Q.(a)
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Imagine you have a zipper. Each tooth on one side fits perfectly only with a specific tooth on the other side — a left tooth always clicks into a right tooth, and they lock together. If you tried to force a left tooth into another left tooth, the zipper would jam. That simple "lock-and-key" fit is the everyday intuition behind complementary base pairing.
In biology, the "zipper" is the DNA molecule, which is made of two long strands twisted together. Each strand is a chain of smaller units called nucleotides. Every nucleotide contains one of four chemical "letters" — Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). The two strands are held together by weak bonds between these letters, but they don't pair randomly. The rule is strict and unchanging:
- A (Adenine) always pairs with T (Thymine)
- G (Guanine) always pairs with C (Cytosine)
This is the complementary base pairing rule. It means that if you know the sequence of letters on one strand, you can instantly write the sequence on the other strand. For example, if one strand reads A–T–G–C, the opposite strand must read T–A–C–G.
The NCERT Class 12 Biology textbook (Chapter 6, "Molecular Basis of Inheritance") states this rule exactly as: "Adenine pairs with Thymine (A = T) and Guanine pairs with Cytosine (G ≡ C)." The double lines (=) and triple lines (≡) indicate the number of hydrogen bonds — A–T has two bonds, G–C has three bonds. This difference in bond strength matters for how DNA unwinds, but the pairing rule itself is what you need to remember.
Why does this matter? Three big reasons:
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Replication: When a cell divides, the two strands of DNA separate. Each strand acts as a template. Using the complementary rule, the cell builds a new partner strand for each old strand. The result is two identical DNA molecules — one for each new cell. Without this rule, copying would be random and life couldn't pass on genetic information accurately.
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Transcription: To make proteins, a cell first makes a temporary copy of a gene in the form of mRNA. The mRNA is built using the complementary rule, but with one change: wherever there is an A on the DNA, the mRNA puts a U (Uracil) instead of T. So the pairing becomes A–U, T–A, G–C, C–G. This ensures the genetic message is faithfully transcribed. …
Part (a): Watson & Crick relied on Wilkins–Franklin's X-ray diffraction data and Chargaff's rule; the double helix is right-handed, antiparallel, complementary (A=T, G≡C), with a 3.4 nm pitch of ~10 bp.
Part (b): template 3'-TAC…AAT-5', mRNA 5'-AUG…UUA-3'; bacteria keep the mRNA unspliced, humans remove the GUA introns to give a 10-codon mRNA coding for 10 amino acids.
(i) The Watson–Crick (1953) double-helix model rested on two key findings:
- The X-ray diffraction studies of DNA by Maurice Wilkins and Rosalind Franklin, which showed DNA to be a regular, helical, two-stranded fibre and gave its dimensions (diameter, pitch, base spacing).
- Chargaff's equivalence rule, established by Erwin Chargaff: for a double-stranded DNA, A = T and G = C (and thus purines = pyrimidines). This ratio pointed directly to specific base pairing.
(ii) Salient features of the double helix (any three):
- DNA consists of two polynucleotide chains with a sugar–phosphate backbone on the outside and nitrogen bases projecting inward; the two chains are antiparallel (5'→3' and 3'→5') and coil into a right-handed helix.
- The chains are joined by hydrogen bonds between complementary bases — adenine pairs with thymine by two H-bonds (A=T) and guanine pairs with cytosine by three H-bonds (G≡C) — so the two strands are exactly complementary.
- The helix is regular: pitch ≈ 3.4 nm containing about 10 base pairs per turn, so consecutive base pairs are 0.34 nm apart, and the uniform pairing of one purine with one pyrimidine keeps the diameter constant (~2 nm).
The uniform A=T / G≡C pairing (a purine always opposite a pyrimidine) is what gives the helix its constant width and its complementary, semi-conservatively replicable strands.
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.
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.
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 …
Part (a): Watson & Crick relied on Wilkins–Franklin's X-ray diffraction data and Chargaff's rule; the double helix is right-handed, antiparallel, complementary (A=T, G≡C), with a 3.4 nm pitch of ~10 bp.
Part (b): template 3'-TAC…AAT-5', mRNA 5'-AUG…UUA-3'; bacteria keep the mRNA unspliced, humans remove the GUA introns to give a 10-codon mRNA coding for 10 amino acids.
The coding strand is 5'-ATG ACC GTA TTT TCT GTA GTG CCC GTA CTT CAG GCA TTA-3'.
(i) RNA polymerase reads the template strand, which is complementary and antiparallel to the coding strand:
- Template: 3'-TAC TGG CAT AAA AGA CAT CAC GGG CAT GAA GTC CGT AAT-5'
The mRNA is complementary to the template, so it matches the coding strand with U in place of T:
- mRNA: 5'-AUG ACC GUA UUU UCU GUA GUG CCC GUA CUU CAG GCA UUA-3'
(ii) The triplet GUA is stated to be an intron.
- (1) In a bacterium: prokaryotes have no introns/splicing (transcription and translation are coupled), so the transcript is used as is: 5'-AUG ACC GUA UUU UCU GUA GUG CCC GUA CUU CAG GCA UUA-3' …
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