Many people confuse bacteria with viruses. They believe that viruses are extremely diminutive forms of bacteria. However, this is not true. In effect, bacteria and viruses are basically so dissimilar that in several aspects a bacterium is more intimately associated with humans compared to a virus. In any case, both humans and bacteria are both composed of living cells. On the contrary, apart from viruses, each and every living thing is composed of cells. Actually, the elements comprising a cell have a variety of functions, such as generating energy, development eating, as well as responding to ecological alterations. However, these aspects are missing in a virus. So long as the viruses remain outside a cell, they are actually nothing, but diminutive elements that are completely inert or lifeless. Unfortunately, viruses are developed with the intension to enter the living cells and when they succeed in penetrating the cells they commence the lethal work of viral contagion.
Each and every virus comprises of two elements - a nucleic acid at the centre and a protein coating that encircles the core. In a number of instances, the viruses have an extra fatty or lipid covering. The role of the protein coating or the lipid covering, if present, is crucial. They are not only responsible for binding the virus to the cell membrane of a living thing, but also to work and enable the virus to penetrate the cell in any way. However, doing so is not an uncomplicated or easy task. In fact, the exterior of the virus coating needs to precisely match with the 'receptor' sites on the membrane or covering of the living cell. The virus is neither able to attach itself to the cell or penetrate it when exterior viral coating does not exactly match with the membrane of the cell. Believe it or not, even when perfect matching occurs, perhaps only one in a number of thousands of viruses is able to bind to as well as penetrate the cell membranes. In other words, the bond and penetration of a virus into a living cell is really a very complicated task. Such accuracy in matching for viruses to bind with living cell membranes and penetrate them perhaps elucidates why they are generally species-specific and will never contaminate any cell from an entirely dissimilar species. However, there are some exceptions to this theory. For instance, the influenza and rabies viruses actually infect an assortment of hosts.
While the viruses will rarely contaminate entirely dissimilar species, but very often they are very particular regarding invading specific types of cells in a living being. For instance, while the hepatitis B virus aims at invading the liver cells, the HIV is on the look for specific sites to bind or marker on particular white blood cells. On the other hand, viruses responsible for common cold attach themselves to the coating of the respiratory tract. It is important to note that when a virus succeeds in binding to a cell membrane, it is able to break through the covering and go into the cell in many different ways. One of the ways is to compel the cell membrane to double up and envelope a small vesicle along with the virus inside the cell. Viruses having a fatty covering are able to mingle their coatings with the cell membrane, thereby breaking through it and allowing the remaining part of the virus to enter the cell.
When a virus enters a cell by any possible means, it is able to do a number of things. And, whatever action the virus takes actually establishes the course of the ailment caused by it. There are several instances when the virus begins to reproduce itself instantly after penetrating the cell membrane of a living organism. On fact, it may be said that rapid replication as well as extensive distribution are the two main causes for its very existence. In order to achieve these two tasks, the virus activates the mechanism of a cell soon after entering it and forces the cell to turn out an increasing number of viral units. Many scientists describe this activity of the virus as an implausible act of piracy that is basically coordinated and regulated by the nucleic acid core of the virus.
Although the nucleic acid present in the core of a virus is an extraordinary substance, the scientists had failed to comprehend its significance till as late as 1953. Actually, it was in that very year that two scientists James Watson and Francis Crick settled on the molecular construction of DNA or deoxyribonucleic acid, which is a long linear polymer found in the nucleus of a cell. In fact, there is another type of nucleic acid known as RNA or ribonucleic acid that is a close cousin of DNA.
