Viruses: Structure & Classification | MDCAT Acellular Life 1.1, NEET & MCAT

MDCAT • ACELLULAR LIFE • TOPIC 1.1

Viruses: structure and classification

A clear route from a virion’s parts to the seven Baltimore groups, with exam reasoning, diagrams, 50 fully explained practice MCQs and a separate 50-question quiz.

MDCAT coreNEET-UG / AIIMSMCAT extension

Jump to Core notes Structure diagrams Classification Baltimore groups Worked reasoning 50 explained MCQs 50-question quiz FAQs

What this lesson covers in each exam

Pathway	Relevant scope	How to study this page
Pakistan MDCAT	PMDC Acellular Life 1.1: classify viruses by structure, strand number, disease and host.	Master the core tables and examples first. HIV/AIDS is the next separate objective (1.2); the HIV genome is introduced here only as a classification example.
India NEET-UG (including AIIMS admissions)	Biological Classification includes viruses and viroids. AIIMS MBBS entry uses NEET-UG.	Prioritize definitions, TMV, viroids, prions, capsid and genome distinctions.
US MCAT	AAMC category 2B includes virus structure and life cycles, retroviruses, prions and viroids.	Use the “MCAT extension” notes and mechanism questions.
UK medical admissions	UCAT tests aptitude, not recall of virus facts; this lesson supports prior biology study or a biology-based graduate-entry pathway where applicable.	Use it for biological literacy, not as a claim that UCAT asks virus classification.

1. What makes a virus acellular?

A virus is a biological entity whose infectious particle, the virion, contains a nucleic-acid genome packaged in a protein capsid. Some virions also have a host-derived lipid envelope. Viruses have no cellular cytoplasm, independent ribosomes or ATP-generating metabolism. They multiply by using a compatible living cell, rather than by cell division. A purified virion can attach to a host but cannot manufacture new virions in nutrient broth by itself.

Do not overlearn a false rule. Viral genomes may be DNA or RNA, single or double stranded. Parvovirus is ssDNA; rotavirus is dsRNA. An individual conventional virus genome is one nucleic-acid type, even though an infected cell may contain both viral RNA and a DNA replication intermediate.

Capsid = protein coat; capsomere = a visible structural unit of the coat; nucleocapsid = genome plus surrounding capsid; envelope = optional outer lipid membrane carrying viral proteins. These names describe different layers, so do not use “envelope” as a synonym for “capsid.”

2. Read the virion before you memorize a list

Figure 1. Original schematic, not to scale. The naked particle has a capsid; the enveloped particle has a capsid plus an outer lipid membrane with attachment spikes.

Why the envelope matters: many enveloped particles are vulnerable to agents that disrupt lipids, including detergents. Naked particles often persist better on surfaces. This is a useful trend, not an absolute claim about every virus. Viral attachment proteins can be embedded in an envelope or displayed from a naked capsid.

Figure 2. Three architectures are compared by capsid organization. TMV is helical; adenovirus is icosahedral; T4-like phage has a complex head-and-tail structure. Shapes are simplified and not to scale.

Helical: protein subunits spiral around the genome (TMV; rabies has a helical nucleocapsid under an envelope). Icosahedral: a roughly spherical polyhedral capsid with 20 ideal triangular faces (adenovirus, poliovirus). Complex: a construction combining structures, such as the head, tail, base plate and tail fibers of T4-like bacteriophages. Envelope status and capsid symmetry are separate classifications: a virus can have an icosahedral capsid and still be enveloped.

3. Five independent ways to classify a virus

1. Genome chemistry: DNA or RNA?
2. Strand number and polarity: ss or ds; if ssRNA, positive-sense or negative-sense? Segmented or continuous?
3. Capsid shape: helical, icosahedral or complex?
4. Envelope: present or absent?
5. Host and disease: bacterial phage, plant or animal virus; the disease it causes is useful clinically but is not a genome class.

