Prokaryotic Cells and Viruses — AQA A-level Biology

SPEC 3.2.1.2 - PROKARYOTES & VIRUSES

Prokaryotic Cell Structure & Viruses

Prokaryote ultrastructure, prokaryote vs eukaryote comparison, virus structure, HIV, and antibiotic mechanisms — with examiner-alert precision

What is a Prokaryotic Cell?

Definition

Prokaryotic cell: A cell that lacks a true nucleus; the DNA is circular, free in the cytoplasm, and not associated with histone proteins. Prokaryotes also lack membrane-bound organelles.

Definition

Murein: A glycoprotein (peptidoglycan) that forms the cell wall of all prokaryotes. It provides structural support and prevents osmotic lysis. It is distinct from cellulose (plants) and chitin (fungi).

Exam Focus

AQA questions on this topic almost always ask you to compare prokaryotes and eukaryotes, or to explain why antibiotics target prokaryotes but not eukaryotic cells. Both require precise structural knowledge.

Prokaryotic Cell — Structure

Note: no nucleus, no membrane-bound organelles — the nucleoid is simply a region of cytoplasm containing the circular DNA

Prokaryote — Key Structural Features

StructurePresent in ALL prokaryotesDetail

Cell-surface membrane✅Phospholipid bilayer; selectively permeable

Cell wall (murein)✅Glycoprotein (peptidoglycan); provides rigidity; distinct from cellulose or chitin

Circular DNA✅Single circular chromosome; free in cytoplasm; NOT associated with histone proteins

70S ribosomes✅Smaller than eukaryotic 80S; target of many antibiotics (e.g. streptomycin)

Plasmids❌ someSmall, circular, double-stranded DNA; separate from main chromosome; often carry antibiotic resistance genes

Capsule❌ someGelatinous polysaccharide layer outside cell wall; provides protection; aids attachment to host cells

Flagellum❌ someProtein filament; rotates to provide motility; powered by proton gradient across membrane

Examiner Alert

Prokaryotic DNA is circular and NOT associated with histone proteins. This is the most commonly dropped mark in comparison questions. Eukaryotic DNA is linear and is associated with histones. Both facts must be stated.

Plasmids

Definition

Plasmid: A small, circular, double-stranded DNA molecule found in prokaryotes, separate from the main circular chromosome. Plasmids often carry genes for antibiotic resistance and can be transferred between bacteria.

Structure

Small and circular

Double-stranded DNA

Separate from main chromosome

Self-replicating

Present in variable copy numbers

Significance

Carry antibiotic resistance genes

Transferred between bacteria (horizontal gene transfer)

Key vectors in genetic engineering

Explain rapid spread of antibiotic resistance

Synoptic Link → Module 8

Plasmids are central to recombinant DNA technology (Lesson 8.05). They act as vectors to introduce foreign genes into bacteria. The AQA spec explicitly names plasmids as vectors — understanding their prokaryotic biology here underpins that topic.

Prokaryote vs Eukaryote — Comparison

FeatureProkaryoteEukaryote

NucleusAbsent — nucleoid region only; no nuclear envelopePresent; bounded by double nuclear envelope with pores

DNASingle circular molecule; NOT associated with histone proteins; free in cytoplasmLinear molecules; associated with histone proteins; enclosed in nucleus

Ribosomes70S (50S + 30S subunits)80S (60S + 40S subunits) in cytoplasm; 70S in mitochondria/chloroplasts

Membrane-bound organellesAbsentPresent (mitochondria, Golgi, ER, etc.)

Cell wallMurein (glycoprotein / peptidoglycan)Plants: cellulose; Fungi: chitin; Animals: absent

Cell size0.5–5 μm (typical)10–100 μm (typical)

PlasmidsPresent (in many)Absent (in most)

CapsulePresent (in some)Absent

Antibiotics — Why They Target Prokaryotes

Antibiotics exploit the structural differences between prokaryotic and eukaryotic cells. They target structures present in bacteria but absent or different in human cells, so they kill bacteria without harming the patient’s own cells.

Antibiotic targetPresent in prokaryotes?Present in eukaryotes?Example antibiotic

70S ribosome✅ Yes❌ No (80S in cytoplasm)Streptomycin, erythromycin

Murein cell wall✅ Yes❌ No (no murein)Penicillin (inhibits murein cross-linking)

Examiner Alert — Antibiotics & Viruses

Antibiotics are completely ineffective against viruses. Viruses lack cell walls, ribosomes, and all other cellular structures that antibiotics target. Never write “antibiotics kill viruses” — this is a zero-mark statement. Viruses use host cell machinery to replicate, so targeting viral replication without harming the host cell is much harder.

Key Exam Point

A 70S-targeting antibiotic would not harm cytoplasmic eukaryotic ribosomes (80S), but could affect mitochondria and chloroplasts (which retain 70S ribosomes). This is a common A-grade synoptic question — revisited in Lesson 2.01.

