Cell Biology, Microbes, Viruses & Replication: Core Concepts

[CELL] What is the defining difference between prokaryotic and eukaryotic cells?

Eukaryotes have a membrane-bound nucleus and membrane-bound organelles; prokaryotes do not. Prokaryotic DNA sits in a nucleoid region, and prokaryotes generally divide by binary fission. Eukaryotes have multiple linear chromosomes in a nucleus and use mitosis; meiosis occurs in relevant life cycles.

[CELL] Compare prokaryotic and eukaryotic cells across DNA, ribosomes, organelles, size, and examples.

Prokaryotes: usually smaller; no membrane-bound nucleus or organelles; usually one main circular chromosome, sometimes plasmids; 70S-type cytoplasmic ribosomes; bacteria and archaea. Eukaryotes: generally larger; nucleus and organelles; multiple linear nuclear chromosomes; 80S-type cytoplasmic/rough-ER ribosomes, while mitochondria and chloroplasts retain bacterial-like ribosomes; fungi, protozoa, algae, plants, animals.

[CELL] Which structures or materials are shared by all cells, and why?

A plasma membrane controls exchange with the environment; DNA stores inherited instructions; ribosomes make proteins; cytoplasm/cytosol provides the internal environment for many reactions. These are fundamental to maintaining a cell, carrying genetic information, and producing proteins. Viruses are not cells and lack ribosomes.

[CELL] How are the plasma membrane and cell wall different?

The plasma membrane is a selectively permeable boundary found in all cells. A cell wall is a more rigid outer support layer found in some organisms, outside the membrane. Walls add shape and protection and help resist osmotic stress; a membrane does not imply a wall. Animal cells and most protozoa lack a rigid wall, while fungi have one.

[CELL] What are the roles of the nucleus, nucleolus, and ribosomes?

Nucleus: stores most eukaryotic DNA and regulates gene expression. Nucleolus: produces rRNA and assembles ribosome subunits. Ribosomes: translate mRNA into proteins; all cells need them, and viruses must use host ribosomes.

[CELL] Compare rough ER, smooth ER, and Golgi apparatus by function.

Rough ER has ribosomes and makes/processes many secreted and membrane proteins. Smooth ER lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage in some cells. Golgi modifies, sorts, and packages proteins and lipids for delivery to their destinations.

[CELL] What do mitochondria and chloroplasts do, and which organisms typically have them?

Mitochondria produce much of a eukaryotic cell's ATP through aerobic respiration and are widespread among eukaryotes. Chloroplasts capture light energy for photosynthesis and occur in algae and plants, not fungi or protozoa.

[CELL] Compare lysosomes, peroxisomes, and vacuoles.

Lysosomes digest ingested material and recycle cellular waste. Peroxisomes break down fatty acids and handle reactive oxygen compounds. Vacuoles store material and help regulate internal conditions; contractile vacuoles pump excess water out of many freshwater protists.

[CELL] What are the functions of the cytoskeleton, cilia, and flagella?

The cytoskeleton supports shape, movement, and intracellular transport. Cilia are usually short and numerous, moving a cell or fluid across a surface. Eukaryotic flagella are generally longer, whip-like projections used for movement. Bacterial and eukaryotic flagella differ structurally.

[CELL] Why might an organism benefit from having a cell wall, and what is the trade-off of lacking one?

A wall provides shape, support, and protection and helps resist osmotic stress; differences in pathogen wall chemistry can be useful drug targets. Lacking a rigid wall allows flexibility, shape changes, engulfment of food, and some forms of movement, but the cell must still handle mechanical stress and water balance.

[ENDOSYMBIOSIS] Explain endosymbiotic theory in sequence.

An ancestral host cell maintained a bacterium inside it. An oxygen-using bacterial ancestor related to alphaproteobacteria supplied energy-yielding respiration and benefited from a protected, nutrient-rich environment. The relationship became obligatory; genes were lost or transferred to the host nucleus, leaving mitochondria. Later, in some photosynthetic lineages, a cyanobacterium-like ancestor became a chloroplast.

[ENDOSYMBIOSIS] What evidence supports the idea that mitochondria and chloroplasts evolved from bacteria?