Example	Genome / group	Structure and host	Why it is memorable
TMV	+ssRNA / IV	Helical, naked; plants	Classic rodlike plant virus.
Adenovirus	dsDNA / I	Icosahedral, naked; humans	A naked animal DNA-virus example.
T4-like phage	dsDNA / I	Complex head and tail; bacteria	Host and complex morphology.
Poliovirus	+ssRNA / IV	Icosahedral, naked; humans	RNA genome can be translated after entry.
Influenza A	−ssRNA / V	Helical nucleocapsids, enveloped; humans and other animals	Eight segments; reassortment.
Rabies virus	−ssRNA / V	Helical nucleocapsid, enveloped; mammals	Negative sense requires viral polymerase.
HIV	ssRNA-RT / VI	Enveloped; humans	RNA-to-DNA reverse transcription.
Hepatitis B virus	partly dsDNA-RT / VII	Enveloped; humans	DNA virus with an RT step.
Rotavirus	dsRNA / III	Icosahedral, naked; humans and animals	Exception to “all RNA is ssRNA”.
Parvovirus	ssDNA / II	Icosahedral, naked; animals	Exception to “all DNA is dsDNA”.

Disease is a clue, not proof: “hepatitis virus” does not specify one genome: hepatitis A is +RNA, hepatitis B is DNA-RT and hepatitis C is +RNA. Different respiratory viruses also differ. Classify the actual agent, not the symptom alone.

Host range and tropism: attachment to a suitable cellular receptor is necessary for many infections, but entry alone does not guarantee productive replication. The cell must also supply a workable environment. “Bacteriophage” means a virus infecting bacteria; “zoonotic” concerns animal-to-human transmission, not an RNA-versus-DNA genome class.

4. Baltimore classification: seven routes to mRNA

Whatever its genome, a virus must cause the host to translate mRNA into viral proteins. Baltimore groups distinguish how that mRNA is obtained. “Sense” compares RNA sequence orientation with readable mRNA: positive-sense RNA can act as mRNA; negative-sense RNA must first be copied to a positive strand. RT means reverse transcription, an RNA-to-DNA step in the replication route.

Figure 3. The seven genome / replication groups converge on mRNA production. The arrows imply a common requirement, not identical reaction pathways.

Group	Genome	Route to mRNA in brief	Anchor example
I	dsDNA	DNA is transcribed.	Adenovirus
II	ssDNA	Usually make a dsDNA template, then transcribe.	Parvovirus
III	dsRNA	Viral RNA polymerase transcribes mRNA.	Rotavirus
IV	+ssRNA	Genome can function as mRNA after entry.	Poliovirus / TMV
V	−ssRNA	Virion-associated RNA polymerase makes +mRNA.	Influenza / rabies
VI	ssRNA-RT	Reverse transcribe RNA to DNA; DNA is transcribed.	HIV
VII	dsDNA-RT	DNA transcribes RNA; an RNA intermediate is reverse transcribed during replication.	Hepatitis B

Two systems, two purposes. Baltimore groups classify genome expression pathways. Formal virus taxonomy and changing species / family assignments are maintained by ICTV. A Baltimore group is not a taxonomic family.

5. Reason through the exam traps

Case A · negative-sense RNA

A new virion has −ssRNA. Can ribosomes directly read it? No. It needs viral RNA-dependent RNA polymerase to make positive-sense mRNA. Therefore it belongs to group V; expect a polymerase in the infecting particle.

Case B · a DNA virus using RT

“DNA” alone suggests group I or II, but a partly dsDNA genome with an RNA-to-DNA replication step is group VII. HBV illustrates why the replication route must be checked.

Case C · shape versus envelope

“Enveloped” says what surrounds the capsid; “icosahedral” describes capsid symmetry. Both can be true at once. Never treat the two labels as exclusive choices.

Case D · acellular agent

A small infectious plant RNA with no coat is a viroid. Infectious misfolded protein without nucleic acid is a prion. Neither is a standard virion.

MCAT extension: the general life cycle

Attachment → entry → uncoating → genome expression and copying → assembly → release. In a lytic phage cycle, assembled particles are released as the bacterial host lyses. A temperate phage genome maintained within a bacterium is a prophage. A retroviral DNA copy integrated into the host genome is a provirus. These terms are related but not interchangeable. Influenza A has a segmented genome; when different strains coinfect a cell, whole segments can reassort.

