Infection STrategies Lytic Cycle: – The virus enters the cell, makes copies of itself and causes the cell to burst. Uses the materials and cell machinery of the host cell to replicate itself. Lysogenic Cycle – The virus enters the cell, integrates itself into host DNA and replicates along with the host. – The imbedded virus is called a prophage – After a period of time, the prophage will enter the lytic cycle

Stages of the Lytic Cycle 1.Attachment: a viral particle and a cell collide randomly. The virus attempts to attach to the receptors on the surface of the host cells. 2.Penetration: The virus enters the cell. The virus has two options: – Trick the cell into endocytosis – Injects itself into the cell » The capsid remains outside the cell. 3.Replication: Viral enzymes use the cell to make all the different pieces of the virus (DNA, capsid, spikes, enzymes, etc…)

Lytic Cycle 4.Assembly: the viral parts are assembled into a complete virus. The enzymes encoded by the viral genes guide the assembly. 5.Lysis: Either: the mature phage particles burst out of the cell, usually because lysozymes degraded the cell wall. The mature viruses then infect more cells. The viral “buds” pinch off the cell membrane and can infect other cells

FIGURE 14.8: Bacteriophage replication

Lysogenic Cycle Some viruses enter a cell, but do not immediately cause cell lysis. The phage DNA integrates into chromosome as a prophage. With this cycle the cell survives the infection, the cell will grow and replicate. – When this happens the prophage is copied and transferred to the cell’s offspring as part of their chromosome. – Each daughter cell after that is “infected”…basically meaning they have the viral genome in the chromosome as a prophage. This will continue until the cell becomes stressed, from lack of nutrients or bad environment. When this happens the prophage will excise itself from the chromosome, and go through the lytic cycle.

FIGURE 14.8: Bacteriophage replication

Figure 10A: Replication of a DNA and RNA

Latent Infections Many DNA viruses will establish latent infections that are characterized by repression of most viral genes, so the virus lies dormant. – For example herpesviruses can generate latent infections, and then when some stress come about the DNA of the virus can produce a new infection. Retroviruses like HIV can also lie dormant – Called a provirus – Each time the host cells then replicates the provirus is copied.

FIGURE 11: The formation of a provirus by HIV

Viruses and Tumors About 60% – 90% of cancers are due to some type of carcinogen. But some viruses can act as carcinogens. Some viruses have been isolated from human cancers and they can cause transformations of normal cells into tumor cells. – Examples: herpes viruses can cause cervical cancers, Epstein-Barr virus is linked to Burkitt lymphoma, Human papilloma virus can cause cervical cancers as well...these viruses are called oncogenic viruses.

Emerging Viruses Many emerging viral infections are due to viruses that appear for the first time in a population, or developing the ability to expand their host range. – Most times this due to the viruses finding new populations in different geographical areas. Some new viruses can form through genetic recombination. – Influenza virus does this that’s why vaccinations only last for one year. Viruses can also mutate to cause changes in their DNA or RNA – These mutations can cause the virus to be resistant to drugs or allow them to affect other populations. Increased contact with animals can also allow viruses to “jump” from one species to another.

Origin of Viruses There Are three hypotheses for the origin of viruses. – The regressive evolution hypothesis Viruses are degenerate life-forms, basically they are derived from intracellular parasites that have lost life functions. – The cellular origins hypothesis Viruses are derived from subcellular components and macromolecules that escaped from cell walls and replicated inside hosts. – The independent entities hypothesis Viruses coevolved with cellular organisms from a self- replicating molecule present on primitive Earth.