a virus particle (sometimes called a virion) is nothing like either a prokaryotic cell or a eukaryotic cell. Viruses are much smaller than even the smallest bacterium. Most are between 0.01 and 0.1 µm in length or diameter. This makes them at least 1000 times smaller than the smallest bacterium and 1 000 000 times smaller than most human cells.
Th particle of a virus is called a virion. All virions contain at least two components: • a protein shell or capsid and DNA or RNA as the genetic material
Some also have a membrane made from lipids and proteins outside the capsid and other proteins and enzymes inside the capsid
Because they do not have the major organelles that are present in living cells, virus particles can’t carry out any of the normal metabolic processes of cells, such as:
• respiration
• protein synthesis
• DNA replication
• photosynthesis
• active transport
• facilitated diffusion
• any other process requiring control by enzymes or the presence of proteins
As a result, all viruses are parasites. Th only way they can reproduce is to invade cells, ‘hijack’ the normal metabolic processes of those cells, and make the cells produce more virus. Once produced, the viruses escape from the cell and infect other cells. Figure below shows how this happens in two different types of viruses. Other viruses adopt different strategies.
Classifying viruses
It is difficult to classify viruses because, even though they have a basic structure, there is a great deal of variation in their shape and the way in which they infect cells. However, they can be classified into three main groups, based on the nature of their genetic material and the way in which it is expressed. These groups are:
• DNA viruses – for example, Herpes simplex (causes cold sores)
• RNA viruses – for example, H1N1 virus (causes swine fl)
• retroviruses – for example, HIV (causes AIDS)
DNA viruses
When a DNA virus infects a cell, the viral DNA can replicate itself and can also control the synthesis of virus proteins, so that the new DNA and new protein can be assembled into new virus particles.
RNA viruses
When an RNA virus infects a cell, its RNA can be used to synthesize more viral proteins, including an enzyme that controls the synthesis of more RNA. Th new RNA and new proteins can be assembled into new virus particles. With the exception of the RNA viruses, all organisms store their permanent information in DNA, using RNA only as a temporary messenger for information. DNA is quite a stable molecule, is not very reactive with other molecules, and replicates very accurately. In contrast, RNA is quite unstable and makes frequent mistakes during copying. But these very properties make RNA ideal for the storage of viral information. Once the host’s immune system has learned to recognize an infecting virus and create antibodies against it, it can quickly destroy it, and the virus needs to change its nature so that the host’s immune system will no longer recognize it – it must mutate. Th unstable nature of RNA allows RNA viruses to evolve far more rapidly than DNA viruses, frequently changing their surface structure.
Retroviruses
Retroviruses also contain RNA, but replicate in a different way. When they infect cells, they release into the cells their RNA and an enzyme that causes it to be ‘reverse-transcribed’ into DNA. This then controls the formation of more viral protein and RNA that can be assembled into new virus particles. Viruses can also be classified by the type of organism they infect:
• animal-infecting viruses
• plant-infecting viruses
• bacteria-infecting viruses – these are called bacteriophages
Bacteriophages have a really unusual shape – they look rather like a lunar landing module!
Virus reproduction.. more of multiplication tho
Yes, they do get busy, grow up.
Much of our knowledge about how viruses are reproduced comes from work on bacteriophages. One bacteriophage in particular, called T4, has been studied more than any other. Its reproductive cycle is shown in figure below
This type of life cycle is called a lytic cycle because it causes the rupture (lysis) of the host cell. From research on bacteriophages, we know that this is not the only type of reproductive cycle in viruses. Sometimes, instead of causing the cell to burst and release the viruses all at once, a few at a time are released by exocytosis through the plasma membrane. This type of life cycle is called a chronic release cycle, because release of new viruses is ongoing (chronic). In other cycles, the virus’s DNA becomes incorporated into the DNA of the host cell. Each time the cell divides, the DNA is replicated, and each daughter cell gets a copy of the cell’s DNA, which now includes the virus DNA. This can continue for many generations until some factor in the environment triggers the virus DNA to start producing virus proteins. Then whole viruses are assembled, which then leave the cell either by causing cell lysis (splitting), or by chronic (ongoing) exocytosis from the plasma membrane. This type of life cycle is called a lysogenic cycle.
Modes of virus transmission As well as having different reproductive strategies inside the host cell, different viruses also enter cells in different ways.
• Th bacteriophage injects just its DNA; the rest of the virus remains outside the cell.
• Many (but not all) animal viruses manage to get the whole virus inside the cell. This is done by tricking the cell into bringing the virus into the cell in the same way as it would with any large protein molecule – using the process of endocytosis.