Viruses and Disease

for proteins that participate in the genome replication process. The DNA virus genome needs to get into the nucleus cells that have them. As you’ll see, bacteria do not have nuclei so this step is not necessary.

RNA replication takes place in the host cell’s cytoplasm. The polarity of the RNA determines how it’s replicated. Positive-sense RNA gets translated into proteins immediately, while negative-sense RNA gets turned into a positive sense by using RNA replicase. In all cases requiring RNA replicase, the enzyme itself comes from the virus that makes the enzyme to create copies of their genome.

Reverse transcribing viruses can be ssRNA (single-stranded RNA) or dsDNA (doublestranded DNA) viruses. Those with RNA genomes, particularly the retroviruses, use a DNA intermediate, while those with DNA genomes use an RNA intermediate. Both use what is called reverse transcriptase (also called RNA-dependent DNA polymerase) that converts RNA to DNA. It is called “reverse transcription” because normal transcription involves DNA going to make RNA. Retroviruses will make the DNA that has been made by reverse transcriptase and will incorporate it into the host DNA. These particular viruses are highly susceptible to drugs like lamivudine and zidovudine, which are reverse transcriptase inhibitors. Normal cells don’t have reverse transcriptase so it only affects the viruses. HIV Is a retrovirus.

VIRUSES AND DISEASE

There are many common diseases that are directly related to viruses, including smallpox, chickenpox, influenza, and the common cold. Very serious diseases are also caused by viruses, such as HIV, Ebola, SARS, and the avian flu. There are potential virally-mediated diseases, such as chronic fatigue syndrome and multiple sclerosis. There are different things that a virus can do to the host; they can exist harmlessly with the host as is the case with the herpes simplex virus, which lies dormant for a long period of time. The chickenpox virus can also lie dormant, leading to herpes zoster or shingles later in life. Other viruses can be mildly protective to the host in order to prevent the presence of other diseases.