Practice provenance. The following are 50 original, past-paper-informed exam-style MCQs with explanations, based on published syllabus areas and recurrent question formats. They are not verbatim MDCAT, NEET, AIIMS, UCAT or AAMC questions, and no unverified examination year is attributed to them. The interactive quiz below has 50 separately worded questions.

6. 50 explained exam-style MCQs

Try the stem first, then open the answer. Each explanation states the reason; deliberate distractors expose common misconceptions.

01 A complete virus particle outside a host cell is a: Show answer

1. prophage
2. virion
3. viroid
4. prion

Answer B: virion. Virion names the complete extracellular particle; a viroid is naked infectious RNA, and a prion is a misfolded protein.

02 The protein shell enclosing a viral genome is the: Show answer

1. ribosome
2. plasma membrane
3. capsid
4. envelope

Answer C: capsid. The capsid protects the genome and consists of virus-encoded protein subunits. An envelope, when present, lies outside it.

03 The building units of a viral capsid are called: Show answer

1. nucleosomes
2. centrioles
3. flagella
4. capsomeres

Answer D: capsomeres. Capsomeres are morphological subunits of the capsid; nucleosomes package cellular chromatin.

04 Which component is absent from a virus particle? Show answer

1. functional ribosomes
2. a genome
3. capsid proteins
4. attachment proteins

Answer A: functional ribosomes. Viruses do not have translation machinery; they rely on host ribosomes. Some virions do carry polymerases.

05 The primary source of a typical viral lipid envelope is: Show answer

1. a ribosome
2. a host-cell membrane
3. the viral capsid
4. a bacterial cell wall

Answer B: a host-cell membrane. Envelopes are acquired from host membranes during egress, with virus-encoded proteins inserted into them.

06 An envelope glycoprotein spike chiefly helps a virus: Show answer

1. translate mRNA
2. form peptidoglycan
3. attach to host receptors
4. make ATP

Answer C: attach to host receptors. Surface proteins mediate attachment and sometimes entry; receptor matching contributes to tropism.

07 Which pair can both describe the SAME virus? Show answer

1. DNA genome and RNA genome
2. host ribosome and viral ribosome
3. viroid and bacteriophage
4. icosahedral capsid and lipid envelope

Answer D: icosahedral capsid and lipid envelope. Capsid geometry and envelope presence are independent axes. A single viral genome is ordinarily DNA or RNA.

08 Tobacco mosaic virus is a classic example of a: Show answer

1. helical RNA virus
2. icosahedral DNA virus
3. complex dsDNA phage
4. protein-only prion

Answer A: helical RNA virus. TMV has a helical capsid built around a positive-sense single-stranded RNA genome.

09 A typical T4-like bacteriophage has: Show answer

1. a cellular nucleus
2. a head, tail and tail fibers
3. an animal-cell envelope
4. no protein coat

Answer B: a head, tail and tail fibers. Its DNA is housed in a head; tail apparatus helps attach to a bacterium and deliver the genome.

10 An icosahedral capsid is characterized by: Show answer

1. a lipid bilayer only
2. a single straight helix
3. 20 triangular faces
4. 12 square faces

Answer C: 20 triangular faces. An ideal icosahedron has 20 triangular faces and 12 vertices; this refers to capsid symmetry.

11 Viral genomes can be: Show answer

1. always double-stranded DNA
2. always single-stranded RNA
3. DNA and RNA together as one ordinary genome
4. DNA or RNA, single or double stranded

Answer D: DNA or RNA, single or double stranded. Genome chemical type and strand number are separate classification properties.

12 Parvoviruses are an important exception because their genome is: Show answer

1. single-stranded DNA
2. double-stranded RNA
3. positive-sense RNA
4. protein only

Answer A: single-stranded DNA. Parvoviruses show that DNA viruses are not necessarily double stranded.

13 Rotavirus belongs to the genome class: Show answer

1. single-stranded negative RNA
2. double-stranded RNA
3. single-stranded DNA
4. double-stranded DNA

Answer B: double-stranded RNA. Rotavirus is a reovirus with a segmented dsRNA genome, contradicting the idea that all RNA viruses are single stranded.

14 Influenza A virus has a genome that is: Show answer

1. positive-sense ssRNA
2. single-stranded DNA
3. segmented negative-sense ssRNA
4. unsegmented dsDNA

Answer C: segmented negative-sense ssRNA. Influenza A has eight negative-sense RNA segments; its polymerase must make readable mRNA.