In fact, the DNA is a genetic material that is present in all types of living cells, barring the mature red blood cells. In other words, this genetic substance is hereditary and responsible for giving any living thing its form as well as characteristics. DNA is the substance that actually makes a human a human, a tree a tree and a bacterium a bacterium. Again this is the genetic substance that is responsible for making one human different from another and one species of trees dissimilar to other species. The structure of the DNA is seen as an extremely long microscopic charm bracelet. For instance, the human DNA comprises three billion charms. Speaking precisely, the DNA is actually two charm bracelets that are arranged in such a manner that their charms are able to attach with each other. Looking at it closely, you will find that there are four different types of charms that are known as nitrogen bases and named by the English language alphabets A, T, C and G. The dual chain also possesses a noticeable coil making it generally appear like a twisted rope ladder or the spiral arrangement of the two complementary strands of DNA having rungs or girdles that are a manifestation of the paired nitrogen bases.
Similar to an alphabet comprising four letters, the arrangement or series of the rungs or base nitrogen pairs makes obvious the functions a particular cell will carry out and what they will eventually turn out to be. In fact, the DNA can be described as a master plan for every life, as it regulates the performance of a cell by deciding on the proteins made by particular cells. However, it is important to note that the DNA does not function individually. In other words, the DNA never directly regulates the synthesis of protein that takes place on the diminutive elements, often called protein factories, which remain buoyant in the region of the cells. In fact, the DNA is found concealed deeply inside the nucleus and is intertwined into the structure of the chromosomes. Actually, the DNA is a very precious element of the cell and, hence, it does not move around all over the cell. As an alternate, the DNA prepares a messenger filament with nucleic acid making use of its base pair series as a guide. This messenger molecule is known as the RNA and its function is to communicate the signals from the DNA to the protein factories located outside the nucleus.
Although viruses do not have much resemblance to the living cells, like the living cells, they also possess nucleic acid as their genetic substance. Any specific virus may enclose DNA or RNA, but no types of viruses contain both. The difference between the DNA contained in a living cell and that in any virus is that while the DNA in living cells transmit codes for manufacture of cellular proteins by the protein factories located outside the nucleus of the cell, in the instance of viruses, the viral nucleic acid signal is sent not for manufacture of cellular proteins, but to produce proteins essential for reproducing more viruses. In fact, the viral nucleic acid actually compels the host cells to produce proteins in large amounts that are required by the virus to replicate it rapidly. This process is somewhat similar to a clever invasion in a phased manner and may be explained as below:
The exact method by which the viruses begin their reproduction largely depends on the genetic substance enclosed by the virus. The manner of functioning of a DNA virus is very much comparable to the DNA of the host cell. Initially, the virus produces a viral RNA making use of the virus' DNA as a model. Next, the RNA carries instructions from the viral nucleic acid to the protein factories of the host cell to manufacture viral proteins - proteins necessary for the replication of the virus. In brief, the virus replication process begins with the viral DNA, passes on to the viral RNA and concludes with the production of viral protein. However, in the instance of majority of the RNA viruses, the first step has to be done away with as they do not contain DNA (viruses may contain DNA or RNA, but not both). In this case, the viral nucleic acid just goes on to produce the requisite viral proteins. In brief, in the case of RNA viruses, the replication process begins with the viral RNA instructing the host cell's protein factory to produce proteins necessary for reproduction of more viruses and it concludes with the production of such viral proteins by the host cells.
The perception that the DNA produces the RNA, which, in turn, produces protein is so fundamental to biology that it today it is widely accepted by scientists as the most vital doctrine or 'central dogma'. It needs to be mentioned here that the RNA never produces DNA, and the process is actually the other way around. However, there is a rare exception to this doctrine and that is in the instance of a retrovirus. This type of RNA virus, includes HIV - the AIDS virus, and has actually amazed the molecular biologists by doing something incredible - by functioning in a retreating manner. It is interesting to note that rather than producing proteins, the RNA of the retrovirus initially goes into a reverse mode and functions as a prototype for the synthesis of DNA. Following the DNA synthesis, the newly formed DNA produces a new RNA that eventually starts mass producing protein molecules needed for virus replication. Precisely speaking, the reproduction process of retrovirus begins with viral RNA that undertakes DNA synthesis and, in turn, the new DNA model produces another RNA, which manufactures viral protein necessary for replication of this microscopic organism.
