A2.3 Viruses HL only

1. A capsid of protein enclosing either DNA or RNA, but not both — The few features common to all viruses are a small, fixed size; nucleic acid, either DNA or RNA, as genetic material; a capsid made of protein; no cytoplasm; and few or no enzymes. No virus contains both DNA and RNA as its genetic material.
2. A cytoplasm in which the virus carries out metabolism — A student who thinks of a virus as a very small cell picks this. Viruses have no cytoplasm and no metabolism of their own; every life function is carried out for them by the host cell.
3. A membrane of phospholipid forming the outermost layer — A student who pictures the capsid as a kind of cell membrane picks this. The capsid is protein. Only enveloped viruses have a lipid layer, and it is host membrane outside the capsid, not a feature of all viruses.
4. A genome of DNA together with RNA, as in any living cell — A student who assumes genetic material must be DNA with RNA as a working copy picks this. A virus has one type of nucleic acid, DNA or RNA, never both; many viruses have RNA as their only nucleic acid.

Syllabus statement A2.3.1 · Read this in Learn

1. Both have an icosahedral head joined to a tail through which the genetic material is injected. — A student whose model of every virus is the bacteriophage diagram picks this. Only phages have a tail; HIV is an enveloped particle with a cone-shaped capsid that enters the cell by fusion of its envelope with the membrane.
2. Both carry reverse transcriptase so that their genetic material can be copied into DNA. — A student who generalises HIV's mechanism to all viruses picks this. Reverse transcriptase is a feature of retroviruses; lambda already has a DNA genome, which is transcribed and replicated by host enzymes.
3. Lambda has double-stranded DNA and no envelope, whereas HIV has RNA and an envelope. — Lambda is a non-enveloped phage with a double-stranded DNA genome in a head-and-tail capsid. HIV is an enveloped retrovirus with two single-stranded RNA molecules. The two illustrate the diversity of viral genetic material and structure.
4. Both have a capsid made of phospholipid taken from the membrane of the host cell. — A student who confuses the capsid with a membrane picks this. The capsid of every virus is protein. HIV has a host-derived lipid envelope outside its protein capsid; lambda has no lipid at all.

Syllabus statement A2.3.2 · Read this in Learn

1. The whole phage is taken into the cell and the capsid is broken down in the cytoplasm. — A student who applies the entry of animal viruses to phages picks this. A phage cannot cross the bacterial cell wall; the capsid stays on the outside and only the DNA enters.
2. The phage DNA passes through the tail into the cell, and the capsid remains outside it. — After attachment, lambda injects its DNA through the tail into the cytoplasm. The head and tail remain outside the cell wall. Only the DNA is needed inside, because the host cell provides everything else for synthesis of new phages.
3. The phage membrane fuses with the bacterial membrane, releasing the DNA inside. — A student who thinks the outer coat of a phage is a membrane picks this. Lambda has no lipid membrane: its outer layer is a protein capsid, and it delivers its DNA by injection through the tail.
4. The phage releases its ribosomes and enzymes into the cell to begin making phage proteins. — A student who thinks the virus brings its own synthetic machinery picks this. Lambda has no ribosomes; phage proteins are made by the host's ribosomes after host RNA polymerase transcribes the phage DNA.

Syllabus statement A2.3.3 · Read this in Learn

1. It is integrated into the host chromosome and is copied along with it. — In lysogeny the lambda DNA is integrated into the E. coli chromosome as a prophage. It is replicated with the chromosome at each cell division and inherited by both daughter cells, while the genes for the lytic cycle are kept switched off.
2. It stays as a separate circle in the cytoplasm, like a plasmid, replicating on its own. — A student who files any extra circle of DNA in a bacterium as a plasmid picks this. The lambda prophage is integrated into the chromosome by integrase and is copied only when the chromosome is copied.
3. It is transcribed slowly, making a few phages that leave the cell without lysis. — A student who imagines lysogeny as a slow lytic cycle picks this. No phages are produced during lysogeny: a repressor keeps the lytic genes off, and phages are only made after induction, when the cell lyses.
4. It is permanently inactivated, so the cell and its descendants can no longer make phages. — A student who thinks a prophage is a dead end picks this. The prophage remains intact and can be induced, for example by ultraviolet damage to the host DNA, to leave the chromosome and enter the lytic cycle.

