If the base sequence of a codon in mRNA is 5'-AUG-3', the sequence of tRNA pairing with it must be: (a) 5'-UAC

NCERT Exemplar · Q25

Q. If the base sequence of a codon in mRNA is 5'-AUG-3', the sequence of tRNA pairing with it must be:

(a) 5'-UAC-3'

(b) 5'-CAU-3'

(d) 5'-GUA-3'

Yanam BieapMCQ· 1mImportance★★★★★

41% · 39/96 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

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 …

Start your 14-day free trial to unlock the full solution →

The tRNA anticodon pairs with the mRNA codon in an antiparallel and complementary fashion, following specific base pairing rules.

To understand the sequence of tRNA that pairs with an mRNA codon, we must first recall the fundamental process of protein synthesis, specifically translation, where the genetic information encoded in mRNA is decoded to build a protein. This process involves two key types of RNA: messenger RNA (mRNA) and transfer RNA (tRNA).

The mRNA carries the genetic code from the DNA in the nucleus to the ribosomes in the cytoplasm. This code is read in sequences of three nucleotides, known as codons. Each codon specifies a particular amino acid or signals the termination of protein synthesis. In our question, the mRNA codon is given as 5'-AUG-3'. This specific codon is significant because it serves as the start codon, initiating translation and coding for the amino acid methionine.

Transfer RNA (tRNA) molecules act as adaptors. They have two crucial sites: one that binds to a specific amino acid and another that contains a three-nucleotide sequence called an anticodon. The anticodon is responsible for recognizing and binding to a complementary codon on the mRNA molecule during translation. This recognition is based on the principle of base pairing.

Important

The pairing between the mRNA codon and the tRNA anticodon is always antiparallel and complementary.

Let's break down the base pairing rules:

Adenine (A) always pairs with Uracil (U).
Guanine (G) always pairs with Cytosine (C).

Now, let's apply these rules to the given mRNA codon: 5'-AUG-3'.

For the tRNA anticodon to pair with this mRNA codon, it must be:

Antiparallel: If the mRNA codon is read 5' to 3', the tRNA anticodon will bind in the 3' to 5' direction.
Complementary: Each base in the mRNA codon will pair with its specific complementary base on the tRNA anticodon. …

Start your 14-day free trial to unlock the full solution →

←Previous questionQ24 — The RNA polymerase holoenzyme transcribes: (a) the promoter, structura…→Next questionQ26 — The amino acid attaches to the tRNA at its: (a) 5'-end (b) 3'-end (c) …

Q.If the base sequence of a codon in mRNA is 5'-AUG-3', the sequence of tRNA pairing with it must be:

🎓Unlock everything free for 14 days

Unlock everything free for 14 days