1.5Unit 1: Lifestyle, Transport, Genes and Health

SNAB Nucleic Acids — IAL Edexcel Modular Revision Notes & Topical Questions

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1.5 Nucleic Acids — IAL EDEXCEL MODULAR notes: Describe the structure of nucleotides, DNA and RNA and the bonds that hold them together.Nucleotide = the monomer of DNA and RNA, made of a pentose sugar, a nitrogenous base and a phosphate group.DNA nucleotide — it contains deoxyrib…

RNA nucleotide
2′ hydroxyl group
Pairing rule
Polynucleotide
Phosphodiester bond
Sugar-phosphate backbone

Section 1 of 10Nucleotide structure

Nucleotide structure

Nucleotide

Nucleotide the monomer of DNA and RNA, made of a pentose sugar, a nitrogenous base and a phosphate group.

DNA nucleotide

DNA nucleotide it contains deoxyribose, which has hydrogen at the 2′ position.
The base is adenine (A), cytosine (C), guanine (G) or thymine (T).

RNA nucleotide

RNA nucleotide it contains ribose, which has a hydroxyl group at the 2′ position.
The base is adenine, cytosine, guanine or uracil (U), and RNA never contains thymine.

2′ hydroxyl group

2′ hydroxyl group it makes RNA more susceptible to hydrolysis, so DNA is the storage molecule and RNA is a shorter-lived transport molecule.

Section 2 of 10Purines and pyrimidines

Type	Structure	Bases
Purine	Double ring	Adenine, guanine
Pyrimidine	Single ring	Cytosine, thymine, uracil

Pairing rule
- a purine always pairs with a pyrimidine, which keeps the width of the helix constant.

Section 3 of 10The phosphodiester bond

Polynucleotide

Polynucleotide a polymer of many nucleotides joined in a long chain.

Phosphodiester bond

Phosphodiester bond covalent bond formed by condensation between the phosphate group of one nucleotide and the pentose sugar of the next.

It is made of a phosphate group and two ester bonds.
It links the 3′ carbon of one sugar to the phosphate attached to the 5′ carbon of the next nucleotide.

Sugar-phosphate backbone

Sugar-phosphate backbone the chain of alternating phosphate groups and pentose sugars formed by many phosphodiester bonds.

Bases

Bases they project out from the backbone.

Section 4 of 10DNA structure

DNA structure

Two strands

DNA is two polynucleotide strands lying side by side in opposite directions, so they are antiparallel.
One strand runs 5′ to 3′ and the other 3′ to 5′, with a 5′ end and a 3′ end on each strand.

Backbone

Backbone each strand has alternating deoxyribose and phosphate joined by phosphodiester bonds.

Complementary base pairing

Complementary base pairing hydrogen bonds join the bases of the two strands.
Adenine pairs with thymine by two hydrogen bonds.
Guanine pairs with cytosine by three hydrogen bonds.
The amounts of A and T are equal, and the amounts of C and G are equal.

Double helix

Double helix the two strands twist to form a three-dimensional helix.

Bond locations

Bond locations phosphodiester bonds hold nucleotides in a strand and hydrogen bonds hold the two strands together.

Section 5 of 10RNA structure

RNA structure

Single strand

RNA is one polynucleotide strand, relatively short compared with DNA.

Sugar and base

Sugar and base it contains ribose and uracil in place of deoxyribose and thymine.

Backbone

Backbone alternating ribose and phosphate are joined by phosphodiester bonds between the 5′ and 3′ carbons, with bases projecting sideways.

Type	Role
mRNA	Transcript copy of a gene that codes for a specific polypeptide
tRNA	Carries amino acids to the ribosome
rRNA	Forms part of ribosomes

Section 6 of 10DNA compared with RNA

Feature	DNA	RNA
Pentose sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Two, antiparallel, in a double helix	One
Length	Long	Relatively short
Role	Stores genetic information	Transfers genetic information and builds proteins

Section 7 of 10More from the specification

Section 8 of 10More from the specification

Section 9 of 10Key terms

Term	Meaning
Nucleotide	Monomer of nucleic acids made of a pentose sugar, a base and a phosphate group.
Phosphodiester bond	Covalent bond linking nucleotides, formed by condensation between phosphate and sugar.
Antiparallel	Running in opposite directions, as the two strands of DNA.
Complementary base pairing	Specific pairing of A with T (or U) and C with G.
Purine	A base with a double ring structure, adenine or guanine.
Pyrimidine	A base with a single ring structure, cytosine, thymine or uracil.
Polynucleotide	A polymer of many nucleotides.
Antiparallel strands	The two polynucleotide chains in DNA run in opposite directions,(5' to 3' and 3' to 5')
Complementary base pairing	Purines pair specifically with pyrimidines: adenine forms two hydrogen bonds with thymine, and cytosine forms three hydrogen bonds with guanine.(or uracil in RNA)
Phosphodiester link	Strong covalent bond linking the phosphate group at the 5' carbon of one pentose sugar to the 3' carbon of the neighbouring pentose sugar in the polynucleotide backbone.
Semi-conservative replication	The process in which the double helix unwinds and each original template strand directs the synthesis of a complementary new strand, ensuring genetic continuity.

Section 10 of 10Exam technique

Saying RNA contains thymine — RNA has uracil instead.

Saying hydrogen bonds join nucleotides in a strand — phosphodiester bonds join them and hydrogen bonds join the two strands.

Pairing adenine with cytosine — adenine pairs with thymine (uracil in RNA) and guanine pairs with cytosine.

Saying purines pair with purines — a purine always pairs with a pyrimidine.

Last updated: 15 October 2026