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."
- TG EAPCET 2025Set ap-2025-04-29-AN1 markMCQQ.In a messenger RNA molecule, untranslated regions (UTRs) are present at I. 5' end before start codon II. 3' end after stop codon III. 3' end before stop codon IV. 5' end after start codon (A) I & III (B) II & III (C) I & II (D) III & IV
›Reveal solutionSolution
Untranslated regions (UTRs) are segments of messenger RNA (mRNA) that are not translated into protein, located at the 5' end before the start codon and at the 3' end after the stop codon. The correct option is (C).
Concept and Intuition
Messenger RNA (mRNA) molecules carry the genetic instructions from DNA to the ribosomes, where these instructions are translated into proteins. While the primary function of mRNA is to serve as a template for protein synthesis, not all parts of an mRNA molecule are actually translated. The region that codes for the protein is called the coding sequence (CDS), which begins with a start codon (typically AUG) and ends with a stop codon (UAA, UAG, or UGA).
Untranslated regions (UTRs) are segments of mRNA that lie outside the coding sequence. They are transcribed from DNA but are not translated into amino acids. Despite not coding for protein, UTRs are crucial for regulating gene expression. They contain sequences that influence mRNA stability, translation efficiency, and localization within the cell. Understanding their precise location relative to the start and stop codons is fundamental to comprehending mRNA structure and function.
Step-by-step explanation
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Understanding mRNA structure:
An mRNA molecule has a linear structure with a 5' end and a 3' end. The central part of the mRNA contains the coding sequence (CDS), which is flanked by a start codon at its 5' end and a stop codon at its 3' end. The ribosome reads the mRNA from the start codon to the stop codon to synthesize a protein.
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Defining Untranslated Regions (UTRs):
Untranslated regions (UTRs) are segments of the mRNA molecule that are not translated into protein. They are present at both ends of the coding sequence.
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Locating the 5' Untranslated Region (5' UTR):
The 5' UTR is located at the 5' end of the mRNA molecule. It extends from the 5' cap to the start codon (AUG). This region is transcribed but not translated. It plays a role in regulating translation initiation and mRNA stability.
- Therefore, statement I, "5' end before start codon," correctly describes the location of the 5' UTR.
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Locating the 3' Untranslated Region (3' UTR):
The 3' UTR is located at the 3' end of the mRNA molecule. It extends from the stop codon (UAA, UAG, or UGA) to the poly-A tail. Like the 5' UTR, this region is transcribed but not translated. It is involved in mRNA stability, localization, and translation termination.
- Therefore, statement II, "3' end after stop codon," correctly describes the location of the 3' UTR.
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Evaluating incorrect options:
- Statement III, "3' end before stop codon," refers to a region that would be part of the coding sequence, specifically the part immediately preceding the stop codon. This region is translated into protein, so it is not an untranslated region.
- Statement IV, "5' end after start codon," refers to the beginning of the coding sequence itself, which is translated into protein. This is also not an untranslated region.
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Identifying the correct combination:
Based on the analysis, only statements I and II correctly describe the locations of untranslated regions in an mRNA molecule.
NoteUTRs are critical for post-transcriptional gene regulation. For example, the 5' UTR can contain regulatory elements that control how efficiently a ribosome binds and initiates translation, while the 3' UTR can contain binding sites for microRNAs (miRNAs) that regulate mRNA stability and translation.
✓Final answerUntranslated regions (UTRs) are present at the 5' end before the start codon and at the 3' end after the stop codon. The correct option is (C).
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- TG EAPCET 2023Set ap-2023-05-10-FN1 markMCQQ.Genetic codon AUG is I. Initiator codon II. Codon for Methionine III. Stop codon IV. Sigma factor (A) I and II (B) I and III (C) II and III (D) III and IV
›Reveal solutionSolution
The genetic codon AUG serves two primary functions: it acts as the initiator codon, signaling the start of protein synthesis, and it codes for the amino acid Methionine. Therefore, statements I and II are correct.
Concept and Intuition
The genetic code is a set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins by living cells. This information is read in groups of three nucleotides, called codons. Each codon typically specifies a particular amino acid, or signals the start or end of protein synthesis.
For protein synthesis (translation) to begin, the ribosome needs a specific signal to identify where to start reading the messenger RNA (mRNA) sequence. This signal is provided by an initiator codon. Similarly, there are specific codons that signal the ribosome to stop translation, known as stop codons. The question asks us to identify the roles of the specific codon AUG.