Syllabus statement A2.3.4 · Read this in Learn

1. As homologous features that all viruses have inherited from a single ancestral virus. — A student who applies the rule 'shared features imply common ancestry' without considering convergence picks this. The IB states that the structural features viruses have in common could be regarded as convergent evolution, not homology.
2. As ancestral features retained from the first living things, from which cells later evolved. — A student who equates simplicity with antiquity picks this. Viruses depend entirely on cells and share the genetic code with them, so they are derived from cellular organisms; their simplicity is a result of parasitism.
3. As convergent evolution: extreme obligate parasitism imposes the same demands on unrelated viruses. — Every virus lives by an extreme form of obligate parasitism, which selects for a minimal particle that can protect its genome and get it into a cell. Features shared for this reason could have arisen independently in several lineages, so they are not evidence of a single origin.
4. As the features of the smallest possible cells, reduced to nothing except their most essential parts. — A student who thinks of viruses as tiny cells picks this. Viruses are not cells: they have no cytoplasm, no ribosomes and no metabolism, which is why they are described as particles rather than as reduced cells.

Syllabus statement A2.3.5 · Read this in Learn

1. Their genome-copying enzymes lack proofreading, and vast numbers of virions are produced quickly. — The RNA-dependent RNA polymerase of influenza and the reverse transcriptase of HIV do not proofread, so the mutation rate is very high. Short generation times and huge populations supply many variants, and immune responses and drugs select among them.
2. Their RNA genomes are chemically unstable, so bases change spontaneously between one infection and the next. — A student who links the short life of RNA with mutation picks this. Mutations are copying errors made during replication by enzymes without proofreading, not spontaneous changes to a genome sitting inside a virion.
3. They alter their genes deliberately whenever they are attacked by antibodies or by antiviral drugs. — A student who reads 'the virus adapts' as intention picks this. Mutations are random and occur regardless of any threat; antibodies and drugs then select the variants that happen to survive.
4. They divide in two every few minutes, as bacteria do, so mutations accumulate over many divisions. — A student who transfers binary fission to viruses picks this. Viruses do not divide; each infected cell releases many assembled virions, and the mutations arise when the genome is copied by error-prone viral enzymes.

Syllabus statement A2.3.6 · Read this in Learn

1. It grows to about twice its size inside the cell and then divides into two identical particles. — A student who transfers binary fission from bacteria to viruses picks this. Viruses have a small, fixed size: they do not grow and do not divide. New particles are assembled at full size from separately made components.
2. Its RNA and proteins are made by the host cell's machinery and then assembled into new particles. — The particle has the features of a virus: fixed size, one nucleic acid, a protein capsid, no cytoplasm. With no ribosomes or metabolism it relies on the host for energy, nutrition and protein synthesis, and new particles are assembled from the components the host makes.
3. It absorbs nutrients from the cell into its cytoplasm and metabolizes them to build copies of itself. — A student who thinks of the particle as a tiny cell picks this. It has no cytoplasm and, with no ribosomes and few or no enzymes, no metabolism; the host cell's cytoplasm is where the copying and protein synthesis happen.
4. It must first acquire a DNA molecule, because one RNA strand cannot act as genetic material. — A student who believes genetic material has to be DNA picks this. Nucleic acid of either type can be a viral genome; single-stranded RNA is the genetic material of coronaviruses and of HIV, and needs no DNA partner to be a genome.

Syllabus statement A2.3.1 · Read this in Learn

1. It has a lipid envelope that it synthesized itself, using enzymes encoded in its own genome. — A student who assumes every part of a virus is made from viral genes picks this. The lipid is correctly identified, but a virus has no metabolism and no enzymes for lipid synthesis; the envelope is a section of host cell membrane taken as the virus buds out of the cell.
2. Its capsid is itself a phospholipid membrane, so the detergent dissolves the coat around its nucleic acid. — A student who thinks the capsid is a lipid membrane picks this. The capsid of every virus, including this one, is a protein shell. What the detergent dissolves is a separate lipid envelope outside the capsid, present only in enveloped viruses; lambda has no lipid layer, so it is unaffected.
3. It is a tiny cell whose plasma membrane, enclosing its cytoplasm, is destroyed by the detergent. — A student who thinks a virus is a small cell picks this. A virus has no cytoplasm and no membrane of its own; the lipid the detergent attacks is borrowed host membrane wrapped around a protein capsid.
4. It is enveloped in host cell membrane, a lipid bilayer lying outside its protein capsid. — Sensitivity to lipid-dissolving detergents shows the particle depends on a lipid bilayer. Viruses do not make lipid; the envelope is host cell membrane acquired as the virus leaves the cell, carrying viral glycoproteins that bind the next host. Lambda is not enveloped, so its protein capsid is its outermost layer and detergents do not inactivate it.

