Which of the following is true with respect to AUG? (a) It codes for methionine only (b) It is an initiation codon (c) It codes for methionine in both prokaryotes and eukaryotes (d) All of the above

Which of the following is true with respect to AUG? (a) It codes for methionine only (b) It is an initiation c

NCERT Exemplar · Q8

Q. Which of the following is true with respect to AUG?

(a) It codes for methionine only

(b) It is an initiation codon

(d) All of the above

Rajasthan RbseMCQ· 1mImportance★★★★★

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Concept understanding — Template Strand Transcription

Template Strand Transcription: A First Look

Imagine you have a master recipe book written in a language only the head chef can read. To share a recipe with the kitchen staff, you don't hand them the original book — you make a working copy on a separate sheet, using the original as your guide. That original page you read from is the template. The copy you produce is the transcript.

In a cell, the master recipe book is DNA. It holds all the instructions for making proteins, which do almost everything in your body. But DNA never leaves the nucleus — it's too precious and too large. So the cell makes a temporary, portable copy of a specific instruction. That copy is called messenger RNA (mRNA).

The process of making this mRNA copy is transcription. And the strand of DNA that is actually read to make the copy is called the template strand.

The Precise Meaning

DNA is a double helix — two strands twisted together. During transcription, the cell unzips a small section of this helix. Only one of the two strands serves as the blueprint. That strand is the template strand. The other strand, called the coding strand (or non-template strand), is not read — it just sits there, matching the sequence of the mRNA that gets made (with one chemical difference: DNA uses T, RNA uses U).

So the template strand is the actual DNA sequence that RNA polymerase (the enzyme that does the copying) reads and uses to build a complementary mRNA molecule.

Note

Think of the template strand as the negative of a photograph. The mRNA is the print made from that negative. The coding strand is like a second print that happens to look almost identical to the final photo — but it wasn't used to make it.

Why It Matters

Accuracy: The cell must read the correct strand. If it read the wrong one, the mRNA would be nonsense and the protein would be wrong or non-functional.
Direction: RNA polymerase reads the template strand in the 3' to 5' direction, and builds mRNA in the 5' to 3' direction. This is a fixed rule — like reading a sentence left to right.
Gene regulation: Which strand is the template for a given gene is fixed. But different genes on the same DNA molecule may use different strands as their template. So a single stretch of DNA can contain genes pointing in opposite directions.

A Simple Example

Suppose a short stretch of DNA has these two strands:

Strand A: ATGCGT
Strand B: TACGCA

If Strand A is the template, the mRNA made will be complementary to it: UACGCA (remember, U replaces T in RNA).

If Strand B is the template, the mRNA will be complementary to B: AUGCGU.

The two mRNAs are completely different. So the cell must know, for each gene, which strand is the template. That information is encoded in the DNA sequence itself — in the promoter region that tells RNA polymerase where to start and which way to go.

Important

The template strand is not the same as the coding strand. The mRNA sequence is identical to the coding strand (with U instead of T), but it is complementary to the template strand. This is a common confusion — the mRNA looks like the coding strand, but it was built from the template strand.

What NCERT Says …

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AUG serves a dual role: it codes for the amino acid methionine and functions as the initiation (start) codon in both prokaryotes and eukaryotes.

The genetic code is universal, meaning the same codons specify the same amino acids across nearly all living organisms. Among the 64 possible triplet codons, AUG occupies a special position because it performs two critical functions simultaneously.

First, AUG codes for methionine. Whenever this codon appears in the coding sequence of mRNA, the translation machinery brings in methionine (or its modified form, N-formylmethionine in prokaryotes, at the start position). This is a straightforward assignment in the genetic code table: AUG → methionine.

Second, AUG acts as the initiation or start codon. Translation cannot begin at just any point on an mRNA molecule; the ribosome must identify where the coding sequence starts. AUG provides that signal. When the ribosome encounters AUG in the correct context (near the ribosome-binding site in prokaryotes, or after scanning from the 5' cap in eukaryotes), it recognizes this as the point to begin protein synthesis. The first amino acid incorporated is therefore methionine, though in prokaryotes this initial methionine carries a formyl group (fMet) that is often removed later.

This dual role holds true across the tree of life. Both prokaryotic and eukaryotic cells use AUG as the start codon and recognize it as the codon for methionine. The universality of the genetic code means that whether you examine a bacterial cell or a human cell, AUG performs these same functions. …

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Q.Which of the following is true with respect to AUG?

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