Viruses are a very interesting phenomenon of nature. Although they exhibit certain characteristics of living organisms in their complex behavior, they are not quite alive because they cannot reproduce on their own. All types of viruses, from animal viruses to bacteriophages, or viruses that attack bacteria, can only replicate within living cells.
The replication process, while slightly difficult to understand, is fascinating and can help us gain knowledge of the part that viruses play in society and in the medical world. All viruses have what's called a "virus genome," or a strand of amino acids.
The virus genome can be either double-stranded DNA, double-stranded RNA, or single-stranded RNA. The replication process differs slightly for each of these virus types. This post will focus on DNA virus replication.
Attachment, Penetration, and Uncoating
Many of the viruses that infect humans are DNA viruses, including the viruses that cause herpes and smallpox. Like all viruses, these infectious agents search for host cells with specific characteristics. When a virus has located an acceptable host cell, it attaches itself to the cell. This is the first step, called attachment.
After attachment, the virus penetrates the cell and goes through a process called "uncoating." Viruses have a protective coat of proteins called a capsid layer. During uncoating, this layer is removed, and the virus' DNA is released into the cell. Attachment, penetration, and uncoating are the three fundamental first steps in virus replication.
Transcription and Translation
After uncoating, the host cell begins the process of replicating the virus. Normally, cells divide and reproduce by copying their own genetic material and making new versions of themselves. However, when a virus has inserted its own DNA into a host cell, the cell will begin copying that DNA (along with some enzymes that the virus requires).
Thus, the virus co-opts the reproductive mechanism of the cell for its own replication. When the host cell finds the virus DNA, it begins to copy the DNA through a process called transcription and translation.
This complicated process involves the organs of the cell and free amino acids. The result is a copy of the virus DNA, as well as copies of proteins that are needed to put the new viruses together with their own capsid layers.
Lytic and Lysogenic Cycles
When the host cell has produced a completed virus, the new virus is released from the cell in a process called "lysis," and the new virus goes on to replicate itself in a different cell in the same manner. The DNA virus replication cycle described is called the lytic cycle.
The lytic cycle is the most common method of virus replication, but there is another method called the lysogenic cycle. Although the two cycles are quite similar, the lysogenic cycle is more efficient.
Whereas, in the lytic cycle, an infected cell replicates and releases a virus without reproducing itself, the lysogenic cycle allows infected cells to reproduce after they have been infected. The host cell copies itself along with the virus DNA, and the copies copy themselves, and so on.
The result is a large number of infected cells, all of which will eventually create copies of the virus. Thus, one virus infecting one cell results in an enormous amount of new virus. Typically, the lysogenic cycle is used by bacteriophages rather than animal viruses.
Vaccines that Trick Viruses
Because viruses are not quite alive, they cannot be killed by antibiotics. For this reason, some viruses present a significant public health hazard because they cannot be easily cured. However, using biotechnology and DNA synthesis, it is possible to create vaccines that prevent people from becoming infected by viruses.
Interestingly, virus vaccines work in a similar way to the viruses themselves. By isolating certain strands of DNA from infected cells, replicating that DNA, and merging it with a vector, or delivery system, people can create vaccines to "trick" cells into not becoming infected.
When the vaccine DNA has been inserted into an individual's cells, any real virus that enters that person's body will not attach itself to any cells there because the cells will not "appear" to the virus to be susceptible.
Although this can be an effective way to prevent the spread of some viruses, such as influenza, it is not easy to discover the appropriate genetic material to prevent every viral disease.
That's why viruses like HIV continue to be such a problem. With enough research, however, scientists may be able to discover ways to vaccinate against even the most troublesome viruses.