They have their own small circular DNA; bacterial-like ribosomes; divide by a fission-like process; have double membranes; have bacterial-like size and biochemistry; and their DNA sequences group with bacterial lineages. Mitochondrial genes group with alphaproteobacteria; chloroplast genes group with cyanobacteria.

[ENDOSYMBIOSIS] Why does the standard introductory model place mitochondria before chloroplasts?

Mitochondria or their descendants occur across most eukaryotic lineages, while chloroplasts occur only in photosynthetic groups. This distribution fits an earlier mitochondrion-associated event and a later chloroplast acquisition in a subset of lineages. Do not claim that the whole eukaryotic cell is simply a bacterium.

[ENDOSYMBIOSIS] A newly discovered organelle has circular DNA, bacterial-like ribosomes, two membranes, and divides by fission. What does this suggest and why?

It supports endosymbiotic theory: the traits resemble those of a once-independent bacterial ancestor that became an organelle. Each trait is evidence; the strongest answer connects the combined bacterial-like features to an evolutionary origin.

[MICROBES] Compare fungi, protozoa, and algae by nutrition, wall, and typical role.

Fungi are eukaryotic absorptive heterotrophs; their walls commonly contain chitin and glucans and their membranes contain ergosterol. Protozoa are mostly unicellular heterotrophs and generally lack a rigid wall, allowing flexible movement or engulfment. Algae are photosynthetic eukaryotes with chloroplasts; wall/outer-covering chemistry varies, and they support aquatic food webs and oxygen production.

[FUNGI] What traits identify fungi and how do their structures help them survive or cause disease?

Fungi are eukaryotic absorptive heterotrophs; their walls commonly contain chitin and glucans and their membranes contain ergosterol. Walls provide structure/protection; hyphae can extend through food or tissue; spores aid dispersal; yeasts can reproduce by budding. Fungi can cause superficial, mucosal, lung, or systemic disease, especially in susceptible hosts, but also support fermentation, decomposition, and biotechnology.

[FUNGI] Distinguish yeast, mold, hyphae, mycelium, and fungal spores.

Yeast: usually unicellular; many reproduce by budding. Mold: multicellular filamentous growth. Hyphae: thread-like fungal filaments. Mycelium: network or mass of hyphae that forms much of a mold's visible body. Fungal spores: reproductive/dispersal structures in many fungi; they are not the same as bacterial endospores or virus particles.

[FUNGI] What is a dimorphic fungus?

A dimorphic fungus can grow in different forms under different conditions; classic examples are mold-like in the environment and yeast-like in host tissues. Do not assume every fungus is dimorphic.

[FUNGI] Why do chitin/glucan and ergosterol matter in identifying fungi?

A chitin- and glucan-containing cell wall plus an ergosterol-containing membrane is a strong fungal clue. These structures distinguish fungi from animal cells and provide selective targets because human cells do not have the same fungal wall.

[FUNGI] What are the high-yield disease associations for Candida, dermatophytes, Aspergillus, Cryptococcus, and Histoplasma?

Candida: thrush, vaginitis, and opportunistic/invasive infection. Dermatophytes: skin, hair, and nail infections such as ringworm/tinea. Aspergillus: allergic or invasive disease, especially in vulnerable patients; spores are inhaled. Cryptococcus: can cause meningitis, especially in immunocompromised people. Histoplasma: inhaled environmental particles can lead to respiratory/systemic disease; it is a classic dimorphic fungus.

[PROTOZOA] What generally characterizes protozoa, and how do these traits help them?

Protozoa are mostly unicellular, heterotrophic eukaryotes and generally lack a rigid cell wall. Flexibility helps some change shape or engulf food; pseudopods, cilia, or flagella enable movement in different groups. Some form resistant cysts that help survival outside a host or transmission. Many are free-living, while some cause intestinal, blood, tissue, or sexually transmitted disease.

[PROTOZOA] Distinguish trophozoite from cyst.

Trophozoite: active feeding/growing form and often the form associated with damage or symptoms. Cyst: more resistant, dormant/transmission form in some protozoa; it can survive harsher conditions and aid spread. Not all protozoa form cysts.

[PROTOZOA] Compare pseudopodia, cilia, and flagella.

Pseudopodia are temporary cell extensions used for amoeboid movement and engulfing food. Cilia are short, numerous, coordinated projections that move the cell or surrounding fluid. Flagella are whip-like projections used for motility in many protists.

