Biology of Viruses and Replication

Replicative Cycles of Bacteriophages

Bacteriophages, or phages, are viruses that infect bacteria. They often feature an elongated capsid head containing DNA and a protein tail used to attach to the host and inject the viral genome. Phages typically follow one of two reproductive pathways:

1. The Lytic Cycle: This cycle results in the death of the host cell. The phage produces new progeny and then lyses (breaks open) the host's cell wall to release them. A phage that reproduces exclusively through this method is called a virulent phage.
2. The Lysogenic Cycle: In this cycle, the phage replicates its genome without destroying the host. The viral DNA is incorporated into the host cell's chromosome, where it is known as a prophage. Every time the host cell divides, it copies the prophage and passes it to daughter cells. Phages capable of using both cycles are called temperate phages.

Bacterial Defenses

Bacteria have evolved sophisticated defenses against phage infection. They use restriction enzymes to identify and cut up foreign DNA. Additionally, many bacteria and archaea utilize the CRISPR-Cas system, an immune mechanism that uses RNA fragments derived from previous infections to target and destroy invading phage DNA.

Animal Viruses and Retroviruses

Animal viruses are classified by their genome type and the presence or absence of a membranous envelope. Unlike phages, many animal viruses are enveloped and possess RNA genomes.

A significant group of animal viruses are retroviruses. These viruses use an enzyme called reverse transcriptase to copy their RNA genome into DNA. This viral DNA then integrates into the host genome, where it is called a provirus. Unlike a prophage, a provirus remains a permanent resident of the host cell, using the host's RNA polymerase to transcribe viral RNA for both protein synthesis and new genomes.

Note: While antibiotics are used to treat bacterial infections, they are ineffective against viruses. Treatment of viral infections typically involves antiviral drugs that inhibit DNA synthesis or interfere with viral assembly.