Retroviruses belong to the family of viruses known as Retroviridae. Viruses are infective particles composed of a nucleic acid, a protein coat called a capsid, and sometimes an enzyme. Viruses are classified as DNA (deoxyribonucleic acid) viruses or RNA (ribonucleic acid) viruses depending on their type of nucleic acid. Retroviruses contain RNA, as well as the enzyme, reverse transcriptase, and a protein coat. Different from other RNA viruses, retroviruses replicate as DNA in the host cell. (Other RNA viruses replicate as RNA.) By using reverse transcriptase, the RNA template synthesizes a DNA copy. This procedure reverses the usual flow of information (DNA to RNA), hence the name retrovirus. The copy enters the host's chromosome as a provirus and may remain dormant, replicating along with the chromosome. It can also become active and replicate more retroviruses. Most retroviruses exist as parasites in animal cells.
HIV (human immunodeficiency virus), the causative agent in AIDS (acquired immunodeficiency syndrome), is a retrovirus. The exterior of the AIDS virus consists of a capsid surrounded by a viral envelope. Glycoproteins on the outside of the viral envelope bind to specific receptors on the host cell membrane. Two identical strands of RNA and the enzyme, reverse transcriptase, reside within the capsid. When the virus attaches to a host cell, enzymes dissolve the capsid and the viral RNA enters the cell. Using reverse transcriptase, the viral RNA produces its complementary DNA.
The new DNA then synthesizes a complementary DNA strand, and the double stranded DNA incorporates into the host's chromosome as a provirus. During protein synthesis, the provirus genes are transcribed into mRNA (messenger RNA) molecules that are translated into HIV proteins. In addition, the provirus genes are transcribed into RNA to create more viruses. Capsids form around the new RNA strands, and newly formed HIV viruses emerge from the host cell.
Scientists are using retroviruses to benefit research and medicine. Retroviruses serve as vehicles to deliver a genetic payload that is therapeutic into cells. The technique involves replacing the harmful parts of retroviruses with genes that could help treat patients with diseases such as cystic fibrosis or brain cancer. Inside the patient's body the retroviruses could enter the cells, form DNA copies, and incorporate their payload into the host's chromosome. The technique has problems, however. For one thing, the entering virus has to bypass the host's immune system. Secondly, the virus has to get past the host's nuclear membrane to reach the chromosomes. Unfortunately this can only occur when the nuclear membrane disintegrates during cell division. Finally, the virus must deliver its payload into the correct place in the host chromosome or it might interfere with an important gene. Researchers are seeking ways to overcome these obstacles. One way is to inject retroviruses carrying a cancer-suppresser gene cancer directly into a patient's cancer. This technique would avoid the immune system, and easily bypass the nuclear membrane because cancer cells divide rapidly. This study and others aim to develop new therapies and research techniques using retroviruses.
This is the complete article, containing 496 words
(approx. 2 pages at 300 words per page).
