Nucleic Acids: Components & Structure

CHENucleic Acids & Vitamins

Nucleic acids (DNA, RNA) store and express genetic information; this subtopic tests their building blocks (base + sugar + phosphate), the nucleoside-vs-nucleotide distinction, base composition, and Chargaff/base-pairing rules. Knowing which bases and sugar belong to DNA vs RNA is essential.

Building-block hierarchy

\text{base} + \text{sugar} = \text{nucleoside}; \quad \text{nucleoside} + \text{phosphate} = \text{nucleotide}

A nucleotide is the phosphate ester (at C5) of a nucleoside.

Chargaff's rule (DNA)

[A] = [T], \quad [G] = [C] \implies [\text{purines}] = [\text{pyrimidines}]

A

T

G

C

are the mole fractions of the four bases in double-stranded DNA.

Base-pair hydrogen bonds

A\text{-}T\ (2\ \text{H-bonds}), \quad G\text{-}C\ (3\ \text{H-bonds})

More H-bonds make

G\text{-}C

pairs more stable than

A\text{-}T

A nucleoside = base + sugar (e.g. adenosine); a nucleotide = base + sugar + phosphate (e.g. adenosine monophosphate, AMP) — a nucleotide is a phosphate ester of a nucleoside.
Purines (two fused rings): adenine (A) and guanine (G), in both DNA and RNA; pyrimidines (one ring): cytosine (C) in both, thymine (T) only in DNA, uracil (U) only in RNA.
Sugar is 2-deoxy-D-ribose in DNA and D-ribose in RNA; nucleotides link through phosphodiester bonds between C3 of one sugar and C5 of the next, forming the sugar-phosphate backbone.
Chargaff's rule: in DNA the amount of A equals T and G equals C (so purines equal pyrimidines); base pairing is A–T (2 H-bonds) and G–C (3 H-bonds), holding the two antiparallel strands together.
DNA is a double helix (Watson–Crick); most RNA is single-stranded; the two DNA strands run antiparallel (one $5'\rightarrow3'$ , the other $3'\rightarrow5'$ ).
Three RNAs in protein synthesis: mRNA carries the code from DNA (template), tRNA brings specific amino acids to the ribosome, rRNA is the structural/catalytic component of ribosomes.
In the base it is the ring nitrogen that links to C1 of the sugar (an N-glycosidic bond); the phosphate esterifies the C5 (and the next C3) hydroxyl.
The bases pair specifically by size/shape: a purine always pairs with a pyrimidine (A with T/U, G with C), keeping the helix diameter constant.
Complete hydrolysis of a nucleic acid yields the nitrogenous bases, the pentose sugar, and phosphoric acid ( $H_3PO_4$ ).
Functions: DNA stores hereditary information and directs protein synthesis and its own replication; RNA chiefly translates that information into proteins.
Naming: adenine + ribose = adenosine (RNA nucleoside); adenine + deoxyribose = deoxyadenosine (DNA nucleoside); adding phosphate gives the corresponding nucleotide (e.g. AMP, dAMP).
The 2-deoxyribose of DNA lacks the C2 $\text{-}OH$ found in RNA's ribose — the single chemical difference that underlies DNA's greater stability.

Where the marks go

Swapping the definitions: a nucleoSide has NO phosphate (base+sugar), a nucleoTide has the phosphate (base+sugar+phosphate).
Putting thymine in RNA or uracil in DNA — thymine is DNA-only, uracil is RNA-only; both pair with adenine.
Writing the sugar wrongly: DNA has 2-deoxy-D-ribose, RNA has D-ribose; the missing C2-OH defines deoxyribose.
Calling the inter-nucleotide link a glycosidic bond — the bases attach by N-glycosidic bonds, but successive nucleotides are joined by phosphodiester bonds (C3-O-P-O-C5).
Forgetting that the two DNA strands are antiparallel, which is essential when writing a complementary strand (read it $3'\rightarrow5'$ opposite the given $5'\rightarrow3'$ ).

How the board asks it

Distinguishnucleoside vs nucleotide; building-block hierarchy
Distinguish between a nucleoside and a nucleotide, giving one example of each.
Define / statechargaff's rule; base composition of DNA
State Chargaff's rule for the base composition of DNA.
Predict the productcomplementary base pairing; antiparallel strands
A single strand of DNA has the base sequence $5'\text{-}ATGCGT\text{-}3'$ . Write the base sequence of its complementary strand.
Give reasonsmissing C2-OH of 2-deoxyribose
Account for the greater stability of DNA as compared to RNA on the basis of the sugar present in each.
Identify / classifycomplete hydrolysis products of a nucleic acid
Name the products obtained on complete hydrolysis of a molecule of DNA.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.