15 A positive-sense RNA genome can generally function immediately as: Show answer

1. DNA polymerase
2. a ribosome
3. a lipid envelope
4. mRNA for translation

Answer D: mRNA for translation. Positive-sense RNA has the same polarity as translatable mRNA, though replication still needs viral machinery.

16 A negative-sense RNA genome first needs: Show answer

1. viral RNA-dependent RNA polymerase
2. host DNA ligase
3. a viral ribosome
4. reverse transcriptase in every case

Answer A: viral RNA-dependent RNA polymerase. Its RNA is complementary to mRNA; an incoming viral polymerase transcribes it into positive-sense mRNA.

17 Which enzyme synthesizes RNA using RNA as a template? Show answer

1. restriction endonuclease
2. RNA-dependent RNA polymerase
3. reverse transcriptase
4. DNA ligase

Answer B: RNA-dependent RNA polymerase. RNA-dependent RNA polymerase copies viral RNA, including the production of mRNA from negative-sense genomes.

18 Reverse transcriptase makes: Show answer

1. protein from RNA
2. RNA from protein
3. DNA from an RNA template
4. RNA from a DNA template only

Answer C: DNA from an RNA template. Reverse transcription reverses the usual transcription direction; retroviruses use an RNA-to-DNA intermediate.

19 HIV is classified in Baltimore group: Show answer

1. IV (+ssRNA)
2. V (-ssRNA)
3. III (dsRNA)
4. VI (ssRNA-RT)

Answer D: VI (ssRNA-RT). Retroviruses carry positive-sense RNA but reproduce through reverse transcription, which makes group VI distinct from group IV.

20 Hepatitis B virus belongs to Baltimore group: Show answer

1. VII (dsDNA-RT)
2. I (dsDNA)
3. III (dsRNA)
4. VI (ssRNA-RT)

Answer A: VII (dsDNA-RT). Hepatitis B has partially double-stranded DNA and uses an RNA intermediate with reverse transcription.

21 Baltimore classification is organized chiefly by: Show answer

1. particle diameter alone
2. the route by which a virus makes mRNA
3. the illness severity
4. host body size

Answer B: the route by which a virus makes mRNA. The seven groups reflect genome type and how each produces mRNA for translation.

22 A dsDNA virus without reverse transcription is Baltimore group: Show answer

1. III
2. VII
3. I
4. II

Answer C: I. Group I comprises dsDNA viruses that do not replicate via reverse transcription; HBV is a group VII exception.

23 A single-stranded DNA virus is Baltimore group: Show answer

1. I
2. IV
3. VI
4. II

Answer D: II. Group II is ssDNA, which typically first becomes a dsDNA template before transcription.

24 A double-stranded RNA virus is Baltimore group: Show answer

1. III
2. II
3. IV
4. V

Answer A: III. Group III includes dsRNA viruses such as rotavirus; incoming polymerase produces mRNA.

25 An ordinary positive-sense ssRNA virus without an RT step is group: Show answer

1. VI
2. IV
3. III
4. V

Answer B: IV. Group IV genome can serve as mRNA; retroviruses belong to VI because their reproductive pathway uses RT.

26 A negative-sense ssRNA virus is group: Show answer

1. VI
2. VII
3. V
4. IV

Answer C: V. Group V viruses must transcribe their negative RNA into mRNA using an RNA-dependent RNA polymerase.

27 The formal authority for virus taxonomy is: Show answer

1. WHO alone
2. IUCN
3. the three-domain scheme
4. ICTV

Answer D: ICTV. The International Committee on Taxonomy of Viruses sets virus taxonomic classifications; Baltimore groups describe replication strategy.

28 A bacteriophage infects: Show answer

1. bacteria
2. plants exclusively
3. fungi exclusively
4. all cells equally

Answer A: bacteria. Phage is the conventional term for a virus of bacteria; a host range depends on compatible receptors and intracellular support.

29 Host range is strongly influenced by: Show answer

1. water solubility only
2. receptor compatibility and cellular support
3. virus size alone
4. the number of capsomeres only

Answer B: receptor compatibility and cellular support. Attachment receptors govern entry, while the host must also permit replication after entry.

