Q.Describe the following-
(क) Punnet square
(ख) Histones OR Write any four salient features of the double helix structure of DNA.

Correct base pairing: G ≡ C (three H-bonds) and A = T (two H-bonds).

In double-stranded DNA the bases pair by complementary hydrogen bonding (Chargaff/Watson-Crick rules). Guanine pairs specifically with Cytosine forming THREE hydrogen bonds (shown as G ≡ C), and Adenine pairs with Thymine forming TWO hydrogen bonds (A = T). Checking the …

DNA follows Chargaff/Watson-Crick complementary base pairing: A-T (2 H-bonds) and G-C (3 H-bonds).

In the DNA double helix, the purine Adenine always pairs with the pyrimidine Thymine through two hydrogen bonds, while Guanine pairs with Cytosine through three hydrogen bonds. This strict complementarity keeps the diameter of the double helix uniform and is t …

DNA is a uniform double helix — the inter-strand distance never changes along its length.

According to the Watson–Crick model, DNA is a right-handed double helix made of two antiparallel polynucleotide chains coiled around a common axis. The two strands are held together by hydrogen bonds between complementary bases (A=T, G≡C), and because every base pair is a purine paired with a pyrimidine (which keeps the width of each 'rung' of the ladder essentially identical, about 20 Å), the …

The two strands of DNA are joined by hydrogen bonds between complementary base pairs, not by covalent, ionic or phosphodiester linkages.

Within a single strand, adjacent nucleotides are joined by strong covalent phosphodiester bonds, forming the sugar-phosphate backbone. The two separate strands, however, are held together across the helix by weak hydrogen bonds between complementary nitrogen bases: adenine pairs with thymine through two hydrogen bonds and guanine pairs with cytosine thr …

In DNA, the purine guanine always base-pairs with the pyrimidine cytosine (three H-bonds); adenine pairs with thymine (two H-bonds).

DNA's two strands are held together by hydrogen bonding between specific, complementary nitrogenous base pairs (Watson-Crick base-pairing rules): a purine always pairs with a pyrimidine of a complementary shape. Adenine (A) pairs with Thymine (T) throug …

G≡C pairing has 3 hydrogen bonds; A=T pairing has 2.

In the double-stranded DNA helix, the purine guanine (G) always pairs with the pyrimidine cytosine (C) through three hydrogen bonds, making the G-C pair relatively more stable than the A-T pair (which is held together by only two hydrogen bonds). This complementary base pairing (Char …

Using the base-pairing rule (A↔T, G↔C) and the antiparallel nature of DNA, the complementary strand read 5'→3' is GCATGCATGCAT.

Given strand: 5'-A T G C A T G C A T G C-3'

Step 1 — Pair each base (A with T, G with C):

T A C G T A C G T A C G (this is the complementary sequence, but it is aligned in the 3'→5' direction relative to the new strand, because DNA strands are antiparallel).

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DNA's nucleotide arrangement is deduced from X-ray crystallography, so the answer is (B).

The precise arrangement of atoms and the double-helical structure of DNA cannot be seen even with an electron microscope. Instead, it is worked out by X-ray crystallography (X-ray diffraction). The famous X-ray diffraction photographs of DNA obtained by Rosalind Franklin and Maurice Wilkins provided the key data (helical shape, spacing) that Watson and Crick used to build their double-helix model.

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Heating DNA to ~82–90°C denatures (melts) it into single strands, so the answer is (B).

The two strands of the DNA double helix are held together by hydrogen bonds between complementary base pairs (A=T two bonds, G≡C three bonds). When DNA is heated to its melting temperature (around 82–90°C, depending on GC content), these hydrogen bonds break and the double helix unwinds and separates into two single strands. This is called denaturation (DNA melting). On slow cooling …

Consecutive base pairs in B-DNA are 0.34 nm apart; one full turn (10 bp) spans 3.4 nm.

According to the Watson and Crick double helix model of DNA (1953), the two polynucleotide strands are antiparallel and coiled in a right-handed helix. The distance between two adjacent (consecutive) base pairs along the helix axis is 0.34 nm, and one complete turn of the helix contains 10 base pairs, giving a helical pitch of 3. …

T = U is the incorrect base pair.

Complementary base pairing follows Chargaff's rule: G pairs with C by three hydrogen bonds (G ≡ C), A pairs with T in DNA by two bonds (A = T, so T = A is correct), and A pairs with U in RNA (A = U is correct). Thymine and …