Q.Define stop codon. Write the codons.
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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). …
During translation, most codons specify an amino acid, but three special codons signal the ribosome to stop making the protein.
…
Stop codons are the three mRNA codons — UAA, UAG, UGA — that code for no amino acid and terminate protein synthesis.
Definition: A stop codon (also called a termination or nonsense codon) is a codon on the mRNA that does not specify any amino acid. When the ribosome reaches it, no tRNA binds; instead release factors bind, and the completed polypeptide is released, ending translation. This is part of the TS Intermediate genetic-code topic, following the NCERT/CBSE curriculum.
The three stop codons:
- UAA (called ochre) …
Showing the 12 most recent of 22 on this concept.
- CBSE 2026Set ANNUAL1 markMCQQ.An example of a non-membrane bound organelle is -(a) Endoplasmic reticulum(b) Ribosome(c) Mitochondria(d) Chloroplast
›Reveal solutionSolution
Ribosomes are the only non-membrane-bound organelle among the four options.
Eukaryotic cells contain both membrane-bound and non-membrane-bound organelles:
- Endoplasmic reticulum — a single-membrane network of tubules and cisternae.
- Mitochondria — double-membrane organelles (outer membrane + inner membrane folded into cristae).
- Chloroplast — also double-membrane, with an internal thylakoid membrane system. …
- CBSE 2026Set ANNUAL1 markQ.Write answer in one word/sentence: Write the name of dual functional (met and initiator) genetic code.
›Reveal solutionSolution
AUG is the codon that both codes for methionine and serves as the start codon.
The genetic code is read in triplets called codons. The codon AUG has two roles: it codes for the amino acid methionine (Met), and it also functions as the initiator or start codon that signals the beginning of translation (protein synthesis). Because it …
- CBSE 2025Set F1 markMCQQ.Which of the following is a soluble RNA?(a) m-RNA(b) r-RNA(c) c-RNA(d) t-RNA
›Reveal solutionSolution
Transfer RNA (tRNA) is the soluble RNA, also called sRNA.
tRNA (transfer RNA) is the smallest of the RNAs and was originally called soluble RNA (sRNA) because it stays dissolved in the cytoplasm. It acts as an adaptor molecule: one end carries a specific amino acid while its anticodon reads the codon on mRNA, thus delivering the correct amino acid to the ribosome during protein synthes …
- CBSE 2024Set E1 markMCQQ.What is the meaning of charging of t-RNA?(a) Linking of amino acid with cognate t-RNA(b) Attachment of t-RNA with ribosome(c) Translation of RNA(d) Modification of RNA
›Reveal solutionSolution
Charging (aminoacylation) of tRNA is the linking of a specific amino acid to its correct (cognate) tRNA.
Before translation, each amino acid must be attached to the correct transfer RNA:
- The enzyme aminoacyl-tRNA synthetase joins a specific amino acid to its cognate (matching) tRNA, using ATP energy.
- This activated, amino-acid-bearing tRNA is said to be 'charged' or aminoacylated. …
- CBSE 2024Set ANNUAL1 markQ.Write the type of RNA which brings aminoacids and reads the genetic code.
›Reveal solutionSolution
tRNA is the adaptor molecule that reads the codon on mRNA (via its anticodon) and delivers the corresponding amino acid during translation.
Transfer RNA (tRNA) has a clover-leaf secondary structure with an anticodon loop at one end that base-pairs with a complementary codon on the mRNA, and an amino-acid-acceptor end (3' CCA end) that is charged with the specific amino acid corresponding to that codon (by the enzyme aminoacyl-tRNA synthetase). By simultaneously reading t …
- CBSE 2024Set ANNUAL1 markMCQQ.For initiation of protein synthesis ribosome binds to the ................ codon of messenger RNA.(a) AUG(b) UGA(c) AGU(d) UAG
›Reveal solutionSolution
Translation begins when the ribosome assembles on the mRNA at the initiation (start) codon AUG.
During translation, the small ribosomal subunit binds to the mRNA and scans for the start codon AUG, which also codes for methionine (or N-formylmethionine in prokaryotes) - the first amino acid of the growing polypeptide. Once the initiator tRNA pairs with AUG, the large ribosomal subunit joins to form the …
- CBSE 2024Set ANNUAL1 markMCQQ.Anticodon is present on :(a) mRNA(b) tRNA(c) rRNA(d) DNA
›Reveal solutionSolution
tRNA carries the anticodon that pairs with the mRNA codon during translation.