[PROTOZOA] What are apicomplexans, and what is a key example?

Apicomplexans are parasitic protozoa with specialized invasion structures; many are not conspicuously motile in adult forms. Plasmodium, which causes malaria, is a key example.

[PROTOZOA] Match Giardia, Entamoeba histolytica, Plasmodium, Toxoplasma gondii, and Trichomonas vaginalis to key disease clues.

Giardia duodenalis: giardiasis with watery/foul diarrhea; contaminated food/water and cysts help transmission. Entamoeba histolytica: amoebic dysentery and intestinal tissue invasion; fecal-oral spread. Plasmodium: malaria, transmitted by Anopheles mosquitoes; infects liver and red blood cells. Toxoplasma gondii: toxoplasmosis; undercooked meat or cat-feces-contaminated material; important in pregnancy and severe immunosuppression. Trichomonas vaginalis: sexually transmitted trichomoniasis.

[ALGAE] Why are algae ecologically important, and how can they also be harmful?

Algae are photosynthetic eukaryotes that convert light energy and carbon dioxide into organic matter, support aquatic food webs, and contribute oxygen. Some blooms produce toxins; decomposition of excess biomass can also lower dissolved oxygen and disrupt water systems. Not every algal bloom is harmless.

[ALGAE] Why is 'cyanobacteria are algae' an incorrect classification?

Algae are eukaryotes with chloroplasts; cyanobacteria are photosynthetic bacteria and therefore prokaryotes. Photosynthesis alone does not make an organism a eukaryotic alga.

[PARASITES] What should you know about parasitic helminths, and what is the course-coverage caveat?

Helminths are multicellular eukaryotic worms. Adults may be visible, while eggs or larvae can be microscopic; specialized life cycles, attachment, feeding, and transmission structures help them persist in hosts. Whether they are emphasized as 'microbes' depends on the course.

[VIRUS] Why are viruses not considered cellular organisms?

Viruses are acellular: they have no cytoplasm or ribosomes and no independent ATP-generating metabolism. Their genome is DNA or RNA inside a protein capsid, sometimes with a lipid envelope. They cannot make proteins alone and must use suitable host-cell machinery to produce new particles.

[VIRUS] What is a virion?

A virion is a complete infectious virus particle outside a host cell. It contains a viral nucleic-acid genome and capsid, plus a lipid envelope and viral proteins if it is enveloped; it is not a cell and may be metabolically inactive outside a host.

[VIRUS] What are the major parts of a virus and their functions?

Genome: DNA or RNA instructions. Capsid: protein shell that protects the genome. Capsomeres: protein subunits that build some capsids. Envelope: host-derived lipid membrane with viral glycoproteins in enveloped viruses. Spikes/surface proteins: bind host receptors and help determine tropism. Some viruses carry enzymes such as reverse transcriptase or RNA-dependent RNA polymerase.

[VIRUS] What is capsid symmetry, and how do you recognize the three main patterns?

Helical: protein subunits spiral around the genome, forming a rod- or thread-like nucleocapsid. Icosahedral: geometric shell with 20 triangular faces, often described as soccer-ball-like. Complex: does not fit either simple pattern; many bacteriophages have a head and tail, and poxviruses are complex.

[VIRUS] What separate questions are answered by viral host type, genome chemistry, strandedness, RNA sense, segmentation, capsid symmetry, and envelope status?

Host type identifies the target organism/cell (e.g., bacterial, animal, plant). Genome chemistry asks DNA or RNA. Strandedness asks single- or double-stranded. RNA sense asks whether RNA can act directly as mRNA. Segmentation asks whether the genome is split into separate segments. Capsid symmetry describes protein arrangement. Envelope status asks whether a host-derived lipid membrane surrounds the nucleocapsid. Do not confuse these classification dimensions.

[VIRUS] Compare enveloped and nonenveloped viruses, including entry, release, and environmental stability.

Enveloped viruses have a lipid membrane with viral proteins; they may enter by membrane fusion or endocytosis and often acquire the envelope while budding through a host/internal membrane. Their lipid envelope is commonly disrupted by detergents, solvents, drying, or harsh conditions. Nonenveloped viruses have capsid as their outer structure, often enter by endocytosis or penetration, and are often more environmentally stable; many are released by lysis. These are tendencies, not universal rules.

