Learning Objectives To explain how viruses reproduce in cells. Viruses are visible only under an electron microscope. They come in a variety of shapes, ranging from spherical to rod shaped. The fact that they contain either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)— but never both —allows them to be divided into two major classes: DNA viruses and RNA viruses (Figure \(\PageIndex{1}\)). Most RNA viruses use their nucleic acids in much the same way as the DNA viruses, penetrating a host cell and inducing it to replicate the viral RNA and synthesize viral proteins. The new RNA strands and viral proteins are then assembled into new viruses. Some RNA viruses, however, called retroviruses (Figure \(\PageIndex{2}\)), synthesize DNA in the host cell, in a process that is the reverse of the DNA-to-RNA transcription that normally occurs in cells. The synthesis of DNA from an RNA template is catalyzed by the enzyme reverse transcriptase. In 1987, azidothymidine (AZT, also known as zidovudine or the brand name Retrovir) became the first drug approved for the treatment of AIDS . It works by binding to reverse transcriptase in place of deoxythymidine triphosphate, after which, because AZT does not have a 3′OH group, further replication is blocked. In the past 10 years, several other drugs have been approved that also act by inhibiting the viral reverse transcriptase. Raltegravir (Isentress) is a newer anti-AIDS drug that was approved by the FDA in October 2007. This drug inhibits the integrase enzyme that is needed to integrate the HIV DNA into cellular DNA, an essential step in the production of more HIV particles. A major problem in treating HIV infections is that the virus can become resistant to any of these drugs. One way to combat the problem has been to administer a “cocktail” of drugs, typically a combination of two reverse transcriptase inhibitors along with a protease inhibitor. These treatments can significantly reduce the amount of HIV in an infected person. Career Focus: Genetics Counselor A genetics counselor works with individuals and families who have birth defects or genetic disorders or a family history of a disease, such as cancer, with a genetic link. A genetics counselor may work in a variety of health-care settings (such as a hospital) to obtain family medical and reproductive histories; explain how genetic conditions are inherited; explain the causes, diagnosis, and care of these conditions; interpret the results of genetic tests; and aid the individual or family in making decisions regarding genetic diseases or conditions. A certified genetics counselor must obtain a master’s degree from an accredited program. Applicants to these graduate programs usually have an undergraduate degree in biology, psychology, or genetics. class of viruses that use reverse transcription to replicate within a host cell. Reverse transcription is a process by which RNA is converted into DNA. Retroviruses, like HIV, use this trick. They carry an enzyme called reverse transcriptase that takes their RNA and makes a complementary DNA strand from it. This new DNA strand then integrates into the host cell's genome, allowing the virus to hijack the host's cellular machinery to reproduce. Properties : Retroviruses are enveloped, meaning they have a lipid membrane derived from the host cell. Their genome consists of RNA, not DNA. They carry an enzyme called reverse transcriptase, which is crucial for their replication. Structure : These viruses have a complex structure with an outer lipid envelope, glycoprotein spikes for attachment, and an inner protein core containing the RNA genome and reverse transcriptase enzyme. Function : Upon entering a host cell, the retrovirus uses reverse transcriptase to convert its RNA genome into DNA. This viral DNA then integrates into the host’s genome, allowing the virus to hijack the host’s machinery to produce new viral particles. Diseases : Retroviruses can cause several diseases, most notably HIV/AIDS. HIV targets and destroys the immune system’s CD4+ T cells, leading to immune deficiency. Other retroviruses can cause certain types of cancers, such as HTLV-1, which is associated with adult T-cell leukemia/lymphoma. The primary treatment for retroviruses, such as HIV, is antiretroviral therapy (ART). ART involves a combination of medications that target different stages of the virus's life cycle to prevent it from replicating Here are the main classes of antiretroviral drugs: Nucleoside Reverse Transcriptase Inhibitors (NRTIs): These drugs inhibit the reverse transcriptase enzyme, preventing the virus from making DNA from its RNA Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs): These drugs bind to and inhibit the reverse transcriptase enzyme without competing with nucleotides Protease Inhibitors (PIs): These drugs inhibit the protease enzyme, which is essential for the virus to produce functional proteins Integrase Inhibitors: These drugs block the integrase enzyme, preventing the viral DNA from integrating into the host cell's genome Entry/Attachment Inhibitors: These drugs prevent the virus from entering the host cell Pharmacokinetic Enhancers (Boosters): These drugs enhance the effectiveness of other antiretroviral drugs by increasing their concentration in the body ART has transformed HIV from a fatal infection to a manageable chronic condition. While it's not a cure, it significantly reduces the viral load, helping people with HIV live longer, healthier lives. These medications are often used in combination to form a regimen that effectively suppresses the virus, reduces viral load, and helps maintain immune function. The specific combination and choice of drugs depend on various factors, including the patient's health status, potential drug interactions, and resistance patterns. Summary Viruses are very small infectious agents that contain either DNA or RNA as their genetic material.