Step-by-step explanation
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Understanding Codons: A codon is a sequence of three nucleotides that forms a unit of genomic information encoding a particular amino acid or signaling the termination of protein synthesis. The codon AUG consists of Adenine (A), Uracil (U), and Guanine (G).
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Role as an Initiator Codon (I): The codon AUG is universally recognized as the start codon or initiator codon in almost all organisms. It signals the ribosome to begin the process of translation, which is the synthesis of a polypeptide chain (protein) from an mRNA template. Without an initiator codon, the ribosome would not know where to start reading the genetic message.
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Role as a Codon for Methionine (II): In addition to its role as an initiator, the AUG codon also codes for the amino acid Methionine (Met). This means that whenever AUG appears in the coding sequence of an mRNA, a Methionine amino acid is incorporated into the growing polypeptide chain. In eukaryotes, the first amino acid in a newly synthesized protein is typically Methionine. In prokaryotes, it is N-formylmethionine (fMet).
ImportantThe AUG codon has a dual function: it initiates translation AND codes for Methionine.
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Is it a Stop Codon (III)? Stop codons, also known as nonsense codons, are specific codons that signal the termination of protein synthesis. There are three primary stop codons:
- UAA (Ochre)
- UAG (Amber)
- UGA (Opal) AUG is not one of these stop codons. Therefore, statement III is incorrect.
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Is it a Sigma Factor (IV)? A sigma factor is a protein subunit required for the initiation of transcription in prokaryotes. It helps RNA polymerase bind to the promoter region of DNA. It is a protein, not a genetic codon (which is a sequence of nucleotides). Therefore, statement IV is incorrect.
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Conclusion: Based on the analysis, statements I (Initiator codon) and II (Codon for Methionine) are correct.
✓Final answerThe genetic codon AUG is both an initiator codon and a codon for Methionine, making the correct option (A).
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- TG EAPCET 2023Set ap-2023-05-11-AN1 markMCQQ.Identify the correct statements I. Prof. V.S Ramdas is a famous taxonomist of India II. 23s rRNA is Ribozyme III. Genetic nature of RNA was discovered by Watson and Crick IV. Cronquist proposed phylogenetic system of classification (A) II and IV (B) II and III (C) I and III (D) I and IV
›Reveal solutionSolution
The question tests factual recall from biology. Statement II (23s rRNA is a ribozyme) is correct; Statement IV (Cronquist proposed a phylogenetic system) is correct. Statements I and III are false. The correct option is (A).
Let’s go through each statement one by one, with the reasoning behind why it is true or false.
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Statement I: Prof. V.S Ramdas is a famous taxonomist of India
This is false. Prof. V.S. Ramdas is known for his work in plant physiology and agricultural science, not taxonomy. The famous Indian taxonomists include figures like C.V. Raman (physics), Birbal Sahni (paleobotany), and E.K. Janaki Ammal (botany), but Ramdas is not a taxonomist. So I is incorrect.
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Statement II: 23s rRNA is Ribozyme
This is true. Ribozymes are RNA molecules with catalytic activity. The 23s rRNA is a component of the large subunit of the bacterial ribosome, and it catalyzes the formation of peptide bonds during protein synthesis. It is indeed a ribozyme. So II is correct.
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Statement III: Genetic nature of RNA was discovered by Watson and Crick
This is false. Watson and Crick are famous for the double-helix structure of DNA (1953). The genetic nature of RNA — that RNA can carry genetic information — was demonstrated later, notably by experiments with tobacco mosaic virus (TMV) by Fraenkel-Conrat and others. Watson and Crick did not discover this. So III is incorrect.
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Statement IV: Cronquist proposed phylogenetic system of classification
This is true. Arthur Cronquist (1919–1992) was an American botanist who developed a widely used phylogenetic system of classification for flowering plants (the Cronquist system), based on evolutionary relationships. So IV is correct.
Watch outA common mistake is to confuse Cronquist with other taxonomists like Linnaeus (artificial system) or Bentham and Hooker (natural system). Cronquist’s system is explicitly phylogenetic (evolutionary).
Thus, the correct statements are II and IV.
✓Final answerThe correct option is (A) II and IV.
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