[VIRUS] What does positive-sense versus negative-sense single-stranded RNA mean?

+ssRNA can function directly as mRNA after uncoating, so host ribosomes can translate it. -ssRNA is complementary to mRNA and cannot normally be translated directly; it must first be copied into +RNA by a viral RNA-dependent RNA polymerase. Negative-sense RNA viruses need that polymerase.

[VIRUS] Why do viral genomes with RNA need special replication logic?

Host ribosomes translate mRNA, but they do not automatically convert every viral genome into mRNA. +ssRNA can serve directly as mRNA; -ssRNA must be copied into +RNA; dsRNA also requires viral RNA-dependent RNA polymerase to make mRNA. Retroviruses copy RNA into DNA using reverse transcriptase. Viral genome type determines the route to protein production.

[VIRUS] What does a segmented viral genome mean, and why is influenza a useful example?

A segmented genome is split among separate nucleic-acid molecules rather than one continuous molecule. Influenza is an enveloped virus with segmented, negative-sense ssRNA and a helical nucleocapsid; its -RNA must be copied into +RNA before translation, and segmentation is a defining classification feature.

[VIRUS] Identify influenza, adenovirus, HIV, coronavirus, poliovirus, and bacteriophage T4 from the guide's genome/structure patterns.

Influenza: enveloped, segmented -ssRNA, helical nucleocapsid. Adenovirus: nonenveloped dsDNA, icosahedral capsid. HIV: enveloped retrovirus with two copies of +ssRNA and reverse transcriptase; DNA can integrate into host DNA. Coronavirus: enveloped +ssRNA, helical nucleocapsid. Poliovirus: nonenveloped +ssRNA, icosahedral capsid. Bacteriophage T4: bacterial virus with complex head-tail structure and dsDNA.

[VIRUS] How do viruses differ from cellular organisms in reproduction?

A cell reproduces by cellular division, whereas viruses assemble new particles from genome and protein components inside a suitable host. Viruses cannot translate proteins independently because they lack ribosomes, and they lack independent energy metabolism for making new components. A virus is therefore an obligate intracellular replicator.

[REPLICATION] What is the standard six-stage viral replication sequence, in order?

Attachment/adsorption -> penetration/entry -> uncoating -> biosynthesis -> assembly/maturation -> release. Attachment finds the right cell; penetration gets the virus or genome in; uncoating exposes the genome; biosynthesis copies genomes and makes viral proteins; assembly packages components; release lets progeny leave.

[REPLICATION] What happens during attachment, and what happens if the host lacks the matching receptor?

A viral surface protein binds a compatible receptor or surface factor on the host cell. Receptor compatibility helps determine host range/tropism. Without the compatible receptor, successful attachment is impaired and infection may fail before entry.

[REPLICATION] How does penetration differ between bacteriophages and many animal viruses?

Many bacteriophages attach to bacteria and inject their nucleic acid while the capsid remains outside. Animal viruses commonly enter as whole particles through membrane fusion or endocytosis and then uncoat; exact mechanisms depend on the virus.

[REPLICATION] What is uncoating, and how is it different from penetration?

Penetration is entry of the virus or its genome into the host. Uncoating is removal/disassembly of the capsid enough to expose or release the genome so replication or transcription can proceed. They are separate stages even if a specific virus's steps overlap.

[REPLICATION] What happens during biosynthesis?

The viral genome is copied and viral proteins are produced. The exact route depends on genome type: viral DNA/RNA must be processed into a form usable for gene expression, and viral proteins are translated by host ribosomes. Some viruses must supply or encode enzymes the host does not normally provide.

[REPLICATION] What happens during assembly/maturation?

New genomes are packaged into capsids and structural components are assembled into mature or maturing particles. Depending on the virus, assembly may occur in the nucleus, cytoplasm, or at membranes.

[REPLICATION] Compare lysis, budding, vesicle/exocytosis, and cell-to-cell spread as release/spread routes.

Lysis ruptures the host cell and releases particles, killing it. Budding lets an enveloped virus pass through a host membrane and acquire an envelope, often without immediate cell death. Some viruses exit in membrane-bound vesicles by exocytosis. Cell-to-cell spread moves into neighbors without relying only on free particles and can help evade extracellular antibodies.

