III.

Viral RNA

Some RNA viruses have double-stranded RNA—that is, their RNA molecules consist of two parallel polynucleotide chains. The base of each RNA nucleotide in one chain pairs with a complementary base in the second chain—that is, adenine pairs with uracil, and guanine pairs with cytosine. For these viruses, the process of RNA replication in a host cell follows the same pattern as that of DNA replication, a method of replication called semi-conservative replication. In semi-conservative replication, each newly formed double-stranded RNA molecule contains one polynucleotide chain from the parent RNA molecule, and one complementary chain formed through the process of base pairing. The Colorado tick fever virus, which causes mild respiratory infections, is a double stranded RNA virus.

There are two types of single-stranded RNA viruses. After entering a host cell, one type, polio virus, becomes double-stranded by making an RNA strand complementary to its own. During replication, although the two strands separate, only the recently formed strand attracts nucleotides with complementary bases. Therefore, the polynucleotide chain that is produced as a result of replication is exactly the same as the original RNA chain.

The other type of single-stranded RNA viruses, called retroviruses, include the human immunodeficiency virus (HIV), which causes AIDS, and other viruses that cause tumors. After entering a host cell, a retrovirus makes a DNA strand complementary to its own RNA strand using the host's DNA nucleotides. This new DNA strand then replicates and forms a double helix that becomes incorporated into the host cell's chromosomes, where it is replicated along with the host DNA. While in a host cell, the RNA-derived viral DNA produces single-stranded RNA viruses that then leave the host cell and enter other cells, where the replication process is repeated.