Q.Select one which is not true for ribosome
Concept understanding — Ribosome Function Translation
Imagine you are in a large library. The shelves are packed with books, but each book is written in a language you cannot read. To understand the book, you need a translator — someone who can read the original script and then explain it to you in plain, usable words.
In a living cell, the "library" is the nucleus, where DNA — the master blueprint — is stored. The "books" are genes, written in the language of nucleic acids. But the cell doesn't work directly with DNA. It needs to build proteins — the actual workers, tools, and building blocks of the body. So it first makes a working copy of a gene, called mRNA (messenger RNA). This mRNA is like a page torn from the book, still in the nucleic-acid language. Now the cell needs a "translator" to convert that message into the language of proteins. That translator is the ribosome.
What is a ribosome?
A ribosome is a tiny, complex molecular machine made of RNA and proteins. It is not a membrane-bound organelle — it floats freely in the cytoplasm or sits on the rough endoplasmic reticulum. Think of it as a workbench that clamps onto the mRNA and reads its sequence, three letters at a time. Each three-letter "word" on the mRNA is called a codon, and each codon specifies one amino acid — the building block of a protein.
The ribosome has two main parts, or subunits — a large one and a small one. The small subunit holds the mRNA in place, while the large subunit does the actual work of joining amino acids together.
The process of translation: from mRNA to protein
Translation happens in three stages, just like reading a sentence: you start, you read word by word, and you stop.
1. Initiation (the start)
The small ribosomal subunit finds a special "start" codon on the mRNA — usually AUG. A special tRNA (transfer RNA) molecule carrying the amino acid methionine binds to that start codon. Then the large subunit clicks into place. The ribosome is now assembled and ready to read.
2. Elongation (the reading and building)
The ribosome moves along the mRNA, one codon at a time. For each codon, a matching tRNA brings the correct amino acid. The ribosome's large subunit then forms a peptide bond between the new amino acid and the growing chain. The ribosome shifts forward, and the empty tRNA is released. This repeats — like a train moving along a track, adding one carriage at a time.
3. Termination (the stop)
When the ribosome reaches a "stop" codon (UAA, UAG, or UGA), no tRNA matches it. Instead, a release factor protein binds, causing the ribosome to let go of the completed protein chain. The two subunits separate, and the new protein is free to fold into its functional shape.
The word "translation" is exact: the cell is translating from the four-letter language of nucleic acids (A, U, G, C) into the twenty-letter language of amino acids. The ribosome is the translator, and tRNAs are the "dictionaries" that match each codon to its correct amino acid.
Why does this matter?
Without translation, the instructions in your DNA would remain useless. Every enzyme that digests your food, every antibody that fights infection, every muscle fibre that lets you move — all are built by ribosomes during translation. If translation stops, the cell dies.
In medicine, many antibiotics work by targeting bacterial ribosomes. For example, tetracycline blocks the binding of tRNA to the ribosome, stopping the bacteria from making proteins. Human ribosomes are slightly different, so the drug affects bacteria but not us — a beautiful example of how understanding this process saves lives.
Key points to remember (as per NCERT)
- Translation is the process of polymerising amino acids into a polypeptide chain, using the sequence of codons on mRNA as a template.
- The ribosome acts as the site of translation and also as a catalyst for forming peptide bonds (the large subunit has an enzymatic activity called peptidyl transferase).
- Each tRNA has an anticodon that base-pairs with the mRNA codon, and it carries the corresponding amino acid at its other end.
- The energy for forming peptide bonds comes from GTP (guanosine triphosphate), not ATP — a small but exam-relevant detail.
In NCERT Class 12 Biology, translation is described as occurring in the cytoplasm (for prokaryotes) or on the rough endoplasmic reticulum (for eukaryotes, especially for proteins destined for secretion). The ribosome moves along the mRNA from the 5' end to the 3' end, and the polypeptide chain grows from the N-terminus to the C-terminus.
A final intuition
Think of the ribosome as a conveyor belt in a factory. The mRNA is the instruction tape that feeds through the belt. Each station along the belt (each codon) calls for a specific part (a tRNA with an amino acid). The belt moves, parts are added, and at the end, a finished product — a protein — rolls off. That protein will go on to do a specific job: some become structural (like collagen in skin), some become enzymes (like lactase to digest milk), and some become hormones (like insulin to regulate blood sugar).
That is translation — the moment when genetic information becomes a working molecule.
This concept is a common exam-prep search query, appearing online as "Ribosome Function Translation diagram and explanation", "Ribosome Function Translation NEET questions", or "Ribosome Function Translation: Definition, Diagram & Examples". This concept is directly part of the Molecular Basis of Inheritance chapter in the NCERT/CBSE Class 12 Biology syllabus, and it is also an important topic for NEET and state medical/CET entrance exams, making it worth mastering for both board and competitive-exam preparation.
The statement that is not true for ribosomes is that they have no role in protein synthesis.
- Ribosomes are, in fact, the very site where proteins are assembled inside a cell — this is their defining job.
- They are genuinely built from two subunits that come together to form the working particle.
- They do attach to messenger RNA during translation.
- Several ribosomes can also gather on a single mRNA strand to form a polysome, allowing many copies of a protein to be made together.
Option D is false — ribosomes are the site of protein synthesis, not structures with no role in it.
Every statement about ribosomes here is true except the claim that they play no role in protein synthesis.
A ribosome is a compact, non-membrane-bound particle whose entire purpose in the cell is to build proteins. Saying it has "no role in protein synthesis" contradicts the most basic fact about what a ribosome does, which makes that statement the false one.