[REPLICATION] Where do viruses replicate: nucleus or cytoplasm? Give the general rule and important cautions.

There is no universal location. Many DNA viruses use the nucleus for genome replication/transcription, while many RNA viruses replicate in the cytoplasm. Poxviruses are a classic DNA-virus exception that replicate in the cytoplasm; influenza has important nuclear steps. Translation uses host ribosomes in the cytoplasm or on rough ER, and assembly/release may occur elsewhere.

[REPLICATION] How is dsDNA viral information generally used to make proteins?

DNA is transcribed to mRNA, and host ribosomes translate mRNA into protein; DNA replication makes new genomes. Animal DNA viruses often use the nucleus, but poxvirus is a classic cytoplasmic exception. The exact route depends on the virus.

[REPLICATION] How does a +ssRNA virus make proteins and more genome?

After uncoating, its genome can act directly as mRNA and be translated by host ribosomes. Replication often uses a complementary -RNA intermediate as a template to make more +RNA genomes.

[REPLICATION] Why must a -ssRNA virus carry or encode RNA-dependent RNA polymerase?

Negative-sense RNA is not directly readable as mRNA by host ribosomes. A viral RNA-dependent RNA polymerase must first make complementary +RNA, which can be translated and used in replication. Host cells do not normally provide the necessary activity for an RNA genome to copy itself into mRNA.

[REPLICATION] How do dsRNA, retroviral RNA, and ssDNA genomes reach a form usable for protein production?

dsRNA viruses use viral RNA-dependent RNA polymerase to make mRNA. Retroviruses use reverse transcriptase to copy RNA into DNA; viral DNA can integrate into host DNA, and host machinery then transcribes viral RNA. ssDNA viruses must first produce a dsDNA or other transcription-compatible intermediate before standard mRNA production.

[HOSTS] What determines viral host range/tropism and whether a cell can support replication?

Important factors include compatible receptors; barriers such as bacterial cell walls or animal-cell membranes; availability of host ribosomes and other machinery; enzymes, metabolism, temperature, innate defenses, and cell type; and whether the cell's nucleus, cytoplasm, or membranes support the virus's specific life cycle. A matching receptor is important but is not the only requirement.

[PHAGE] What is a bacteriophage, and how does it usually enter a bacterial cell?

A bacteriophage (phage) is a virus that infects bacteria. Many phages bind a bacterial surface receptor and inject their nucleic acid through/into the bacterial envelope while the capsid remains outside. Phages require compatible receptors and host machinery; lytic phage growth commonly ends with bacterial lysis.

[PHAGE] Compare the lytic and lysogenic cycles of bacteriophages.

Lytic: phage genes direct synthesis of many progeny particles, which are assembled and commonly released when the bacterium lyses. Lysogenic: a temperate phage genome integrates into bacterial DNA as a prophage or persists in a stable state; the host survives and copies the prophage as it divides, with no immediate burst of new phages.

[PHAGE] What are virulent and temperate phages, and what is induction?

In the classic simplified model, a virulent phage follows the lytic cycle, while a temperate phage can enter lysogeny. Induction is a change that moves a stable prophage/lysogenic state into active lytic replication, leading to production and release of new phages.

[PHAGE] A phage genome is found in a bacterial chromosome while the bacterium continues dividing. What is this called and what happens next?

This is lysogeny; the integrated phage genome is a prophage. It is copied with the bacterial chromosome as the host divides without immediate lysis or a burst of phage particles. If induced, it can switch into lytic replication.

[VIRAL EFFECTS] What are cytopathic effects (CPE), and what are examples beyond simply killing a cell?

CPE are visible or functional cell changes caused by viral infection, including rounding, swelling, detachment, altered shape, syncytium formation, inclusion bodies, shutdown of host protein synthesis, altered gene expression/metabolism, persistent infection, latency, cell transformation, and immune-mediated tissue injury. Infection can damage a cell or alter its function without immediately killing it.

[VIRAL EFFECTS] What is a syncytium, and what causes it?

A syncytium is one large multinucleated cell formed when several cells fuse. In some infections, viral fusion proteins displayed on infected-cell surfaces cause neighboring cells to fuse.

