Three Domains

8 MCQs9-step worked example
Source: NCERT The Living WorldPYQ coverage: NEET 2020, 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

The three-domain system of classification, proposed by Carl Woese in 1990, sits above the kingdom level and divides all life into Archaea, Bacteria, and Eukarya. This system was built on differences in ribosomal RNA (rRNA) sequences, not on visible morphology — and that distinction is where NEET questions pull their traps.

Why three, not two? Before Woese, prokaryotes were lumped into one group. rRNA sequencing revealed that Archaea and Bacteria, though both prokaryotic, are as genetically distant from each other as either is from eukaryotes. The three-domain model reflects evolutionary divergence that morphology alone cannot capture.

Domain characteristics at a glance:

  • Bacteria — prokaryotic, peptidoglycan cell wall, ester-linked membrane lipids, circular DNA, 70S ribosomes.
  • Archaea — prokaryotic, lack peptidoglycan (pseudopeptidoglycan or protein/polysaccharide walls instead), ether-linked membrane lipids, many thrive in extreme environments (methanogens, halophiles, thermoacidophiles), 70S ribosomes.
  • Eukarya — eukaryotic, membrane-bound nucleus, 80S ribosomes, includes Protista, Fungi, Plantae, Animalia.

The high-frequency confusion: students conflate Archaea and Bacteria because both are prokaryotic. NEET exploits this by offering distractors that assign peptidoglycan walls or ester-linked lipids to Archaea. The reliable discriminator is cell wall chemistry (peptidoglycan present in Bacteria, absent in Archaea) and membrane lipid linkage (ester in Bacteria, ether in Archaea).

NCERT anchor: The three-domain concept is introduced in NCERT Class 11 Biology Chapter 2, page 16, where the need for a classification system above kingdom level is discussed.

Watch out: questions may ask "which domain includes methanogens?" — the answer is always Archaea, never Bacteria, despite both being prokaryotic.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

The three-domain system of life classification was proposed by:

MCQ 2Easy RecallPractice

Which molecule's sequence analysis formed the basis for Carl Woese's three-domain classification?

MCQ 3Easy RecallPractice

The three domains of life are:

MCQ 4Direct ApplicationPractice

A prokaryotic organism is found to lack peptidoglycan in its cell wall and possesses ether-linked membrane lipids. This organism belongs to domain:

MCQ 5Direct ApplicationPractice

Methanogens, halophiles, and thermoacidophiles are classified under domain:

MCQ 6Direct ApplicationPractice

Which of the following is a key biochemical difference between domains Bacteria and Archaea?

MCQ 7Concept TrapPractice

Before the three-domain system, Archaea and Bacteria were grouped together under kingdom Monera. Woese separated them primarily because:

MCQ 8CalculationPractice

A student lists four features of an unknown organism: (i) prokaryotic, (ii) cell wall lacks peptidoglycan, (iii) found in hot springs at 80°C, (iv) ether-linked membrane lipids. The organism most likely belongs to which domain, and which specific group within that domain?

Worked Example

Pattern: Identification and classification across domains (from PYQ pattern: classification and taxonomy questions across major groups)

  1. 1

    Given

    - No nuclear membrane (prokaryotic) - No peptidoglycan in cell wall - Thrives at 25% NaCl (extreme halophilic environment)

  2. 2

    Required

    Domain classification and specific group within Archaea.

  3. 3

    Concept

    The three-domain system classifies all life based on rRNA sequence divergence. Prokaryotes split into Bacteria (peptidoglycan walls, ester-linked lipids) and Archaea (no peptidoglycan, ether-linked lipids). Within Archaea, three major groups are recognised: methanogens, halophiles, and thermoacidophiles.

  4. 4

    Decision tree (no formula needed for this classification problem)

    1. Nuclear membrane absent → prokaryote → domain is either Bacteria or Archaea. 2. Peptidoglycan absent → eliminates Bacteria → domain Archaea. 3. Extreme salt tolerance → halophile group within Archaea.

  5. 5

    Application

    - No nuclear membrane eliminates Eukarya. - No peptidoglycan eliminates Bacteria. - Remaining domain: Archaea. - 25% NaCl environment → halophile (salt-loving archaeon).

  6. 6

    Reasoning

    This is a pure classification problem with no arithmetic. Each feature eliminates one possibility. The key discriminator between Bacteria and Archaea is peptidoglycan — its absence is diagnostic for Archaea.

  7. 7

    Final answer

    Domain **Archaea**, group **halophiles**.