Syllabus statement A2.3.2 · Read this in Learn

1. 3 × 10² — A student who adds 100 phages for each of the three cycles picks this. Every phage released infects a new cell, so the number is multiplied by 100 in each cycle, not increased by 100.
2. 1 × 10² — A student who pictures the original infected cell as the only source of phages, releasing one batch of 100 at each lysis, picks this. Every released phage infects a fresh cell, so each cycle multiplies the number by 100.
3. 1 × 10⁶ — Lysis at 40 minutes releases 100 phages, which infect 100 cells; lysis at 80 minutes releases 100 × 100 = 10⁴ phages, which infect 10⁴ cells; lysis at 120 minutes releases 10⁴ × 100 = 1 × 10⁶ phages. Each cycle multiplies the number by the burst size.
4. 1 × 10⁴ — A student who stops after two cycles, or who counts the lysis at 120 minutes as the second rather than the third, picks this. Lysis events occur at 40, 80 and 120 minutes, so three rounds of ×100 give 1 × 10⁶.

Syllabus statement A2.3.3 · Read this in Learn

1. The prophage had been releasing phages all along, and the ultraviolet light merely burst the cells. — A student who thinks lysogeny involves slow phage production picks this. A prophage produces no phages; the culture had been growing normally with none released, and the phages appeared only after induction.
2. Damage to the host DNA induced the prophage to be excised from the chromosome and enter the lytic cycle. — Ultraviolet light damages DNA, and the host's damage response leads to destruction of the phage repressor. The prophage is excised, the lytic genes are expressed, new phages are assembled and the cells lyse. This is induction, the switch from the lysogenic to the lytic cycle.
3. A prophage cannot cause lysis, so the culture must have been contaminated by a separate lytic phage. — A student who treats lytic and lysogenic as two fixed kinds of phage picks this. Lambda is a temperate phage: the same prophage that was dormant can be induced into the lytic cycle, so no second phage is needed.
4. The ultraviolet light made the prophage grow and divide repeatedly until the cells burst. — A student who thinks viruses multiply by growth and division picks this. The prophage does not grow; after induction the phage DNA is replicated and phage proteins are made by the host, and complete phages are assembled before lysis.

Syllabus statement A2.3.4 · Read this in Learn

1. It is DNA or RNA, and either type may be single-stranded or double-stranded. — Viral genomes are highly diverse: double-stranded DNA in bacteriophage lambda, single-stranded RNA in coronaviruses and HIV, and single-stranded DNA or double-stranded RNA in other groups. Each virus has one type of nucleic acid.
2. It is DNA in every virus, with RNA also present, as in a living cell. — A student who assumes the cellular pattern of DNA genome plus RNA copies applies to viruses picks this. Many viruses have only RNA, and no virus has both DNA and RNA as its genetic material.
3. It is RNA in every virus, and is copied into DNA by reverse transcriptase. — A student who generalises from HIV picks this. Only retroviruses use reverse transcriptase; coronaviruses copy RNA directly to RNA, and lambda and many other viruses have DNA genomes.
4. It is double-stranded DNA in every virus, as it is in bacteriophage lambda. — A student who takes the phage as the model for all viruses picks this. Lambda does have double-stranded DNA, but coronaviruses and HIV have single-stranded RNA genomes.

Syllabus statement A2.3.2 · Read this in Learn

1. The phage's own enzymes and ribosomes work only once they are activated inside a cell's cytoplasm. — A student who thinks the virus carries its own synthetic machinery picks this. Lambda has no ribosomes and very few enzymes to activate; the machinery that makes phage proteins belongs to the host.
2. The phage cannot absorb dissolved nutrients from the broth, but it can feed on the contents of a cell. — A student who treats the phage as a tiny cell that feeds picks this. A virus does not absorb or metabolize nutrients at all; the host cell's own metabolism does the work using the phage's genetic instructions.
3. The phage has to attach to a cell wall before it can grow large enough to divide in two. — A student who thinks viruses reproduce by growth and division picks this. Phages have a fixed size and do not divide; new phages are assembled from proteins and DNA made inside the host cell.
4. The phage lacks ribosomes and most enzymes, so it needs a cell's protein synthesis machinery. — Raw materials and ATP are not enough. A virus has no ribosomes, transfer RNA or RNA polymerase and few or no enzymes, so its DNA can only be transcribed, translated and replicated inside a cell that supplies these. This is the obligate parasitism the IB describes.

