A2.3.4_

Lysogenic Cycle of a Virus

The lysogenic cycle is a form of viral reproduction where the virus integrates its genetic material into the host cell's genome, allowing for a stable and dormant presence within the host. This cycle contrasts sharply with the lytic cycle, where the host cell is immediately killed upon viral reproduction.

Key Points on Viruses (A2.3.2)

All viruses aim to hijack cellular machinery to achieve three fundamental objectives:

  1. Make more viral nucleic acid (DNA or RNA): The virus uses the host's nucleotides and enzymes to replicate its genome.

  2. Produce more virus proteins: Viral proteins are synthesized using the host's ribosomes and translation machinery.

  3. Assemble new virus particles: The viral components are assembled into new virions that can be released to infect other cells.

Viruses can be classified based on their RNA structure:

  • Positive-sense RNA viruses: These viruses possess single-stranded RNA that can be directly translated into proteins by the host's ribosomes without any prior transcription. Examples include poliovirus and hepatitis C virus.

  • Negative-sense RNA viruses: These viruses have RNA that must first be transcribed into a complementary strand before translation. This process requires an RNA-dependent RNA polymerase to create a positive-sense strand. Examples include influenza and rabies viruses.

  • Retroviruses: Retroviruses, such as HIV, possess RNA and utilize reverse transcription to convert their RNA into DNA after entering the host cell. This DNA is then integrated into the host's genome, allowing the virus to replicate and produce new virions.

Bacteriophage (Lambda)

Bacteriophages (or phages) are viruses that specifically target bacteria. The Lambda virus, for example, infects the bacterium E. coli and has the capability to switch between two distinct types of infection:

  1. Lysogenic cycle: In this phase, the virus integrates its genome with the host DNA and can replicate quietly as bacteria divide, thus facilitating horizontal gene transfer.

  2. Lytic cycle: Here, the virus replicates itself by commandeering the host's machinery and ultimately causes cell lysis, leading to the death of the bacterial cell and the release of new viral particles.

Steps of the Lysogenic Cycle

  1. Phage attachment to host cell: The attachment begins when the phage recognizes and binds to a specific receptor protein on the bacterial membrane, usually through random collisions. The specificity of this interaction is critical; the receptor compatibility dictates whether the phage can infect the bacterial cell.

  2. Phage DNA entry into host cell: The phage's tail structure, which varies among different phages, includes a tube often encased in contractile proteins. Upon binding, the tail contracts, injecting the phage's genetic material directly into the bacterial cell's cytoplasm.

  3. Phage DNA integration: Once inside, the linear phage DNA is circularized and integrates into the bacterial genome at a specific site with the help of an enzyme called integrase. This integrated viral DNA is referred to as a prophage, and it remains dormant and replicated along with the host's DNA.

  4. DNA replication: During subsequent cell divisions, the prophage is copied along with the host bacterial DNA. Due to its relatively smaller genome, the prophage does not typically disrupt the host's vital functions, allowing for a long-term dormant state.

  5. Cell division: As the host cell divides, it produces two daughter cells, each containing the prophage within their own genomic material. In this prophage state, the virus is considered temperate and remains quiescent, causing no immediate harm to the host.

Transition to Lytic Cycle

Under specific conditions, such as exposure to stressors like UV light, nutrient deprivation, or chemical signals, a prophage may excise from the bacterial DNA, transitioning back into the lytic cycle. This process is known as induction, where the virus becomes active, leading to the production of progeny viruses and the eventual destruction of the host cell.