Viruses: MDCAT Biology notes

What a virus is

Viruses are acellular infectious agents. They have no cytoplasm, no ribosomes and no metabolic machinery of their own, so they cannot respire, excrete or grow outside a host. They reproduce only inside living cells, which makes every virus an obligate intracellular parasite. Viruses infect all groups of organisms, including bacteria (bacteriophages), plants and animals.

They are on the border of living and non-living:

• Living features: they contain genetic material (DNA or RNA, never both), they reproduce inside a host, and they mutate.
• Non-living features: they can be crystallized (Stanley crystallized TMV in 1935), lack cellular respiration and lack their own enzymes for biosynthesis.

Size: about 20–250 nm. They pass through porcelain (Chamberland) filters that hold back bacteria, so they are studied with the electron microscope. Tobacco mosaic virus (TMV) was the first virus to be isolated, crystallized and seen under the electron microscope.

Structure of a virion

• Virion: the complete, mature, infectious virus particle.
• Capsid: the protein coat around the genome, built from subunits called capsomeres.
• Nucleocapsid: the nucleic acid core together with its capsid.
• Envelope: an outer lipid-protein (lipoprotein and glycoprotein) layer found in some animal viruses. It comes from the host cell membrane as the virus buds out.
• Spikes: glycoprotein projections on the envelope that recognize and bind specific receptors on the host cell. This receptor matching is why most viruses infect only particular hosts or tissues.

Enveloped viruses (for example influenza, HIV, herpes, measles) are sensitive to drying, heat, sunlight, detergents and lipid solvents, so they do not survive long outside the host.

Capsid shapes

DNA and RNA viruses

Retroviruses (for example HIV) are ss RNA viruses placed in a separate class because they carry reverse transcriptase, which copies single-stranded RNA into double-stranded DNA. That DNA then integrates into the host chromosome.

Bacteriophage life cycle

A T-phage has a polyhedral head holding ds DNA, a tail sheath, an end plate and six tail fibres.

1. Adsorption: tail fibres attach to specific receptor sites on the bacterial wall.
2. Penetration: the phage releases lysozyme, which digests a hole in the cell wall. The tail sheath contracts and the DNA is injected; the empty capsid stays outside.
3. Replication: phage DNA takes over host machinery to make phage DNA and proteins.
4. Assembly and maturation: new phages are put together.
5. Release: the cell bursts (lysis), freeing new phages.

Virulent phages always follow this lytic cycle. Temperate phages can follow the lysogenic cycle: their DNA joins the bacterial chromosome as a prophage and is copied with it until it is induced into the lytic cycle. In animal viruses the removal of the capsid inside the host is called uncoating.

Common MDCAT traps

• Lysozyme acts at penetration, not at release or adsorption.
• Parvovirus is the only common ss DNA example; pox, herpes and adeno are ds DNA.
• The envelope comes from the host membrane, not from the capsid.
• Capsid is the protein coat, capsomere is one subunit, and nucleocapsid is the capsid plus nucleic acid.
• Viruses can crystallize and mutate but cannot excrete or respire.

Quick revision

• Virion is the complete infectious particle.
• Adenovirus is polyhedral with 252 capsomeres; an icosahedron has 20 faces.
• A bacteriophage has six tail fibres.
• Measles is caused by a paramyxovirus.
• Reverse transcriptase copies ss RNA into ds DNA.
• Temperate phages show the lysogenic cycle; virulent phages are lytic.