  8. 8

    Common trap

    Students may select Bacteria because the organism is prokaryotic, forgetting that "prokaryotic" encompasses two domains, not one. The absence of peptidoglycan is the decisive feature that separates Archaea from Bacteria. Another trap: confusing halophiles with thermoacidophiles — the environmental clue (salt lake vs hot spring) distinguishes them.

  9. 9

    Similar NEET-style question

    "An organism is found in a volcanic vent at 100°C. It is prokaryotic and lacks peptidoglycan. Which domain and group does it belong to?" (Answer: Archaea, thermoacidophiles.) ---

Before solving, remember these

Definition

Three domains of life

Carl Woese proposed three-domain classification: Archaea, Bacteria, and Eukarya — based on rRNA sequence differences. Replaces the old prokaryote/eukaryote dichotomy.

-- NCERT Class 11 Biology, Ch. 2, p. 16

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Monera vs Protista confusion

Category: Similar Terms

Both kingdoms include single-celled organisms but Monera is prokaryotic; Protista eukaryotic.

When it triggers

Question lists organisms and asks for kingdom; tempts placing all unicellulars in same kingdom.

How to avoid

Test for nucleus: prokaryote → Monera; eukaryote → Protista regardless of being unicellular.

Trap

Genus and species capitalisation/italicisation reversed

Category: Similar Terms

Binomial nomenclature: genus capitalised, species lowercased; both italicised. Reversing or non-italicising changes the entity.

When it triggers

Question presents a Latin name and asks for the correct typography or which name is genus.

How to avoid

Genus is ALWAYS the first word, capitalised; species is the second, lowercase; both italicised in print.

Past Year Questions

62 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2025

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : All vertebrates are chordates but all chordates are not vertebrate. Reason (R) : The members of subphylum vertebrata possess notochord during the embryonic period, the notochord is replaced by cartilaginous or bony vertebral column in adults. In the light of the above statements, choose the correct answer from the options given below:

1(A) is false but (R) is true
2Both (A) and (R) are true and (R) is the correct explanation of (A)
3Both (A) and (R) are true but (R) is not the correct explanation of (A)
4(A) is true but (R) is false
NTA Answer: Option 2(final)
NEET 2025

Given below are two statements : One is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : A typical unfertilised, angiosperm embryo sac at maturity is 8 nucleate and 7-celled. Reason (R) : The egg apparatus has 2 polar nuclei. In the light of the above statements, choose the correct answer from the options given below :

1A is false but R is true
2Both A and R are true and R is the correct explanation of A
3Both A and R are true but R is NOT the correct explanation of A
4A is true but R is false
NTA Answer: Option 4(final)
NEET 2025

Each of the following characteristics represent a Kingdom proposed by Whittaker. Arrange the following in increasing order of complexity of body organization. A. Multicellular heterotrophs with cell wall made of chitin. B. Heterotrophs with tissue/organ/organ system level of body organization. C. Prokaryotes with cell wall made of polysaccharides and amino acids. D. Eukaryotic autotrophs with tissue/organ level of body organization. E. Eukaryotes with cellular body organization. Choose the correct answer from the options given below :

1C, E, A, B, D
2A, C, E, B, D
3C, E, A, D, B
4A, C, E, D, B
NTA Answer: Option 3(final)
NEET 2025

Which one of the following statements refers to Reductionist Biology?

1Behavioural approach to study and understand living organisms
2Physico-chemical approach to study and understand living organisms
3Physiological approach to study and understand living organisms
4Chemical approach to study and understand living organisms
NTA Answer: Option 2(final)
NEET 2024

Given below are two statements: Statement I : Parenchyma is living but collenchyma is dead tissue. Statement II : Gymnosperms lack xylem vessels but presence of xylem vessels is the characteristic of angiosperms. In the light of the above statements, choose the correct answer from the options given below:

1Both Statement I and Statement II are true
2Both Statement I and Statement II are false
3Statement I is true but Statement II is false
4Statement I is false but Statement II is true
NTA Answer: Option 4(final)
NEET 2023

Given below are two statements : One labelled as Assertion A and the other labelled as Reason R: Assertion A : The first stage of gametophyte in the life cycle of moss is protonema stage. Reason R : Protonema develops directly from spores produced in capsule. In the light of the above statements, choose the most appropriate answer from options given below:

1Both A and R are correct but R is NOT the correct explanation of A
2A is correct but R is not correct
3A is not correct but R is correct
4Both A and R are correct and R is the correct explanation of A
NTA Answer: Option 4(final)
NEET 2023

Given below are two statements : One labelled as Assertion A and the other labelled as Reason R : Assertion A : In gymnosperms the pollen grains are released from the microsporangium and carried by air currents. Reason R : Air currents carry the pollen grains to the mouth of the archegonia where the male gametes are discharged and pollen tube is not formed. In the light of the above statements, choose the correct answer from the options given below :

