Miller-Urey Experiment: Setup, Conditions and Results
Biology · Evolution · NEET
In 1953, S.L. Miller made an electric discharge (like lightning) inside a closed flask holding CH4, H2, NH3 and water vapour at 800°C. This copied early Earth's air, and after some days he got amino acids. Memory hook: "4 gases + a spark = amino acids" — remember the gases as "My House Needs Water" (Methane, Hydrogen, ammonia = N, Water).
Gases (CH4, H2, NH3, water vapour) at 800°C are sparked by electric discharge (lightning), and amino acids collect in the trap — proving life's molecules can form from non-living gases.
Your doubts, answered
What exactly did Miller put inside the flask?
He put four things: methane (CH4), hydrogen (H2), ammonia (NH3) and water vapour. There was NO free oxygen, because early Earth had a reducing (oxygen-free) atmosphere. For NEET, memorise these exact four — a common wrong option writes CH3 instead of CH4.
Why was the flask kept at 800°C and where did water vapour come from?
The mixture was maintained at 800°C to keep water as vapour and to give energy, just like the hot early Earth. Water was boiled in one part of the apparatus so its vapour joined the gas mixture, copying the ocean-to-cloud cycle. The exact value tested by NEET is 800°C (not 600°C).
What did the electric discharge (spark) stand for?
The electric discharge (electric sparks between electrodes) represented the lightning of primitive Earth. This energy broke and rejoined the simple gases into more complex organic molecules. So the spark is the energy source in the experiment.
What was actually formed at the end?
After about a week, Miller found amino acids (like glycine and alanine) collected in the trap. Amino acids are the monomers (building blocks) of proteins. This showed that life's molecules can form from non-living gases without any living cell — supporting chemical evolution.
How is this different from Oparin-Haldane theory?
Oparin and Haldane only PROPOSED (as an idea) that life's molecules formed from simple gases in a reducing atmosphere. Miller PROVED it by experiment. So Oparin-Haldane = the theory; Miller-Urey = the laboratory proof of that theory.
⚠️ The NEET trap ✗ CH3, H2, NH3 and water vapour at 600°C ✓ CH4, H2, NH3 and water vapour at 800°C 🧠 NTA swaps CH4 with CH3 and 800°C with 600°C in the options. The gas is methane = CH4 (not CH3), and the temperature is 800°C. Lock both exact values.
Real NEET questions
NEET 2020
From his experiments, S.L. Miller produced amino acids by mixing the following in a closed flask:
A · CH4, H2, NH3 and water vapor at 600°C
B · CH3, H2, NH3 and water vapor at 600°C
C · CH4, H2, NH3 and water vapor at 800°C ✓
D · CH3, H2, NH3 and water vapor at 800°C
Solution: In 1953 S.L. Miller created an electric discharge in a closed flask containing methane (CH4), hydrogen (H2), ammonia (NH3) and water vapour at 800°C, copying the reducing atmosphere of early Earth, and obtained amino acids. Options with CH3 or 600°C are wrong. (NCERT Ch 6, p.111)
NEET 2016 (Phase 2)
Which of the following is the correct sequence of events in the origin of life?
(i) Formation of protobionts
(ii) Synthesis of organic monomers
(iii) Synthesis of organic polymers
(iv) Formation of DNA-based genetic systems
A · (i), (ii), (iii), (iv)
B · (i), (iii), (ii), (iv)
C · (ii), (iii), (i), (iv) ✓
D · (ii), (iii), (iv), (i)
Solution: Chemical evolution goes simple to complex: first organic monomers such as amino acids form from inorganic gases (this is exactly what Miller's experiment demonstrated), then monomers join into polymers, which aggregate into protobionts, and finally DNA-based genetic systems arise. Order: (ii) → (iii) → (i) → (iv).
Solved Evolution NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