Syllabus statement A2.3.3 · Read this in Learn

1. Viruses all descend from one ancestral virus, just as all cells descend from a single common ancestor. — A student who assumes every group must have a single root picks this. The shared code links viruses to cells, not to each other, and the diversity of genome types points away from a single ancestral virus.
2. Viruses evolved before cells existed, and cells later inherited the genetic code from them. — A student who treats the simplest replicator as the oldest picks this. Viruses are obligate parasites with no metabolism, so they could not have existed before the cells they depend on; the evidence is for origins of viruses from other organisms.
3. Viruses with RNA genomes must convert them into double-stranded DNA before the code can be used. — A student who generalises HIV's reverse transcription to all RNA viruses picks this. Coronavirus RNA is translated directly by host ribosomes using the same code; no conversion to DNA is needed, and this says nothing about origins.
4. Viruses are derived from cellular organisms, and probably arose on several separate occasions. — The shared genetic code indicates that viruses are derived from living organisms rather than having an independent origin. The diversity of their genomes and structures is hard to reconcile with a single ancestral virus, so the IB describes the evidence as suggesting several possible origins.

Syllabus statement A2.3.5 · Read this in Learn

1. Memory cells against influenza die within a year, so a new vaccine is needed to replace them. — A student who thinks the jab simply 'wears off' picks this. Memory cells persist for years; the problem is that the virus has changed, which is why the vaccine must be reformulated rather than merely repeated.
2. Point mutations change influenza's surface antigens each year, so earlier antibodies no longer bind to them. — Antigenic drift, the accumulation of point mutations in the haemagglutinin and neuraminidase genes, alters the antigens of circulating strains. Antibodies and memory cells raised against last year's strains fail to recognise the new ones, so the vaccine is changed to match. Measles virus antigens have barely changed.
3. Reassortment of the genome segments produces a completely new influenza subtype every year. — A student who confuses antigenic shift with drift picks this. Reassortment between strains in a co-infected cell is rare and produces pandemic subtypes occasionally; the annual changes are due to point mutations.
4. The influenza virus mutates in response to the vaccine so that it can escape the antibodies produced. — A student who thinks the virus changes on purpose picks this. Mutations arise at random during error-prone replication; vaccination only selects, from the variants already present, those that antibodies fail to bind.

Syllabus statement A2.3.6 · Read this in Learn

1. One drug would eliminate the virus if it were taken for long enough; three are used only to shorten the course of treatment. — A student who models antiviral treatment on a course of antibiotics picks this. A single drug fails because resistant variants survive and multiply; the combination is needed to prevent resistance, not to save time.
2. HIV carries its own enzymes for every step of its replication, so all of them have to be blocked at the same time. — A student who thinks the virus is self-sufficient in enzymes picks this. HIV carries only a few enzymes and uses host machinery for most steps; blocking any one viral enzyme stops replication of sensitive virions. The combination is about resistance, not completeness.
3. Resistance to one drug arises readily in the rapidly mutating population, but one virion is unlikely to resist all three. — HIV's reverse transcriptase is error-prone and the virus population in a patient is enormous, so virions resistant to any single drug appear and are selected. Simultaneous resistance to three drugs with different targets would need several independent mutations in one virion, which is far less likely, so the combination keeps the virus suppressed.
4. Each drug makes HIV mutate, and three drugs together cause so many mutations that the virus is destroyed. — A student who thinks drugs cause the mutations picks this. Mutations arise at random during replication whether or not a drug is present; the drugs select among variants, and the combination works by making a fully resistant variant improbable.

Syllabus statement A2.3.6 · Read this in Learn

1. Gradual accumulation of point mutations in the haemagglutinin gene of one strain. — A student who runs antigenic drift and shift together picks this. Point mutations alter existing antigens gradually; they cannot bring whole genome segments from swine and avian viruses into a human strain.
2. Reverse transcription of the viral RNA followed by insertion of host genes. — A student who generalises HIV's mechanism to all RNA viruses picks this. Influenza has no reverse transcriptase and no DNA stage; its new segments came from other influenza strains, not from host genes.
3. Mutation of the virus in response to widespread vaccination against the earlier strains. — A student who thinks viruses change purposefully picks this. Vaccination does not cause genetic change, and no mutation can copy segments from other viruses; the strain arose by reassortment in a co-infected cell.
4. Reassortment of RNA genome segments in a cell infected by two different strains. — The influenza A genome is in eight separate RNA segments. When two strains infect the same cell, new virions can be assembled with segments from both, producing antigenic shift: a novel subtype to which few people have immunity, which is why it spread as a pandemic.

Syllabus statement A2.3.6 · Read this in Learn