1Both A and R are true but R is NOT the current explanation of A
2A is true but R is false
3A is false but R is true
4Both A and R are true and R is the correct explanation of A
NTA Answer: Option 2(final)
NEET 2023

Given below are two statements : One is labelled as Assertion A and the other is labelled as Reason R : Assertion A : A flower is defined as modified shoot wherein the shoot apical meristem changes to floral meristem. Reason R : Internode of the shoot gets condensed to produce different floral appendages laterally at successive node instead of leaves. In the light of the above statements, choose the correct answer from the options given below :

1Both A and R are true but R is NOT the correct explanation of A
2A is true but R is false
3A is false but R is true
4Both A and R are true and R is the correct explanation of A
NTA Answer: Option 4(final)
NEET 2023

Given below are two statements: Statement I: In prokaryotes, the positively charged DNA is held with some negatively charged proteins in a region called nucleoid. Statement II: In eukaryotes, the negatively charged DNA is wrapped around the positively charged histone octamer to form nucleosome. In the light of the above statements, choose the correct answer from the options given below:

1Both Statement I and Statement II are false.
2Statement I is correct but Statement II is false.
3Statement I is incorrect but Statement II is true.
4Both Statement I and Statement II are true.
NTA Answer: Option 3(final)
NEET 2023

Match List I with List II with respect to human eye. List I List II A. Fovea I. Visible coloured portion of eye that regulates diameter of pupil. B. Iris II. External layer of eye formed of dense connective tissue. C. Blind spot III. Point of greatest visual acuity or resolution. D. Sclera IV. Point where optic nerve leaves the eyeball and photoreceptor cells are absent. Choose the correct answer from the options given below:

1A-IV, B-III, C-II, D-I
2A-I, B-IV, C-III, D-II
3A-II, B-I, C-III, D-IV
4A-III, B-I, C-IV, D-II
NTA Answer: Option 4(final)
NEET 2023

Match List I with List II. List I List II (Interacting species) (Name of interaction) A. A Leopard and a Lion in a forest/grassland I. Competition B. A Cuckoo laying egg in a Crow’s nest II. Brood parasitism C. Fungi and root of a higher plant in Mycorrhizae III. Mutualism D. A cattle egret and a Cattle in a field IV. Commensalism Choose the correct answer from the options given below.

1A-I, B-II, C-IV, D-III
2A-III, B-IV, C-I, D-II
3A-II, B-III, C-I, D-IV
4A-I, B-II, C-III, D-IV
NTA Answer: Option 4(final)
NEET 2023

The unique mammalian characteristics are:

1hairs, pinna and mammary glands
2hairs, pinna and indirect development
3pinna, monocondylic skull and mammary glands
4hairs, tympanic membrane and mammary glands
NTA Answer: Option 1(final)
NEET 2022

Identify the correct set of statements : (a) The leaflets are modified into pointed hard thorns in Citrus and Bougainvillea (b) Axillary buds form slender and spirally coiled tendrils in cucumber and pumpkin (c) Stem is flattened and fleshy in Opuntia and modified to perform the function of leaves (d) Rhizophora shows vertically upward growing roots that help to get oxygen for respiration (e) Subaerially growing stems in grasses and strawberry help in vegetative propagation Choose the correct answer from the options given below :

1(a), (b), (d) and (e) Only
2(b) and (c) Only
3(a) and (d) Only
4(b), (c), (d) and (e) Only
NTA Answer: Option 4(final)
NEET 2022

Match the plant with the kind of life cycle it exhibits: List-I List-II (a) Spirogyra (i) Dominant diploid sporophyte vascular plant, with highly reduced male or female gametophyte (b) Fern (ii) Dominant haploid free-living gametophyte (c) Funaria (iii) Dominant diploid sporophyte alternating with reduced gametophyte called prothallus (d) Cycas (iv) Dominant haploid leafy gametophyte alternating with partially dependent multicellular sporophyte Choose the correct answer from the options given below :

1(a)-(ii), (b)-(iv), (c)-(i), (d)-(iii)
2(a)-(iv), (b)-(i), (c)-(ii), (d)-(iii)
3(a)-(ii), (b)-(iii), (c)-(iv), (d)-(i)
4(a)-(iii), (b)-(iv), (c)-(i), (d)-(ii)
NTA Answer: Option 3(final)
NEET 2022

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : All vertebrates are chordates but all chordates are not vertebrates. Reason (R) : Notochord is replaced by vertebral column in the adult vertebrates. In the light of the above statements, choose the most appropriate answer from the option given below :

1(A) is not correct but (R) is correct
2Both (A) and (R) are correct and (R) is the correct explanation of (A)
3Both (A) and (R) are correct but (R) is not the correct explanation of (A)
4(A) is correct but (R) is not correct
NTA Answer: Option 2(final)
NEET 2022

In the taxonomic categories which hierarchical arrangement in ascending order is correct in case of animals?