30 Which comparison is best supported? Show answer

1. envelopes are made of peptidoglycan
2. only enveloped viruses have proteins
3. enveloped viruses are often less stable against detergents
4. all non-enveloped viruses are destroyed by soap faster

Answer C: enveloped viruses are often less stable against detergents. Detergents disrupt lipid envelopes; naked capsids often tolerate environmental conditions better, although specific viruses differ.

31 Which agent has infectious RNA but no protein capsid? Show answer

1. virion
2. prion
3. bacteriophage
4. viroid

Answer D: viroid. Viroids are small infectious noncoding RNA molecules chiefly associated with plant disease.

32 Which infectious agent contains no nucleic acid? Show answer

1. prion
2. viroid
3. retrovirus
4. adenovirus

Answer A: prion. Prions are misfolded protein agents; viroids contain RNA and viruses have nucleic acid genomes.

33 Why do antibiotics that block bacterial ribosomes generally fail against viruses? Show answer

1. viral RNA is always double stranded
2. viruses use host ribosomes
3. viruses have peptidoglycan
4. all viruses are larger than bacteria

Answer B: viruses use host ribosomes. Viruses have no own ribosomes or peptidoglycan synthesis to target. Antibiotic choice depends on the disease cause.

34 The first step of a generalized viral infection is: Show answer

1. release of virions
2. host cell lysis
3. attachment to a compatible receptor
4. assembly of daughter capsids

Answer C: attachment to a compatible receptor. Attachment enables subsequent entry, uncoating, synthesis, assembly and release.

35 Uncoating releases the: Show answer

1. newly made virions into air
2. host ribosome from nucleus
3. bacterial cell wall
4. viral genome from its protective coat

Answer D: viral genome from its protective coat. Uncoating exposes the genome so that expression and replication can proceed.

36 A strictly lytic phage cycle ends by: Show answer

1. host-cell rupture and virion release
2. permanent dormant prophage only
3. mitosis of the phage
4. binary fission of virus

Answer A: host-cell rupture and virion release. In lytic growth, particles assemble then host lysis releases progeny phages.

37 A prophage is: Show answer

1. an envelope spike
2. phage DNA maintained in a bacterial host
3. an isolated free phage particle
4. an RNA-only viroid

Answer B: phage DNA maintained in a bacterial host. Temperate phage DNA maintained in the host, often integrated, is termed a prophage.

38 A provirus in the retrovirus context is: Show answer

1. a free capsid
2. a bacterial plasmid only
3. viral DNA integrated into the host genome
4. the free viral RNA particle

Answer C: viral DNA integrated into the host genome. Retroviral reverse-transcribed DNA can integrate as a provirus and be copied with the cell genome.

39 A zoonotic virus is one that: Show answer

1. infects plants only
2. has two capsids
3. must be a DNA virus
4. passes from animals to humans

Answer D: passes from animals to humans. Zoonosis denotes an infection transmitted from an animal to a human; genome chemistry does not define it.

40 The genetic material of adenovirus is: Show answer

1. double-stranded DNA
2. double-stranded RNA
3. negative-sense RNA
4. single-stranded DNA

Answer A: double-stranded DNA. Adenoviruses are non-enveloped icosahedral dsDNA viruses.

41 Poliovirus is best described as: Show answer

1. protein-only
2. non-enveloped positive-sense ssRNA
3. enveloped negative-sense ssRNA
4. enveloped dsDNA

Answer B: non-enveloped positive-sense ssRNA. Poliovirus is a picornavirus: a non-enveloped icosahedral virus with +ssRNA.

42 Rabies virus has a: Show answer

1. dsRNA genome
2. ssDNA genome
3. negative-sense ssRNA genome
4. dsDNA genome

Answer C: negative-sense ssRNA genome. Rabies is an enveloped rhabdovirus whose helical nucleocapsid contains negative-sense RNA.

43 Measles virus is: Show answer

1. a naked dsDNA virus
2. a viroid
3. a ssDNA phage
4. an enveloped negative-sense RNA virus

Answer D: an enveloped negative-sense RNA virus. Measles is a paramyxovirus with a negative-sense RNA genome and envelope.

