VirusINTRODUCTIONViruses are tiny organisms that may lead to mild to severe illnesses in humans, animalsand plants. Ex- flu or a cold to something more life threatening like HIV/AIDS.OBJECTIVESHow big are viruses?Are viruses alive?Structure of a virusReceptorsHow do viruses infect?Life cycle of a basic virus
Size of virusesThe virus particles are 100 times smaller than a single bacteria cell. The bacterial cellalone is more than 10 times smaller than a human cell and a human cell is 10 timessmaller than the diameter of a single human hair.Are viruses alive?Viruses by themselves are not alive. They cannot grow or multiply on their own and needto enter a human or animal cell and take over the cell to help them multiply. Theseviruses may also infect bacterial cells.The virus particle or the virions attack the cell and take over its machinery to carry outtheir own life processes of multiplication and growth. An infected cell will produce viralparticles instead of its usual products.Structure of a virusA virion (virus particle) has three main parts: Nucleic acid – this is the core of the virus with the DNA or RNA (deoxyribonucleic acid and ribonucleic acid respectively). The DNA or RNA holds all of the information for the virus and that makes it unique and helps it multiply. Protein Coat (capsid) – This is covering over the nucleic acid that protects it.Lipid membrane (envelope) – this covers the capsid. Many viruses do not have thisenvelope and are called naked viruses.ReceptorsViruses are not simply taken into cells. They must ﬁrst attach to a receptor on the cellsurface. Each virus has its speciﬁc receptor, usually a vital component of the cell surface.It is the distribution of these receptor molecules on host cells that determines the cell-
preference of viruses. For example, the cold and flu virus prefers the mucus lining cellsof the lungs and the airways.How do viruses infect?Viruses do not have the chemical machinery needed to survive on their own. They, thusseek out host cells in which they can multiply. These viruses enter the body from theenvironment or other individuals from soil to water to air via nose, mouth, or any breaksin the skin and seek a cell to infect.A cold or flu virus for example will target cells that line the respiratory (i.e. the lungs) ordigestive (i.e. the stomach) tracts. The HIV (human immunodeficiency virus) that causesAIDS attacks the T-cells (a type of white blood cell that fights infection and disease) ofthe immune system.VideoLife cycle of a basic virusThere are a few basic steps that all infecting viruses follow and these arecalled the lytic cycle. These include: 1.A virus particle attaches to a host cell. This is called the process of adsorption 2.The particle injects its DNA or RNA into the host cell called entry. 3.The invading DNA or RNA takes over the cell and recruits the host’s enzymes 4.The cellular enzymes start making new virus particles called replication 5.The particles of the virus created by the cell come together to form new viruses. This is called assembly 6.The newly formed viruses kill the cell so that they may break free and search for a new host cell. This is called release.Viral life cycleViruses are similar to living organisms, however there are differences. One of the ways avirus can be seen as living is that a virus needs to replicate and create progeny. However,unlike organisms, a virus cannot survive on its own. It is only active when replicatingwithin a host, using a hosts resources and food. Once inside a host, a viruss sole purposeis to make as many copies of itself, and infect other host cells; everything it does is tobenefit its fitness and increase the number of its offspring.
Objectives 1 Overview 2 Exposure of host 3 Viral Entry 4 Viral replication 5 Viral shedding 6 Viral latencyOverviewA virus is totally dependent on a host cell. Most viruses are species specific, and theyonly infect a narrow range of plants, animals, bacteria or fungi.Exposure of hostUsually viral infection occurs when a virus enters the host, either: through a physical breach (a cut in the skin) direct inoculation (e.g.mosquito bite) direct infection of the surface itself (inhalation of the virus into trachea)It is usually only after a virus enters a host that it can gain access to possible susceptiblecells.Viral EntryMain article: Viral entryFor the virus to reproduce and thereby establish infection, it must enter cells of the hostorganism and use those cells materials. To enter the cells, proteins on the surface of thevirus interact with proteins of the cell. Attachment, or adsorption, occurs between theviral particle and the host cell membrane. A hole forms in the cell membrane, then thevirus particle or its genetic contents are released into the host cell, where viralreproduction may commence.Viral replicationMain article: Viral replicationNext, a virus must take control of the host cells replication mechanisms. It is at this stagea distinction between susceptibility and permissibility of a host cell is made.Permissibility determines the outcome of the infection. After control is established andthe environment is set for the virus to begin making copies of itself, replication occursquickly.
Viral sheddingMain article: Viral sheddingAfter a virus has made many copies of itself, it usually has exhausted the cell of itsresources. The host cell is now no longer useful to the virus, therefore the cell often diesand the newly produced viruses must find a new host. The process by which virusprogeny are released to find new hosts, is called shedding. This is the final stage in theviral life cycle.Viral latencyMain article: Viral latencySome viruses can "hide" within a cell, either to evade the host cell defenses or immunesystem, or simply because it is not in the best interest of the virus to continually replicate.This hiding is deemed latency. During this time, the virus does not produce any progeny,it remains inactive until external stimuli—such as light or stress—prompts it to activate. Two Life Cycles of a Virus Lytic versus lysogenic life cycles:In the lytic stage, many viral particles are made and copies are sent back into theenvironment.A virus is found in this phase when conditions are favorable, i.e. when bacteria is"growing like crazy"
The virus attaches to bacteria (host) The virus inserts its DNA into the bacteria The virus takes over the cells machinery The virus reproduces itself and self- assembles. The host cell is destroyedIn the lysogenic phase there is no pathology.Under certain conditions the lysogenic lifestyle can switch to a lytic lifestyle. A virus is found at this stage under harsh conditions.
The virus is a prophage at this stage. The virus binds to bacteria (host) The virus inserts its DNA into thebacteria The viral DNA gets incorporated into thecells chromosome Viral DNA is replicated along with chromosomal material