Virus Structure

Definition

Virus: An acellular, non-living entity consisting of genetic material (DNA or RNA, never both) enclosed in a protein capsid, with attachment proteins on the surface that determine host specificity.

All viruses have:

Genetic material — DNA or RNA (never both); may be single or double stranded

Capsid — protein coat surrounding and protecting the genetic material; made of protein subunits called capsomeres

Attachment proteins — on capsid surface; complementary shape to receptors on host cell surface → determines host specificity

Some viruses also have:

Lipid envelope — derived from host cell membrane; e.g. HIV, influenza

Examiner Alert

Viruses are acellular (not made of cells) and non-living — they cannot carry out any of the seven life processes independently. They are entirely dependent on host cell machinery. Never describe viruses as “reproducing” independently — say they “replicate using host cell machinery.”

HIV — Structure & Replication

AQA Spec Detail

HIV (Human Immunodeficiency Virus) is an RNA retrovirus with a lipid envelope. It specifically infects helper T cells (T-helper lymphocytes) via CD4 surface receptors, progressively destroying the immune system.

HIV componentStructureFunction

Genetic materialTwo identical strands of RNACarries genetic information for viral replication

Reverse transcriptaseEnzyme within capsidConverts viral RNA → DNA inside the host cell (retrovirus mechanism)

CapsidProtein coat (cone-shaped)Protects genetic material and reverse transcriptase

Lipid envelopeDerived from host cell membraneProtects virus; helps evade host immune system

gp120 glycoproteinAttachment protein in envelopeBinds specifically to CD4 receptors on helper T cells → determines host cell specificity

HIV Replication Sequence

1. gp120 binds to CD4 receptor on helper T cell → 2. Fusion of viral envelope with cell membrane → 3. RNA + reverse transcriptase enter cell → 4. Reverse transcriptase makes DNA from viral RNA → 5. Viral DNA integrates into host genome → 6. Host cell transcribes & translates viral genes → 7. New viruses assembled and bud off, destroying helper T cell

HIV & the Immune System

HIV specifically targets helper T cells (CD4+ T lymphocytes) — the cells that coordinate the entire immune response. Progressive destruction of helper T cells leads to AIDS (Acquired Immune Deficiency Syndrome).

Why helper T cells?

gp120 attachment proteins on HIV are complementary to CD4 receptors exclusively found on helper T cells. This precise molecular complementarity determines HIV’s host cell specificity.

Consequence for immunity

Helper T cells activate B cells (antibody production) and cytotoxic T cells (cell-mediated killing). Without helper T cells, neither arm of the specific immune response can be activated. Patients become vulnerable to opportunistic infections.

Synoptic Link → Lesson 2.06/2.07

The role of helper T cells in activating B lymphocytes and cytotoxic T cells is examined in detail in Lessons 2.06–2.07 (Immune System). This slide provides the cell biology foundation for that topic. AQA frequently combines HIV structure with immune response in multi-part questions.

Common Error

Do not say “HIV destroys white blood cells.” Be specific: HIV destroys helper T cells (T-helper lymphocytes / CD4+ T cells). Generic statements about white blood cells score zero on AQA mark schemes.

Binary Fission in Prokaryotes

Definition

Binary fission: The method of asexual reproduction in prokaryotes. The circular DNA and any plasmids are replicated, the cell elongates, and the cytoplasm divides to produce two genetically identical daughter cells.

Binary fission involves:

1

Replication of the circular DNA chromosome

2

Replication of any plasmids

3

Cell elongation — the two DNA copies are pulled to opposite ends

4

Formation of a new cell wall and membrane across the middle

5

Separation into two genetically identical daughter cells

Never write

“Bacteria undergo mitosis” — mitosis requires a nucleus and spindle fibres. Prokaryotes have neither. Binary fission does not involve a spindle; the DNA copies are separated by growth of the cell membrane.

Contrast with Eukaryotes

Eukaryotes reproduce by mitosis (cell division with spindle fibres and condensed chromosomes). Binary fission is simpler, faster, and does not involve chromosomes condensing — there are no linear chromosomes or histones.

Comparison Questions — Exam Technique

AQA “compare” questions require explicit statements of both similarities and differences — describing each separately earns no marks. Every comparison point must name both organisms.

FeatureProkaryoteEukaryoteMark-scheme phrase

NucleusNo nuclear envelope; nucleoid regionDouble nuclear envelope + pores“Prokaryotes lack a membrane-bound nucleus; eukaryotes have a nucleus bounded by a double envelope”

DNACircular; not associated with histonesLinear; associated with histone proteins“Prokaryotic DNA is circular and not histone-associated; eukaryotic DNA is linear and histone-associated”

Ribosomes70S80S (cytoplasm)“Prokaryote ribosomes are 70S; eukaryote cytoplasmic ribosomes are 80S”

OrganellesNone membrane-boundMembrane-bound organelles present“Prokaryotes lack membrane-bound organelles; eukaryotes possess them”

Cell wallMureinPlants: cellulose; Fungi: chitin; Animals: noneName the specific material for both

Scoring Rule

For a 4-mark compare question: make 4 explicit comparison statements. Each one must name the feature AND state what it is like in both organisms. Listing features of one organism only scores zero.

