A2.3 Viruses Flashcards
A2.3.1 (HL) Structural features common to viruses - small size
Most are between 20-300 nanometres in diameter.
Must be smaller than their host cells to enter. Also small bc lack cytoplasm + other structural features.
A2.3.1 (HL) Structural features common to viruses - fixed size
Viruses do not grow so they do not increase in size.
A2.3.1 (HL) Structural features common to viruses - genetic material
All viruses have genes made of nucleic acid and they use the universal genetic code.
A2.3.1 (HL) Structural features common to viruses - Protein capsid
Before viruses are released from their host cell, their genetic material is enclosed in a protein coat.
A2.3.1 (HL) Structural features common to viruses - Cytoplasm and enzymes
Have no cytoplasm and contain no (or few) enzymes. Rely on the hosts metabolism.
The viral enzymes that are produced are required for genetic replication, infecting host cells or bursting host cells.
A2.3.2 (HL) Diversity of Structure in Viruses - diversity of genetic material
DNA or RNA
Circular or linear
Double-stranded (ds) or single-stranded (ss)
A2.3.2 (HL) Diversity of Structure in Viruses - diversity of genetic material replication in single-stranded RNA
Positive-sense: use their genes directly as mRNA.
Negative-sense: transcribe genes to make mRNA.
Retroviruses: make double-stranded DNA copies of RNA and then transcribe the negative-sense strand to produce mRNA.
A2.3.2 (HL) Diversity of Structure in Viruses - Enveloped viruses
In lysis, some viruses become covered in a membrane from the cell. Some go through “budding”.
More sensitive to extreme pH, heat, dryness, and simple disinfectants.
A2.3.5 (HL) Evidence for Several Origins of Viruses - Virus-first hypothesis
Viruses predate or coevolved with their current cellular hosts.
❌All viruses are intracellular parasites, requiring a cell to replicate.
❌ Share genetic code w/ cells. It would need either an ancestral virus evolving into the first cell or independent evolution of cells w/ the same code.
A2.3.3 (HL) Lytic Cycle of a Virus - Step 1
The phage attaches itself to a receptor protein (CM). This happens through random collisions as it is a selective process.
A2.3.3 (HL) Lytic Cycle of a Virus - Step 2
The phage injects its genetic material into the host cell via the phage tail tube, which is surrounded by a sheath of contractile proteins.
A2.3.3 (HL) Lytic Cycle of a Virus - Step 3
The ends of the linear phage DNA come together to form a circle. Then, a process called rolling-circle replication occurs. One strand is cut, the 3’ end grows while the 5’ end is peels off, creating a new strand.
A2.3.3 (HL) Lytic Cycle of a Virus - Step 4
Phage DNA is then used to synthesis viral proteins, using the host machinery.
The host RNA polymerase transcribes phage DNA into RNA. Its ribosomes translate phage RNA into phage proteins.
A2.3.3 (HL) Lytic Cycle of a Virus - Step 5
Capsid proteins form empty heads for DNA to be packaged in. Tails assemble separately. Filled heads are joined to the tails in the final step of synthesis.
A2.3.3 (HL) Lytic Cycle of a Virus - Step 6 + 7
Enzymes produced by the phage weaken the cell wall causing lysis to happen.
The new virus particles are now “virulent”, able to infect other cells.
A2.3.4 (HL) Lysogenic Cycle of a Virus - Step 1
The phage attaches itself to a receptor protein (CM). This happens through random collisions as it is a selective process.
A2.3.4 (HL) Lysogenic Cycle of a Virus - Step 2
The phage injects its genetic material into the host cell via the phage tail tube, which is surrounded by a sheath of contractile proteins.
A2.3.4 (HL) Lysogenic Cycle of a Virus - Step 3 (prophage)
The phage DNA ends join to form a circle.
Then, the DNA is inserted into the the bacterial DNA. The viral enzyme integrase catalyzes the integration.
A2.3.4 (HL) Lysogenic Cycle of a Virus - Step 4
The prophage genome is replicated passively along with the host genome during DNA replication.
A2.3.4 (HL) Lysogenic Cycle of a Virus - Step 5
The host cell divides, creating two daughter cells which each contain the prophage (dormant).
A2.3.4 (HL) Lysogenic Cycle of a Virus - Step 6
If exposed to stressors, e.g. UV light or chemicals, prophage may spontaneously extract themselves and enter the lytic cycle in a process called induction.
A2.3.5 (HL) Evidence for Several Origins of Viruses - Progressive hypothesis
Viruses arose by taking and modifying cell components (“escape hypothesis”).
✔ Evidence supports for the origin of some virus, e.g. retroviruses.
A2.3.5 (HL) Evidence for Several Origins of Viruses - Regressive hypothesis
Viruses arose by loss of cellular components.
✔ Evidence supports for the origin of some virus, e.g. mimivirus.
A2.3.5 (HL) Evidence for Several Origins of Viruses - Convergent Evolution in Viruse
Different viral lineages have likely independently evolved similar adaptations due to selection for function.
A2.3.6 (HL) Rapid Evolution in Viruses - Generations
Evolutionary change is limited by generation time. In humans, the avg. GT is 25 years, but in viruses it can be less than an hour.
A2.3.6 (HL) Rapid Evolution in Viruses - Genetic variation
Mutation rates are high, particularly in RNA viruses, who don’t checks replication errors.
Beneficial mutations become more prevalent in the population, as those virus are more prone to survive.
A2.3.6 (HL) Rapid Evolution in Viruses - Natural selection
Host defenses give selection pressure, e.g. antibodies targeting proteins. By changing these proteins, viruses can evade the immune system and multiply, while older virus variants are destroyed.
A2.3.6 (HL) Rapid Evolution in Viruses - Influenza (antigenic drift)
Small mutations that can lead to changes in the surface proteins of the virus. Antibodies won’t effectively recognize and neutralize the antigenically different flu viruses.
A2.3.6 (HL) Rapid Evolution in Viruses - Influenza (antigenic shift)
An abrupt, major change. It can happen if a flu from an animal population gains the ability to infect humans. This introduces surface proteins which can evade the immune systemö
A2.3.6 (HL) Rapid Evolution in Viruses - HIV
One of the highest viral mutation rates. Natural selection can rapidly select for any new variation that help the virus evade the immune system or drug treatments.
