very difficult to develop drugs which take advantage of differences between viruses and the host. For example, viruses uses the ribosomes of their host to replicate and thus drugs like antibiotics which work well with bacteria would not work well with viruses.

Viruses come in a variety of different sizes, morphologies and types of genomes such as the following.

• single stranded linear DNA (parvovirus) or RNA (like poliovirus)

• double-stranded linear DNA (like herpes viruses) or RNA (reovirus)

• circular single stranded DNA or double stranded circular DNA (polyoma viruses)

• double-stranded DNA with covalently linked proteins at the end of the DNA strands (adenovirus) or with each end covalently sealed (poxvirus)

• segmented where the virion contains multiple RNA genomic fragments (influenza) or non-segmented where the virion contains a single genome (HIV)

• polarity of genome in that the RNA can be sense (+) stranded or antisense (-) stranded. This will be important later in in respect to replication of the virus.

• nonenveloped or having an envelope surrounding a capsid which surrounds the genome.

3 types of proteins are common to all retroviruses (like HIV): (1) GAG proteins for the capsid, (2) Env proteins for the envelope and (3) Pol proteins for reverse transcriptase and integrase.

Growth of virus under benign laboratory conditions lacks the selective pressures of the body and allows weaker strains to survive. This process is used to develop attenuated virus strains for use in vaccines.

How Viruses Replicate

How Viruses Cause Disease

Treatment