Viruses
Page 5

Other RNA viruses, called retroviruses, act different) addition to RNA, retroviruses contain an enzyme called re~ transcriptase. Reverse transcriptase is an enzyme that makes [ from RNA. In normal cells, you recall, DNA makes RNA. RNA in turn makes proteins. In retroviruses RNA makes L with the aid of reverse transcriptase. The DNA then make~ new RNA. This RNA in turn makes the proteins that become of the new viruses. The AIDS virus is a retrovirus.

Viroids and Prions

In the last three decades scientists have isolated certain disease causing particles that are smaller and simpler than viruses. Or these is the viroid. A viroid (VIE-R OlD) is a short, single stran RNA with no surrounding capsid. Look at the viroid picture Figure 19-2. The strand of RNA shown there looks like a tan piece of string. It does not contain enough RNA to make protc Scientists suggest that this RNA strand somehow interferes normal cell functions and causes the production of new vi strands by using the host cell's enzymes. Diseases caused viroids include those that harm potato, coconut, chrysanthern and citrus crops, as well as other plants.

A prion (PREE-AHN) is a glycoprotein particle containing a polypeptide of about 250 amino acids. Even without nucleic acids prions are capable of reproducing in some mammalian cells. Prions are implicated in diseases with long incubation periods. For instance, prions cause scrap ie, a slow degeneration of the nervous system in sheep and goats. Another pri on disease is kuru, a degenerative nerve disease that can be contracted by touching the brains of deceased individuals. It occurs among some New Guinea high-land tribes whose funeral rites include touching the brains of deceased ancestors.

Section Review

1.  What is a capsid?

2.  What are the similarities between viruses and cells? What are the differences?

3.  Compare and contrast the structures of a polio virus and a tobacco mosaic virus.

4.  What is reverse transcriptase? What is its function?

5.  Why might the study of viroids and prions lag behind the study of viruses?

BIOGRAPHY
LOUIS PASTEUR