1Kingdom, Order, Phylum, Class, Family, Genus, Species
2Kingdom, Phylum, Class, Order, Family, Genus, Species
3Kingdom, Class, Phylum, Family, Order, Genus, Species
4Kingdom, Order, Class, Phylum, Family, Genus, Species Answer (2*) 178. Given below are two statements : Statement I : Fatty acids and glycerols cannot be absorbed into the blood. Statement II : Specialized lymphatic capillaries called lacteals carry chylomicrons into lymphatic vessels and ultimately
NTA Answer: Option 2(final)
NEET 2021

Gemmae are present in (4) 2, 4-D

1Some Liverworts Answer (4)
2Mosses 125. Which of the following algae produce Carrageen?
3Pteridophytes (1) Blue-green algae
4Some Gymnosperms (2) Green algae
NTA Answer: Option 1(final)
NEET 2021

With regard to insulin choose correct options. (b) Echinoderms are triploblastic and coelomate animals. (a)C-peptide is not present in mature insulin. (c) Round worms have organ-system level of body (b)The insulin produced by rDNA technology has C- organization. peptide. (d) Comb plates present in ctenophores help in (c)The pro-insulin has C-peptide digestion. (d)A-peptide and B-peptide of insulin are (e) Water vascular system is characteristic of interconnected by disulphide bridges. Echinoderms. Choose the correct answer from the option given Choose the correct answer from the options given

1(a) and (d) only (1) (b), (c) and (e) are correct
2(b) and (d) only (2) (c), (d) and (e) are correct
3(a), (b) and (c) are correct (3) (b) and (c) only
4(a), (d) and (e) are correct (4) (a), (c) and (d) only
NTA Answer: Option 1(final)
NEET 2021

Match List - I with List - II (b) 'A' band widens List - I List - II (c) 'I' band reduces in width (a) Adaptive (i) Selection of (d) Myosine hydrolyzes ATP, releasing the ADP and radiation resistant varieties Pi due to excessive use of herbicides (e) Z-lines attached to actins are pulled inwards and pesticides Choose the correct answer from the options given (b) Convergent (ii) Bones of forelimbs beloow evolution in Man and Whale

1(b), (d), (e), (a) only (c) Divergent (iii) Wings of Butterfly evolution and Bird
2(a), (c), (d), (e) only (d) Evolution by (iv) Darwin Finches
3(a), (b), (c), (d) only anthropogenic
4(b), (c), (d), (e) only action
NTA Answer: Option 2(final)
NEET 2020

ÁŸêŸÁ‹Áπà ◊ ‚ ∑§ÊÒŸ ‚Ê ÿÈÇ◊ ∞∑§ ∑§Ê Á‡Ê∑§Ëÿ ‡ÊÒflÊ‹Ê ∑§Ê 91. Which of the following pairs is of unicellular „Ò? algae ?

1Chlorella and Spirulina (1) Ä‹Ê ⁄ U‹Ê •ÊÒ⁄U S¬ÊßL§‹ËŸÊ
2Laminaria and Sargassum (2) ‹ÒÁ◊Ÿ Á⁄UÿÊ •ÊÒ⁄U ‚Ê⁄UªÊ‚◊
3Gelidium and Gracilaria (3) ¡ Á‹Á«Uÿ◊ •ÊÒ⁄U ª˝ÊÁ‚‹ Á⁄UÿÊ
4Anabaena and Volvox (4) ∞ ŸÊ’ËŸÊ •ÊÒ⁄U flÊÚÀflÊÚÄ‚
NTA Answer: Option 1(final)
NEET 2020

Á∑§‚ ÁflÁœ mÊ⁄UÊ ’Ë∑§ÊŸ ⁄UË ∞ flË¡ ∞fl ◊Ò⁄UËŸÊ ⁄ Uê‚ ‚ ÷ «∏ ∑§Ë 94. By which method was a new breed ‘Hisardale’ of sheep formed by using Bikaneri ewes and Marino Ÿß¸ ŸS‹ “Á„‚Ê⁄U« U‹” ÃÒÿÊ⁄U ∑§Ë ªÿË „Ò? rams ?

