Viruses

• A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism.
• Viruses infect all life forms, including animals, plants and bacteria.
• Viruses have common features such as a small fixed size, typically 20 to 200 nm in diameter, genetic material in the form of DNA or RNA, a protective protein coat, known as a capsid, surrounding the genetic material, no cytoplasm, few or no enzymes, and do not have cells, and most scientists do not consider them to be alive.
• The Lysogenic Cycle of Lambda Bacteriophage is a HL Content Only.
• An E. coli lysogen is triggered to enter the lytic cycle.
• Phage DNA Replication begins, leading to lysis and the release of many copies of the lambda bacteriophage to infect other E. coli cells.
• All offspring of the infected bacteria will contain the lambda bacteriophage DNA.
• Coli reproduces, the lambda bacteriophage DNA is replicated along with the bacterial chromosome.
• The prophage (lambda bacteriophage DNA) is excised from the bacterial chromosome.
• Viruses are a very diverse group, but all viruses have the following features in common: small fixed size, typically 20 to 200 nm in diameter, genetic material in the form of DNA or RNA, a protective protein coat, known as a capsid, surrounding the genetic material, no cytoplasm, few or no enzymes, and do not have cells, and most scientists do not consider them to be alive.
• Viruses such as the bacteriophage lambda are not alive, and do not have metabolism.
• Bacteriophage lambda depend on their host cell for energy supply, nutrition, protein synthesis, and all other life functions.
• The bacteriophage lambda infects Escherichia coli bacterial cells, by the means of two different cycles: Lytic Cycle and Lysogenic Cycle.
• The genetic material of bacteriophage lambda is a double-stranded DNA molecule.
• The Lytic Cycle of the lambda bacteriophage involves the replication of the genetic material, the formation of progeny viruses, and the destruction of the host cell.
• During the lytic cycle, the bacteriophage lambda DNA remains separate from the host cell’s DNA.
• Evidence for several origins of viruses from other organisms can be found in the genetic code shared between viruses and living organisms.
• E. coli reproduction in the lysogenic cycle of lambda bacteriophage involves the infected E. coli lysogen reproducing.
• Viruses share an extreme form of obligate parasitism as a mode of existence, so the structural features that they have in common could be regarded as convergent evolution.
• The DNA from the bacteriophage lambda takes control of the cell’s metabolism.
• The genetic code is shared between viruses and living organisms.
• Most viruses can be classified as having an icosahedral shape (a 3D-shape with 20 faces) or a helical shape.
• The lytic cycle of a virus involves attachment, penetration, phage DNA replication, phage assembly, host cell lysis, and lysogenic cycle.
• The lysogenic cycle of lambda bacteriophage involves attachment, penetration, prophage formation, and lysogenic cycle.
• Rapid evolution in viruses can be exemplified by the evolution of influenza viruses and of HIV.
• Viruses are highly diverse in their shape and structure, with differences including shape, genetic material, and envelope status.
• The infected bacterium in the lysogenic cycle of lambda bacteriophage is known as a lysogen.
• The diversity of viruses suggests several possible origins.
• The Lysogenic Cycle of the lambda bacteriophage involves the replication of the genetic material, the formation of progeny viruses, and the persistence of the host cell.
• Viruses are obligate parasites, that require host cells for replication.
• Viruses use the same genetic code as all organisms.
• There are several competing hypotheses on the origin of viruses, but it is possible that all viruses do not share a common ancestor, and viruses could have developed in different ways.
• The similarities between viruses could result from convergent evolution, as similar adaptations are required for being obligate parasites.
• The spike proteins on the Covid-19 virus are an example of antigens.
• All modern viruses can only replicate using cells, suggesting that viruses could not have existed before cells, which is a weakness of the Virus First Hypothesis.
• Some viruses have an unstable genome and mutate very rapidly due to the reasons mentioned in the Rapid Evolution of Flu section.
• The Virus First Hypothesis proposes that viruses existed before cells, as they are much simpler than cells.
• The Regressive Hypothesis suggests viruses were once small cells that parasitized larger cells.
• If the genetic material of the virus mutates, this can cause the shape of the protein coat, including the antigens, to change.
• Viruses do not have a proofreading mechanism during replication, making it more likely that a mutation happens.