12 min readConcept sheet2 or 3-mark questions on composition, nucleoside vs nucleotide, DNA vs RNA and functions; sometimes a 5-mark question; also a NEET favourite

Nucleic acids: DNA and RNA

Nucleic acids are polynucleotides: chains of sugar, phosphate and base units. DNA is a double helix with A-T and G-C pairing; RNA is usually single-stranded.

NucleosideBase + pentose sugar; the base is joined at C1′ of the sugar

NucleotideNucleoside + phosphoric acid; the phosphate is joined at C5′ of the sugar

SugarsDNA: β-D-2-deoxyribose. RNA: β-D-ribose

Base pairing in DNAAdenine pairs with thymine, guanine pairs with cytosine (Watson and Crick double helix)

Composition of nucleic acids

The particles in the cell nucleus that carry heredity are the chromosomes. They are made of proteins and another kind of biomolecule, the nucleic acids. There are two kinds: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are long chains of nucleotides, so they are also called polynucleotides. Complete hydrolysis of DNA or RNA gives three kinds of product: a pentose sugar, phosphoric acid, and nitrogen-containing heterocyclic bases.

DNA and RNA compared

Feature	DNA	RNA
Sugar	β-D-2-deoxyribose	β-D-ribose
Bases	Adenine (A), guanine (G), cytosine (C), thymine (T)	Adenine (A), guanine (G), cytosine (C), uracil (U)
Strands	Two, wound into a double helix	Usually one
Kinds	One	Three: messenger RNA (m-RNA), ribosomal RNA (r-RNA), transfer RNA (t-RNA)
Main job	Stores genetic information and passes it to daughter cells	Carries out protein synthesis

From base to polynucleotide

1
Base + sugar → nucleoside
The base is joined at the C1′ position of the sugar.
2
Nucleoside + phosphoric acid → nucleotide
The phosphate is joined at the C5′ position of the sugar.
3
Nucleotide + nucleotide → dinucleotide
They are joined by a phosphodiester linkage between C5′ of one sugar and C3′ of the next.
4★
Many nucleotides → polynucleotide (a nucleic acid)
The sequence of nucleotides is the primary structure; the double helix of DNA is its secondary structure.

★ key step

How to draw a nucleotide and the DNA double helix

Nucleotide: draw a pentagon for the sugar with the ring O at the top. Join a box labelled 'base' at C1′ (right corner) and a circle labelled 'P' to C5′ (the CH₂ group outside the ring on the left).
Double helix: James Watson and Francis Crick proposed that two polynucleotide chains wind round each other.
Draw two ribbons twisting round a common axis. These are the sugar-phosphate backbones, on the outside.
Draw short rungs between the ribbons for the bases, which point inwards and are joined by hydrogen bonds.
Label the rungs to show complementary pairing: A with T and G with C. So the base sequence of one strand fixes the sequence of the other.

Using base pairing

Problem

(a) One strand of a DNA molecule has the base sequence 5′-ATGCCTAG-3′. Write the sequence of the other strand. (b) A DNA sample contains 30% adenine. Find the percentage of each of the other bases.

Working

(a) Pair each base: A with T, T with A, G with C, C with G.
Complementary strand: 3′-TACGGATC-5′ (the two chains run in opposite directions).
(b) A pairs with T, so T = A = 30%.
A + T = 60%, so G + C = 100% − 60% = 40%.
G pairs with C, so G = C = 40% ÷ 2 = 20% each.

Answer

(a) 3′-TACGGATC-5′ (b) thymine 30%, guanine 20%, cytosine 20%.

Biological functions of nucleic acids

DNA is the chemical basis of heredity: it stores the genetic information of the cell.
DNA is responsible for keeping the identity of each species over millions of years.
DNA can copy itself (self-duplication) during cell division, so identical DNA strands pass to the daughter cells.
Nucleic acids direct protein synthesis. The message for each protein is in the DNA, but the proteins are actually made by the various RNA molecules in the cell.

In real life

Forensic labs in India use DNA fingerprinting to identify people in crime cases and to settle parentage disputes. It works because the base sequence of each person's DNA is unique, except in identical twins.

Exam angle

2 marks: “Differentiate between a nucleoside and a nucleotide.” Nucleoside: base + pentose sugar (base at C1′). Nucleotide: nucleoside + phosphate group (at C5′).

3 marks: “Write three structural differences between DNA and RNA.” Sugar: deoxyribose in DNA, ribose in RNA. Bases: thymine in DNA, uracil in RNA. Strands: DNA is a double helix, RNA is usually single-stranded.

2 marks: “What is complementary base pairing? A DNA sample has 20% guanine. Find the percentage of adenine.” A always pairs with T and G with C. G = C = 20%, so A + T = 60% and A = 30%.

Remember it

DNA: Deoxyribose, Double strand, thymine. RNA: Ribose, one strand, Uracil. Pairs: A-T and G-C.