DEEP Answer — Antibiotic Selectivity

“Explain why antibiotics that inhibit 70S ribosome function can be used to treat bacterial infections in humans without harming human cells. [4 marks]”

DEEP Framework Answer

D

DefineBacteria are prokaryotes with 70S ribosomes. Human cells are eukaryotes with 80S ribosomes in the cytoplasm. The two ribosome types differ sufficiently in structure for some antibiotics to bind selectively to 70S. [1]

E

Explain mechanismThe antibiotic (e.g. streptomycin) binds to the 30S subunit of the bacterial 70S ribosome, preventing translation of bacterial mRNA into protein. Without protein synthesis, bacterial cells cannot grow or function, and die. [1]

E

Evidence / precisionHuman cytoplasmic ribosomes are 80S and have a different structure; the antibiotic does not bind to them. Therefore human protein synthesis is unaffected and the drug causes no direct harm to human cells at therapeutic doses. [1]

P

Practical / caveatImportant caveat: human mitochondria and chloroplasts (in plant cells) contain 70S ribosomes. High doses of these antibiotics can inhibit mitochondrial protein synthesis, explaining some antibiotic side effects. [1]

Top Examiner Errors — Lesson 2.02

Error 1 — Forgetting “not associated with histones”

Saying “prokaryotic DNA is circular” alone is incomplete. The full mark-scheme answer requires: “circular DNA that is not associated with histone proteins.” Missing the histone point loses a mark in every comparison question.

Error 2 — “Antibiotics kill viruses”

Viruses have no ribosomes, no murein cell wall, and no cellular machinery. Antibiotics have no target in a virus. This error appears in roughly 30% of student answers on this topic and always scores zero.

Error 3 — “Bacteria reproduce by mitosis”

Prokaryotes reproduce by binary fission, not mitosis. Mitosis requires a nucleus with condensed chromosomes and a spindle apparatus — none of which prokaryotes possess.

Error 4 — HIV “destroys white blood cells”

Must be specific: HIV destroys helper T cells (CD4+ T lymphocytes). “White blood cells” is far too vague and will not be credited on AQA mark schemes.

Practice Question

A bacterium is observed under a transmission electron microscope. The following structures are identified: a cell wall, a cell-surface membrane, ribosomes, a region of circular DNA, plasmids, and a capsule. The bacterium reproduces rapidly in the presence of the antibiotic penicillin but is inhibited by streptomycin.

(a) State three structural features that distinguish this bacterium from a eukaryotic cell. [3 marks]

(b) Penicillin inhibits the synthesis of murein. Explain why penicillin could be effective against bacteria without harming human cells. [2 marks]

(c) Streptomycin inhibits 70S ribosomes. A student claims this means streptomycin has absolutely no effect on human cells. Evaluate this claim. [2 marks]

D:

(a) Any three from: circular DNA not associated with histone proteins [1]; 70S ribosomes (not 80S) [1]; no membrane-bound nucleus / no nuclear envelope [1]; murein cell wall (not cellulose) [1]; presence of plasmids [1].

E:

(b) Human cells have no cell wall containing murein (animals have no cell wall at all) [1]; therefore penicillin has no target in human cells and cannot harm them [1].

E:

(c) The claim is partially correct — eukaryotic cytoplasmic ribosomes are 80S and would be unaffected [1]; however, mitochondria (and chloroplasts in plant cells) contain 70S ribosomes, so streptomycin could inhibit protein synthesis in these organelles, potentially disrupting aerobic respiration [1].

Summary

Prokaryote vs Eukaryote — Key Differences

FeatureProkaryoteEukaryote

DNACircular; no histones; free in cytoplasmLinear; histone-associated; in nucleus

Ribosomes70S80S cytoplasm; 70S in mito/chloroplast

NucleusAbsent (nucleoid only)Present (double envelope + pores)

OrganellesNone membrane-boundMembrane-bound present

Cell wallMureinCellulose / chitin / absent

Virus Components

All viruses: genetic material (DNA or RNA, never both) + capsid (protein) + attachment proteins. Some also have a lipid envelope (e.g. HIV). Viruses are acellular and non-living.

HIV Key Facts

RNA virus; reverse transcriptase; lipid envelope; gp120 binds CD4 receptors on helper T cells. Antibiotics ineffective — viruses have no 70S ribosomes or murein wall.

Three Most Dropped Marks

(1) Omitting “not associated with histone proteins” for prokaryotic DNA. (2) Writing “antibiotics kill viruses.” (3) Saying HIV destroys “white blood cells” rather than “helper T cells / CD4+ T lymphocytes.”