Double Helical Structure of DNA

Secondary structure: (Double helical structure of DNA or Watson and Crick model of DNA)

It was proposed by J.D. Watson and F.H.C. Crick in 1953, and for this work, they were awarded the Nobel Prize in 1962. This model of DNA is based on the X-ray photographic technique of Wilkins & Franklin.

1. DNA is a polynucleotide, double stranded, Right-handedly coiled, and helical. The backbone of each strand consists of sugar-phosphate units.
2. The two strands of DNA double helix are anti-parallel, complementary, and not identical.
3. The diameter of the helix is about 2.0 nm or 20Å.
4. The length of each coil (pitch) or the distance between any two successive turns of the helix is 3.4 nm or 34Å.
5. There are 10 nitrogen base pairs in each turn. The distance between any two adjacent base pairs is 0.34 nm.
6. The nitrogen base pairs of the two strands are linked together through hydrogen bonds.
7. A purine base of one strand is always paired with a pyrimidine base of the other strand.
8. Guanine and cytosine are paired with three hydrogen bonds (G≡C) and Adenine and Thymine are paired through two hydrogen bonds (A=T).
9. Unlike DNA, RNA typically consists of a single strand.

NOTE: G≡C base pairs form three hydrogen bonds, making them more stable than A=T base pairs with two hydrogen bonds. The DNA molecule with higher number of G≡C pairs will have higher melting point than the DNA molecule with higher number of A=T pairs (i.e., lower number of G≡C pairs).
Because of lower stability and lower melting temperatures (T_m) of A=T-rich segments of DNA are sometimes used in biological processes, such as the opening of the DNA double helix for transcription or replication.

Comparison: DNA vs. RNA

Feature	DNA (Deoxyribonucleic Acid)	RNA (Ribonucleic Acid)
Structure	Double-stranded helix; anti-parallel strands.	Usually single-stranded; can form complex 3D shapes.
Pentose Sugar	Deoxyribose (Lacks -OH group at 2' carbon).	Ribose (Contains -OH group at 2' carbon).
Nitrogenous Bases	Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).	Adenine (A), Guanine (G), Cytosine (C), and Uracil (U).
Base Pairing	A=T (2 H-bonds), G≡C (3 H-bonds).	A=U (2 H-bonds), G≡C (3 H-bonds).
Function	Long-term storage of genetic information; inheritance.	Transfers genetic code for protein synthesis; catalytic roles.
Location	Primarily in the nucleus (and mitochondria/chloroplasts).	Nucleolus, cytoplasm, and ribosomes.
Stability	High; chemically stable and resistant to alkaline hydrolysis.	Low; more reactive and easily degraded by enzymes.

UPSC Prelims: Science & Tech PYQs

UPSC Prelims 2022

Q1. Consider the following statements: DNA Barcoding can be a tool to:

1. Assess the age of a plant or animal.
2. Distinguish among species that look alike.
3. Identify undesirable animal or plant materials in processed foods.

Which of the statements given above is/are correct?

(a) 1 only
(b) 3 only
(c) 1 and 2
(d) 2 and 3

Correct Answer: (d) 2 and 3

Note: DNA Barcoding is used for identification/taxonomy. Age assessment (Statement 1) is typically done via Carbon Dating or Dendrochronology.

UPSC Prelims 2019

Q2. With reference to the recent developments in science, which one of the following statements is not correct?

(a) Functional chromosomes can be created by joining segments of DNA taken from cells of different species.
(b) Pieces of artificial functional DNA can be created in laboratories.
(c) A piece of DNA taken out from an animal cell can be made to replicate outside a living cell in a laboratory.
(d) Cells taken out from plants and animals can be made to undergo cell division in laboratory petri dishes.

Correct Answer: (a)

While DNA segments can be joined (Recombinant DNA), creating a fully functional, stable chromosome from different species is currently not achievable as per standard biological definitions.

Chhatrapati Shahu Ji Maharaj University Kanpur (CSJMU) 2018, 2016, 2014, 2012