Q.What does 'S' refer in a 70S & an 80S ribosome?
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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). …
In a 70S or 80S ribosome, 'S' stands for the Svedberg unit.
- The Svedberg unit is the sedimentation coefficient, a value that reflects how fast a particle settles out when it is spun in a centrifuge.
- Heavier and larger particles sediment faster, so the S value gives an indirect measure of a ribosome's density and size. …
The 'S' in 70S and 80S ribosomes stands for the Svedberg unit, a measure of how fast the particle sediments in a centrifuge.
Ribosomes are described by size using a value expressed in Svedberg units. This value is technically the sedimentation coefficient, obtained by observing how quickly a particle settles out of solution when spun at high speed in a centrifuge. Because heavier and larger particles move down through the solution faster, the resulting S value works as an indirect way of gauging a ribosome's overall density and size, rather than measuring its mass directly. …
Method: Define the Unit First, Then Explain Why the Numbers Don't Simply Add
When a question asks "what does X refer to" about a unit or abbreviation, answer the definition directly and specifically before adding any extra detail — don't bury the core fact (Svedberg unit = sedimentation coefficient) inside a longer explanation where it might get missed by a grader skimming for the key term.
Once the definition is stated, it helps to explain what the number physically measures: it comes from spinning a sample at high speed in a centrifuge and observing how quickly a particle moves through the solution, with larger/denser particles moving faster and so getting a bigger S value. Framing it this way makes clear that S is an experimental measurement of behaviour, not a count of subunits or a mass value. …
- 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 …
- 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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