44 A virus with a segmented genome may generate new combinations by: Show answer

1. reassortment during coinfection
2. binary fission
3. meiosis in a virion
4. translation without a host

Answer A: reassortment during coinfection. Reassortment exchanges whole genome segments when distinct strains infect one cell; influenza A is the classic example.

45 Which feature is shared by naked and enveloped virions? Show answer

1. peptidoglycan walls
2. a genome within a capsid
3. a host-derived lipid envelope
4. their own ribosomes

Answer B: a genome within a capsid. Both virus designs have a genome and a protective protein capsid; only one has an external lipid envelope.

46 The expression “obligate intracellular parasite” means a virus: Show answer

1. contains a nucleus
2. can perform independent metabolism
3. depends on a living cell for reproduction
4. always grows on nutrient agar alone

Answer C: depends on a living cell for reproduction. Viruses reproduce by directing host-cell processes; outside a cell, a virion does not grow or divide.

47 In a phage experiment, a membrane filter removes bacteria but the filtrate remains infectious. What follows? Show answer

1. a complete host cell passed through
2. all bacterial cells became viruses
3. the filtrate necessarily contains prions
4. a smaller infectious agent passed through

Answer D: a smaller infectious agent passed through. Filtration established that tobacco mosaic disease agent was filterable, although filtration alone did not reveal its full nature.

48 Which of these is a classification by HOST rather than genome? Show answer

1. plant virus
2. dsRNA virus
3. ssDNA virus
4. negative-sense RNA virus

Answer A: plant virus. “Plant virus” refers to host range, while the other terms specify genome chemistry or polarity.

49 In a +RNA virus, inhibiting host translation most directly blocks: Show answer

1. preexisting virion genome identity
2. viral protein synthesis
3. initial receptor recognition only
4. the lipid bilayer composition immediately

Answer B: viral protein synthesis. Positive-sense viral RNA can be translated by host ribosomes to produce viral proteins, including replication enzymes.

50 The most defensible statement about viral disease and classification is: Show answer

1. all respiratory viruses are enveloped
2. all human viruses are DNA viruses
3. one disease name alone cannot reveal genome or structure
4. all hepatitis viruses share one genome type

Answer C: one disease name alone cannot reveal genome or structure. Clinical syndrome is not a reliable substitute for genome, capsid and envelope data; distinct viruses may cause similar disease.

7. Interactive quiz · 50 fresh questions

This is a separate, differently worded set. Answer at your own pace; submit to see your score and a question-by-question review. Nothing is sent to a server.

Question 1 of 50

Frequently asked questions

Are viruses living organisms?

They carry genetic information and evolve, but virions are acellular and cannot reproduce or metabolize independently. Their place at the boundary of “living” depends on how life is defined.

Is every RNA virus single stranded?

No. Rotavirus has double-stranded RNA. HIV, influenza and poliovirus each have single-stranded RNA but follow different expression strategies.

Do all viruses have envelopes?

No. Adenovirus, poliovirus, TMV and rotavirus are non-enveloped examples. A capsid is distinct from an optional envelope.

Is a bacteriophage always shaped like a T4 phage?

No. Bacteriophage describes the host, not a required shape. The T4-like head-and-tail particle is only a familiar example.

Is HIV the same as AIDS?

No. HIV is the virus; AIDS is an advanced clinical syndrome caused by untreated HIV infection. Transmission and symptoms belong to the next MDCAT Acellular Life objective.

Does AIIMS have a separate undergraduate biology entrance exam?

AIIMS undergraduate medical admissions use NEET-UG. Consult the current official admissions and syllabus notices for the relevant year.

Sources and further reading

• PMDC official MDCAT syllabus listing — Acellular Life objective 1.1.
• NTA NEET-UG — undergraduate medical entry in India.
• AAMC MCAT content outline — category 2B.
• UCAT official format — aptitude subtests.
• OpenStax Biology 2e, section 21.1 — morphology, genomes and Baltimore scheme.
• International Committee on Taxonomy of Viruses — formal taxonomy.

Related PreachBio biology: Three-domain classification · Transcription and mRNA processing. The latter concerns cellular transcription; compare the shared need to produce mRNA without confusing cellular and viral pathways.