[VIRAL EFFECTS] What are inclusion bodies and host-cell shutdown?

Inclusion bodies are distinct accumulations of viral components or altered cell material in the nucleus or cytoplasm. Host-cell shutdown occurs when a virus suppresses host protein synthesis or alters gene expression to redirect resources toward viral production.

[VIRAL EFFECTS] Distinguish persistent/chronic infection from latency.

Persistent/chronic infection: virus remains and may be produced over a long period rather than immediately killing the cell. Latency: viral genome persists with limited gene expression or production and can later reactivate. Do not assume all viruses use latency.

[VIRAL EFFECTS] How can viruses cause cancer or immune-mediated injury without directly destroying every infected cell?

Some viruses change cell-cycle regulation or signaling, increasing risk of uncontrolled growth in particular infections. Separately, immune responses against infected cells can cause symptoms and tissue damage; injury is not always solely due to direct viral destruction.

[VIRUS BENEFITS] What beneficial roles can viruses have?

Bacteriophages can kill bacteria and are being explored/used in selected phage-therapy settings; phages can transfer genes by transduction; viral vectors deliver genetic material in research and some gene-therapy/vaccine approaches; viruses shape microbial populations and ecosystem nutrient cycles; viral proteins and replication systems are tools for studying genes, immunity, and cell biology.

[VIRUS BENEFITS] What is transduction, and why can it be beneficial or harmful?

Transduction is transfer of bacterial genes between bacteria by bacteriophages. It contributes to bacterial evolution and can spread useful or harmful traits, including virulence or antimicrobial resistance. The mechanism can be scientifically useful while also contributing to pathogen evolution.

[VIRUS BENEFITS] Does phage therapy universally replace antibiotics?

No. Bacteriophages can kill bacteria and are explored or used in selected phage-therapy settings, but they are not a universal replacement for antibiotics. The correct answer includes both the potential benefit and the limitation.

[INFECTIOUS PARTICLES] Distinguish a virus, viroid, prion, and satellite/virusoids by infectious material and dependence.

Virus: DNA or RNA genome protected by a protein capsid; some also have an envelope; reproduces in a suitable host cell. Viroid: small infectious RNA, classically circular ssRNA without a capsid and associated with plant disease. Prion: infectious misfolded protein with no DNA or RNA genome. Satellite/virusoids: agents that depend on a helper virus for some functions.

[INFECTIOUS PARTICLES] What is a prion, how does it propagate, and what disease examples are associated with it?

A prion is an abnormally folded infectious protein with no DNA or RNA genome. It propagates by inducing normal versions of the protein to misfold and is unusually resistant to many conventional decontamination methods. Associated diseases include Creutzfeldt-Jakob disease and bovine spongiform encephalopathy.

[INFECTIOUS PARTICLES] What is a viroid, and how does it differ from a virus?

A viroid is a small infectious RNA, typically circular single-stranded RNA, without a protein capsid and without protein-coding genes in the classic model; it is primarily associated with plant diseases. A virus has a DNA or RNA genome protected by a capsid and relies on host cells to make new particles.

[INFECTIOUS PARTICLES] What does it mean for a satellite/virusoid to depend on a helper virus?

The agent cannot complete the relevant replication cycle alone and relies on a helper virus for some functions. This is a more specialized distinction and should be prioritized if it appeared in the lecture material.

[INFECTIOUS PARTICLES] Distinguish fungal spores, bacterial endospores, and virions.

Fungal spores are generally reproductive/dispersal structures. Bacterial endospores are dormant survival structures made by certain bacteria. Virions are complete infectious virus particles containing a viral genome and capsid, sometimes plus an envelope. Do not treat these as interchangeable just because all are associated with survival or spread.

[APPLICATION] A cell has no rigid wall and can alter its shape to engulf particles. Which eukaryotic microbial group does this suggest, and why?

Protozoa are the best fit: many are flexible, lack a rigid wall, and use pseudopodia or other movement structures to engulf food or move. A rigid fungal wall would constrain this kind of shape change.

[APPLICATION] An organism has a chitin/glucan wall, ergosterol in its membrane, and grows as budding single cells or filamentous networks. Identify it and interpret the structures.