The other three statements are all accurate. A ribosome genuinely is made of two subunits that fit together to form the complete working particle. It does attach to messenger RNA, reading the genetic message strand by strand as it builds a protein chain. And several ribosomes commonly gather on one mRNA molecule at the same time, forming a chain called a polysome or polyribosome, which lets multiple copies of the same protein be produced simultaneously.
Ribosomes may occur free in the cytoplasm or attached to the rough endoplasmic reticulum, but in either location their function is always protein synthesis.
The false statement is option D — ribosomes are precisely the site of protein synthesis, so saying they have no role in it is incorrect.
Method: Spotting the Statement That Contradicts the Organelle's Defining Job
When an MCQ lists several claims about an organelle and asks which is false, the fastest route is to anchor first on that organelle's single defining job, then check whether any option directly contradicts it. For the ribosome, that defining job is protein synthesis — it is, almost by definition, the site where amino acids get strung into polypeptides.
Once that anchor is in place, a statement claiming "no role in protein synthesis" doesn't need any further evidence-gathering to reject — it collides head-on with the one fact you're most sure of. The remaining statements (two subunits, attaching to mRNA, forming polysomes) are all secondary structural/functional details that are consistent with, and supportive of, that same core job, so they don't need to be independently verified once you've confirmed they don't contradict the anchor fact.
This "anchor on the defining function, then scan for contradictions" approach is generally faster than trying to independently confirm all four statements from memory."
- KCET 2021Set C-31 markMCQQ.In which region of the t-RNA molecule is the amino-acid binding site located? (A) 5′ end (B) anticodon loop (C) 3′ end (D) None of the above
›Reveal solutionSolution
The amino acid is attached to the free 3′-OH of the terminal adenosine (the CCA end) of the acceptor arm — so the binding site is the 3′ end.
Step 1 — The structure of tRNA (the adapter molecule)
tRNA is the adapter molecule postulated by Francis Crick: it must read the genetic code on one side and carry the amino acid on the other. Its cloverleaf secondary structure (an inverted-L in 3-D) has:
- an anticodon loop, bearing the three bases complementary to the mRNA codon — this is the reading end;
- a DHU loop (binds aminoacyl-tRNA synthetase);
- a TψC loop (binds the ribosome);
- an acceptor (amino-acid) arm — the base-paired stem formed by the 5′ and 3′ ends, whose 3′ end protrudes as a single-stranded ...C-C-A-3′-OH tail.
Step 2 — Where the amino acid actually attaches
The enzyme aminoacyl-tRNA synthetase ("charging" the tRNA) forms an ester bond between the –COOH group of the amino acid and the free 3′-OH group of the terminal adenosine of that CCA tail.
amino acid–COOH+tRNA–3’OHaminoacyl-tRNA synthetaseATPaminoacyl-tRNA
Hence the amino-acid binding (acceptor) site is at the 3′ end.
Step 3 — Why the geometry makes sense
The anticodon and the acceptor arm sit at opposite ends of the L-shaped molecule (~7–8 nm apart). This is essential: the anticodon must sit in the ribosome's decoding centre pairing with the mRNA codon, while the amino acid must simultaneously reach the peptidyl transferase centre where the peptide bond is made. A single molecule with its two functional ends far apart is exactly what an "adapter" needs to be.
Step 4 — Eliminate
- (A) 5′ end — this end is base-paired within the acceptor stem and carries a phosphate, not the amino acid.
- (B) anticodon loop — this reads the codon; if the amino acid sat here it would physically block codon–anticodon pairing.
- (D) None of the above — unnecessary, since (C) is right.
✓Final answerThe correct option is (C) — 3′ end.
ANSWER: C
- KCET 2019Set A-11 markMCQQ.In Prokaryotes, the transcription of DNA is initiated with the help of (A) Elongation factor (B) Termination factor (C) Rho factor (D) Sigma factor
›Reveal solutionSolution
In prokaryotes, transcription initiation requires the sigma factor to guide RNA polymerase to the promoter — the correct option is (D).
The question asks which factor initiates transcription in prokaryotes. The key is to understand that transcription happens in three phases: initiation, elongation, and termination. Each phase uses different protein factors. The initiation phase is all about getting RNA polymerase to bind correctly to the promoter and start making RNA. That job belongs to the sigma factor.
Let’s walk through the options one by one.
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Elongation factor (A) — This is a trick. In eukaryotes, elongation factors help RNA polymerase move along the DNA during elongation. In prokaryotes, the core RNA polymerase itself handles elongation without needing extra factors. So this is not involved in initiation.
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Termination factor (B) — This is for the end of transcription. In prokaryotes, termination can be Rho-dependent (using the Rho factor) or Rho-independent (using a hairpin loop). Either way, termination factors act after the RNA is made, not at the start.
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Rho factor (C) — This is a specific termination factor. It binds to the growing RNA transcript and pulls it off the DNA template, ending transcription. Again, this is an ending mechanism, not a starting one.
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Sigma factor (D) — This is the correct answer. The sigma factor is a subunit that temporarily binds to the core RNA polymerase (forming the holoenzyme). Its job is to recognise and bind to the promoter sequence on the DNA. Once a few nucleotides are added, sigma dissociates, and the core enzyme continues elongation.
TipA helpful memory aid: Sigma starts, Rho stops. Sigma factor initiates transcription by finding the promoter; Rho factor terminates it by pulling the RNA off.
Watch outA common mistake is to confuse the sigma factor with the Rho factor because both are Greek letters and both are associated with RNA polymerase. Remember: sigma is for start (initiation), Rho is for release (termination).
✓Final answerThe correct option is (D) Sigma factor.
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