Test: Virus structure and reproductive cycle - MCAT MCQ

Viruses are acellular particles composed of a protein coat called a capsid and a nucleic acid core (DNA or RNA). Some viruses may also possess a lipid membrane called an envelope. However, viruses do not contain ribosomes, which are cellular organelles involved in protein synthesis.

The protein coat, or capsid, of a virus serves as a protective layer around the viral genome. It helps shield the genetic material from damage, such as enzymatic degradation, while the virus is outside the host cell.

The envelope of a virus is derived from the host cell's plasma membrane or an internal membrane, such as the endoplasmic reticulum or Golgi apparatus. As the virus exits the host cell during the budding process, it acquires a portion of the host's membrane, which becomes part of its envelope.

Viruses lack the necessary cellular machinery to replicate on their own. Instead, they hijack the host cell's machinery to reproduce. They utilize the host cell's enzymes, ribosomes, and metabolic pathways to synthesize viral proteins and replicate their genetic material.

In the lytic cycle, the virus enters the host cell, replicates its genetic material, synthesizes viral proteins, assembles new viral particles, and finally lyses (bursts) the host cell, releasing the newly formed viral progeny.

Lysogeny is the process in which a virus integrates its genetic material, specifically its DNA, into the host cell's genome. The viral DNA, known as a prophage, becomes a permanent part of the host cell's DNA and can be replicated along with the host cell's genome.

Retroviruses are RNA viruses that replicate their RNA genome using an enzyme called reverse transcriptase. Reverse transcriptase converts the viral RNA into DNA, which can then integrate into the host cell's genome. This unique characteristic sets retroviruses apart from other types of viruses.

Enveloped viruses exit the host cell through a process called budding. During budding, the virus acquires an envelope derived from the host cell's membrane, and it buds off from the cell surface, taking a portion of the membrane with it. This process allows the virus to exit the cell without immediately lysing it.

The immune response primarily targets viral RNA or DNA within the host cell. Immune cells recognize and respond to viral genetic material as a sign of infection, triggering a cascade of immune responses to eliminate the infected cells and control the spread of the virus.

Antiviral drugs are designed to target specific viral proteins involved in viral replication or other essential viral functions. By inhibiting these viral proteins, antiviral drugs can interfere with viral replication, preventing the spread of the infection and reducing symptoms.