It is a fungus. Budding single-celled forms are yeasts; filamentous molds grow as hyphae, and a network of hyphae is a mycelium. The wall provides support/protection, and spores can aid dispersal.

[APPLICATION] A photosynthetic microscopic organism is eukaryotic and has chloroplasts. What group fits, and what classification trap should you avoid?

An alga fits. Do not classify cyanobacteria as eukaryotic algae: cyanobacteria are photosynthetic bacteria, so they are prokaryotes.

[APPLICATION] A patient acquired a protozoan after consuming contaminated water; the organism has a resistant cyst stage. What does the cyst explain?

The cyst can survive outside the host and aid transmission through contaminated water or food. The active trophozoite is the feeding/growing form and may be responsible for symptoms, depending on the organism.

[APPLICATION] An RNA virus can immediately be translated by host ribosomes after uncoating. What genome sense does it have, and what about a negative-sense virus?

It has +ssRNA, which can function as mRNA. A -ssRNA virus must first use viral RNA-dependent RNA polymerase to make +RNA before translation can proceed.

[APPLICATION] A virus has RNA that is reverse-transcribed into DNA, and that DNA integrates into the host genome. What class of virus is suggested?

A retrovirus, with HIV as a classic example. Reverse transcriptase copies RNA into DNA, and viral DNA can integrate into host DNA; host machinery then transcribes viral RNA.

[APPLICATION] A virus binds a host receptor but cannot enter the cell. Which replication stages are complete or failing?

Attachment has occurred because receptor binding happened; penetration/entry is the failing next stage. Remember the order: attachment -> penetration -> uncoating -> biosynthesis -> assembly/maturation -> release.

[APPLICATION] A bacterium bursts and releases many new phage particles. Which cycle is occurring, and what is the host outcome?

The lytic cycle. The phage directs progeny production and the bacterium is commonly lysed to release new particles; the host cell dies.

[APPLICATION] Several infected cells fuse into one multinucleated cell. Name the effect and mechanism.

Syncytium formation, a cytopathic effect. Viral fusion proteins at infected-cell surfaces can cause neighboring cells to fuse.

[APPLICATION] An infectious agent is a misfolded protein and has no nucleic acid. Identify it and explain its unusual propagation.

A prion. It induces normal versions of the protein to misfold; it has no DNA or RNA genome and can resist many conventional decontamination methods.

[APPLICATION] A plant pathogen consists of small circular RNA without a capsid. Identify it and distinguish it from a prion.

A viroid. Viroids are small infectious RNAs, classically plant pathogens; prions are infectious misfolded proteins with no nucleic-acid genome.

[APPLICATION] A virus loses infectivity after damage to its lipid envelope. What property does this illustrate?

Enveloped viruses depend on a host-derived lipid membrane containing viral proteins for entry and infection. Their envelopes are commonly disrupted by detergents, solvents, drying, or harsh conditions, although environmental stability varies by virus.

[APPLICATION] What can be inferred if an organelle is present in algae/plants but absent from fungi and protozoa?

If it is the chloroplast, it enables photosynthesis and captures light energy. Fungi and protozoa do not use chloroplast photosynthesis as their main energy strategy; mitochondria, by contrast, are broadly distributed among eukaryotes.

[EXAM STRATEGY] How should you answer a 'why is this structure useful?' question for microbiology?

Give the structure -> function -> advantage -> consequence chain. State what the structure does, how that helps the organism survive/reproduce/move/feed/transmit, and how its presence or absence can affect host interaction or disease. Include trade-offs rather than listing a definition only.

[EXAM STRATEGY] How should you answer a compare-and-contrast question about two microbes or viruses?

State one meaningful similarity, then 2-3 discriminating differences tied to the question: cell type, genome, wall/envelope, nutrition, movement, replication, host, or disease effect. Avoid unrelated fact lists and do not treat tendencies as universal laws.

[EXAM STRATEGY] What absolute-word traps should you watch for in virus questions?

Be skeptical of statements that all viruses replicate in the nucleus or all replicate in the cytoplasm, that all enveloped/nonenveloped viruses follow one entry/release route, that every protozoan forms cysts, or that every alga has the same wall. The guide emphasizes general patterns with exceptions; answer using the named virus and lecture-specific details when provided.