A codon is a triplet of bases present on mRNA that specifies an amino acid. The complementary triplet that recognises and base-pairs with this codon is called the anticodon, and it is located in the anticodon loop of transfer RNA (tRNA). During translation, the tRNA charged with a specific amino aci …
- CBSE 2024Set ANNUAL1 markMCQQ.During protein synthesis, peptide bonds are formed at the –(a) Nucleus(b) Nucleolus(c) Ribosomes(d) Lysosomes
›Reveal solutionSolution
Peptide bonds are formed on ribosomes during translation, catalysed by the peptidyl transferase activity of the large ribosomal subunit.
Protein synthesis (translation) occurs on ribosomes — ribonucleoprotein particles made of a large and small subunit assembled from rRNA and proteins. The mRNA is read codon by codon at the ribosome; aminoacyl-tRNAs bring in specific amino acids that base-pair with codons at the ribosomal A site. The peptidyl transferase activity (a catalytic function of the large subunit's rRNA, making the ribosome a ribozyme) then catalyses formation of the peptide bond between the amino acid held in the A site and the growing polypeptide chain in the P site …
- CBSE 2024Set ANN1 markMCQQ.Which of these cell organelles is not covered by a membrane ?(a) Golgi bodies(b) Endoplasmic reticulum(c) Ribosomes(d) Vacuoles
›Reveal solutionSolution
Ribosomes are not membrane-bound; Golgi bodies, endoplasmic reticulum and vacuoles are all surrounded by a membrane.
Cell organelles are broadly grouped into membrane-bound (Golgi apparatus, endoplasmic reticulum, mitochondria, plastids, vacuoles, lysosomes) and non-membrane-bound (ribosomes, cytoskeleton, centrioles). Ribosomes are minute particles made of ribosomal RNA and proteins, found free in the cytoplasm or attached to the rough ER and the outer mitochondrial/plastid membranes. They lack any …
- CBSE 2024Set ANNUAL1 markMCQQ.Which of the following organelles does not surrounded by any membrane ?(a) Ribosome(b) Nucleus(c) Mitochondria(d) Endoplasmic reticulum
›Reveal solutionSolution
Ribosomes are non-membranous cell organelles made of RNA and protein (ribonucleoprotein particles).
Cell organelles are broadly divided into membrane-bound (nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, plastids, vacuoles) and non-membrane-bound organelles. Ribosomes, found either freely in the cytoplasm/matrix or attached to the endoplasmic reticulum or the outer membrane of the nuclear envelope, are composed of ribosomal RNA and proteins and are NOT bound by any membrane. …
- CBSE 2023Set CG1 markMCQQ.The process of protein synthesis with the help of m-RNA is called(a) Translation(b) Transcription(c) Reverse transcription(d) Replication
›Reveal solutionSolution
Protein synthesis using an mRNA template is translation — option (a).
Concept — the three stages of gene expression:
- Transcription (b): DNA → mRNA (copying a gene into messenger RNA).
- Translation (a): mRNA → protein. Ribosomes read the mRNA codons (three bases at a time), tRNA brings the matching amino acids, and a polypeptide is built.
- Reverse transcription (c): RNA → DNA (in retroviruses, by reverse transcriptase). …
- CBSE 2023Set ANNUAL1 markMCQQ.In which cell organelles synthesis of protein takes place(a) Golgi complex(b) Ribosomes(c) Mitochondria(d) Plastids
›Reveal solutionSolution
Ribosomes are the cell organelles where protein synthesis (translation) takes place.
This is a fundamental fact from the Cell: The Unit of Life chapter regarding the functions of different cell organelles.
Ribosomes and protein synthesis: ribosomes are small, dense, granular structures made of ribosomal RNA (rRNA) and proteins, and are not bound by any membrane. They occur as free particles in the cytoplasm or attached to the endoplasmic reticulum (rough ER) and outer membrane of the nuclear envelope. During translation, the ribosome reads the sequence of codons on a messenger RNA (mRNA) molecule and, with the help of transfer RNA (tRNA) bringing in specific amino acids, links these amino acids together by peptide bonds to build a polypeptide chain. Several ribosomes can attach to a single mRNA strand simultaneously, forming a polyribosome (polysome), which increases the rate of protein production.
Why the other options are wrong: …
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