GCSE BIOLOGY REVISION: INFECTION AND RESPONSE
Please note: Text in bold is what the AQA GCSE biology specification requires an understanding of.
Measles is a viral disease showing symptoms of fever and a red skin rash. Measles is a serious illness that can be fatal if complications arise. For this reason most young children are vaccinated against measles. The measles virus is spread by inhalation of droplets from sneezes and coughs. HIV initially causes a flu-like illness. Unless successfully controlled with antiretroviral drugs the virus attacks the body’s immune cells. Late stage HIV infection, or AIDS, occurs when the body’s immune system becomes so badly damaged it can no longer deal with other infections or cancers. HIV is spread by sexual contact or exchange of body fluids such as blood which occurs when drug users share needles. Tobacco mosaic virus (TMV) is a widespread plant pathogen affecting many species of plants including tomatoes. It gives a distinctive ‘mosaic’ pattern of discolouration on the leaves which affects the growth of the plant due to lack of photosynthesis.
The first important thing to recognise is the fact that viruses are not alive. Why? They do not complete all of the seven life processes; movement, respiration, sensitivity, growth, reproduction, excretion and nutrition (MRS GREN). When discussing viruses, we refer to them as ‘strains’ rather than species. Viruses are comprised of relatively short genetic material enclosed in a protein coat. They have the same life cycle as other pathogens and can often live outside of a host for significant periods of time.
Viruses operate by infecting a suitable host (a single cell or multiple cells) and then proceed to replicate themselves within the cell. They replicate by thousands, they do not divide or reproduce. Instead they replicate their DNA and protein coats, these then assemble into new virus particles. The mass replication of viral DNA causes bursting of host cells leading to infection of nearby cells. This is a very quick process, it can be as quick as twelve hours. Ebola, for example, takes several days, whilst influenza takes approximately two days.
It is important to note than viral infections cannot be treated by antibiotics. Bacterioa and viruses have different mechanisms for survival and replication. Antibiotic treatment works on bacterial infections as it relies on recognition of receptors outside of the cell to kill infection. As viruses are found inside of cells, antibiotic treatment would not be able to recognise the infected cells. The antibiotic has no target to attack in a virus.
Tobacco Mosaic Virus
Tobacco mosaic virus is a plant viral infection where the virus infects chloroplasts in the leaves. Such plant species are tobacco plants and other closely related species such as tomatoes and peppers. Transmission occurs through plant to plant contact (naturally occurring or through farmer handling). Infection leads to a colour change from green to yellow to white in a mosaic pattern. Additionally, leaves can crinkle and curl up. These changes inhibit the plants ability to photosynthesise and therefore affects growth. Consequently, the crop yield is reduced. As there is no cure, farmers must make a substantial effort to prevent the spread of the virus or reduce infection.