1•Ã —¬¡˝ ŸŸ (1) Inbreeding
2’Á„—¬˝¡ŸŸ (2) Out crossing
3©à¬Á⁄UfløŸ ¬˝¡ŸŸ (3) Mutational breeding
4‚∑ §⁄UáÊ (4) Cross breeding
NTA Answer: Option 4(final)
NEET 2020

ÿÁŒ ŒÊ ‹ªÊÃÊ⁄U ˇÊÊ⁄U ÿÈÇ◊Ê ∑ § ’Ëø ∑§Ë ŒÍ⁄UË 0.34 nm „Ò •ÊÒ⁄U 106. If the distance between two consecutive base pairs is 0.34 nm and the total number of base pairs of a ∞∑§ S߬ÊÿË ∑§Ê Á‡Ê∑§Ê ∑§Ë DNA ∑§Ë Ám∑È §«U‹Ë ◊ ˇÊÊ⁄U ÿÈÇ◊Ê DNA double helix in a typical mammalian cell is ∑§Ë ∑ȧ‹ ‚ ÅÿÊ 6.6×109 bp „Ò– Ã’ DNA ∑§Ë ‹ê’Ê߸ 6.6×109 bp, then the length of the DNA is „Ê ªË ‹ª÷ª — approximately :

12.7 ◊Ë≈U⁄U (1) 2.7 meters
22.0 meters (2) 2.0 ◊Ë≈U⁄U
32.5 meters (3) 2.5 ◊Ë≈U⁄U
42.2 meters (4) 2.2 ◊Ë≈U⁄U
NTA Answer: Option 4(final)
NEET 2020

ÁŸêŸÁ‹Áπà ◊ ‚ ‚„Ë ÿÈÇ◊ ∑§Ê øÈÁŸ∞ —

1∞Ä‚Ê ãÿÍÁÄ‹ÿ ¡ - «Ë.∞Ÿ.∞. ◊ ÁflÁ‡Êc≈U 133. Choose the correct pair from the following : SÕÊŸÊ ¬⁄U ∑§Ê≈U (1) Exonucleases - Make cuts at specific ‹ªÊÃÊ „Ò positions within DNA
2‹Êߪ ¡ - ŒÊ «UË.∞Ÿ.∞. ∑ § (2) Ligases - Join the two DNA •áÊÈ•Ê ∑§Ê ¡Ê «∏ÃÊ „Ò molecules
3¬ÊÚÁ‹◊⁄ U¡ - «UË.∞Ÿ.∞. ∑§Ê πá«UÊ ◊ ÃÊ «∏ÃÊ „Ò (3) Polymerases - Break the DNA into
4ãÿÍÁÄ‹ÿ ¡ - «UË.∞Ÿ.∞. ∑ § ŒÊ fragments ⁄UîÊÈ∑§Ê ∑§Ê ¬ÎÕ∑§ (4) Nucleases - Separate the two strands ∑§⁄UÃÊ „Ò of DNA
NTA Answer: Option 2(final)

How NEET usually asks this

Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.

Identification, classification, taxonomy questions across kingdoms and major phyla

RecallMedium

Common distractors

plant life cycle ploidy confusion

Double fertilization produces two products; students assign 2n to both zygote and endosperm nucleus, forgetting the two polar nuclei each contribute 1n.

gymnosperm angiosperm feature swap

Both groups produce seeds; students attribute angiosperm features (flowers, fruit) to gymnosperms or deny gymnosperm-exclusive features (naked seeds, cones).

disease causative organism mismatch

Many diseases are associated with look-alike organism classes; students swap bacterial, fungal, protozoan, and viral agents for well-known diseases.

coelomate pseudocoelomate acoelomate confusion

Porifera has a spongocoel water canal; students interpret any internal space as a coelom and classify sponges as pseudocoelomate rather than acoelomate.

chordate nerve cord reversal

Students recall 'dorsal' but forget 'hollow single'; canonical distractor 'mid-dorsal, solid and double' catches the partially-right recall.

fungi classification non criterion

Mode of nutrition is used to define kingdom Fungi overall; students incorrectly extend it as a criterion for classification WITHIN fungi.

assertion reason unchecked reason

In A-R format, a true assertion with a plausible-sounding reason tempts students into selecting option A without independently verifying whether the reason is correct and whether it genuinely explains the assertion.

prokaryote membrane organelle swap

Students learn prokaryotes lack membrane-bound organelles but incorrectly exclude ribosomes (not membrane-bound) from prokaryotes, or assume prokaryotes have some organelles simply because they are complex organisms.

Sources

NCERT refs: Class 11 Biology Chapter 2, p.16

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