Viruses (AHL)

Flashcards about Viruses (AHL)

Structural features common to viruses

Small, fixed size; nucleic acid (DNA or RNA) as genetic material; a capsid made of protein; no cytoplasm; and few or no enzymes.

Diversity of structure in viruses

Genetic material may be RNA or DNA, which can be either single- or double-stranded. Some viruses are enveloped in host cell membrane and others are not enveloped.

Lytic cycle of a virus

Viruses rely on a host cell for energy supply, nutrition, protein synthesis and other life functions.

Evidence for several origins of viruses from other organisms

Viruses share an extreme form of obligate parasitism as a mode of existence, so the structural features that they have in common could be regarded as convergent evolution. The genetic code is shared between viruses and living organisms.

Rapid evolution in viruses

Evolution of influenza viruses and of HIV.

Size of viruses

Between 20 – 30nm and smaller than their hosts.

Genetic material of viruses

A nucleic acid like RNA or DNA is used as the genetic code.

Capsid

This protein coat encloses the genetic material. Often composed of multiple different proteins, sometimes only one type.

Enzymes in viruses

Viruses use the metabolism of their host cells and most of their enzymes. Only few enzymes are required for the replication of the virus’ genetic material.

Growth of viruses

Viruses don’t grow, so they don’t increase in size. A virus is assembled inside a host cell with a fixed number of components.

Diversity of structure in viruses - genetic material

Viruses are diverse in shape and structure. In addition they have genetic material made of either DNA or RNA. It can be single or double stranded, the length of DNA varies and it can be circular or linear.

Diversity of structure in viruses - Envelope

The virus become covered in a membrane when they are breaking free from their host cell. The phospholipid membrane surrounding the virus originates from the host cell, while the proteins come from the virus itself.

Non-enveloped viruses

Viruses which affect plants or bacteria are non-enveloped viruses. They do not become enclosed in a membrane.

Lytic cycle of a virus

The virus reproduces and then bursts out of the host cell, killing it.

Lysis

Viral proteins make holes through the wall and membranes of the host cell and spread to infect other host cells.

Lysogenic cycle of a virus

When a virus remains in the lysogenic cycle it is “temperate” It does not cause harm and causes minimal damage.

Regressive hypothesis

Viruses develop from cells in a series of steps by loss of cell components.

Progressive hypothesis

Viruses are built up in a series of steps by taking and modifying cell components.

Generation rate in viruses

The number of generations produced in viruses is very fast (less than 1 hour).

Mutation rate in viruses

The mutation rate is very high in viruses.

Natural selection in viruses

There are many defense mechanism in place by host cells which increases selective pressure on viruses. Natural selection is powerful and encourages evolution.

Influenza virus replication

The RNA replicase used to replicate does not have a proofreading ability (unlike DNA polymerase). This leads to a high mutation rate.

Novel virus

A novel virus (a strain) has the potential to create a pandemic.

HIV

HIV is a retrovirus using the enzyme reverse transcriptase to convert the single stranded RNA genome into DNA.

Reverse transcriptase in HIV

The enzyme, does not have any profreading ability (unlike DNA polymerase), resulting in a high mutation rate.

Mutations in HIV

Most of the mutations that occur in HIV are harmful to the virus. However, due to the high mutation rate and new generation of strains the virus can easily evade the immune system.

HIV infection

Individuals vary in their response to HIV infection.