Unit 1.5: Nucleic Acids
and Their Functions
Part 1: Nucleotides and ATP
1. What is a nucleotide?
Key definition
A nucleotide is the monomer of nucleic acids. Nucleotides join to form polynucleotides (DNA, RNA),
and single nucleotide derivatives include ATP and ADP.
• Each nucleotide has three parts, joined by condensation reactions: a phosphate
group, a pentose sugar, and a nitrogen-containing organic base.
The three parts
Part Details
Phosphate group Contains phosphorus and oxygen. Forms the sugar-phosphate backbone, is
involved in ATP energy transfer, removed by hydrolysis, added during
phosphorylation
Pentose sugar 5-carbon sugar. DNA = deoxyribose, RNA = ribose. Deoxyribose has one less
oxygen than ribose
Nitrogenous base Contains nitrogen; the base sequence stores genetic information. Bases: A,
C, G, T (DNA) and A, C, G, U (RNA). RNA has no thymine
2. Forming a polynucleotide
• The phosphate group of one nucleotide joins to the sugar of the next, in a
condensation reaction releasing water, forming a phosphodiester bond.
Bond Where it is found
Phosphodiester bond Between sugar and phosphate groups, within one strand
(the sugar-phosphate backbone)
, Hydrogen bond Between complementary bases on opposite strands
Say it like this in the exam
Nucleotides are joined by condensation reactions between the phosphate group of one nucleotide and
the pentose sugar of the next, forming phosphodiester bonds.
3. Purines and pyrimidines
Group Ring structure Bases
Purines Double ring Adenine, Guanine
Pyrimidines Single ring Cytosine, Thymine, Uracil
A purine always pairs with a pyrimidine, keeping the width of the DNA molecule constant.
4. ATP: adenosine triphosphate
Key definition
ATP is the universal energy currency: it provides immediate energy for metabolic reactions in all
living things.
• Contains: the base adenine, the sugar ribose, and three phosphate groups.
"Adenosine" = adenine + ribose; "triphosphate" = three phosphates.
• Structure shown as: adenine–ribose–P–P–P. ATP hydrolysis breaks the bond
between the middle and terminal phosphate.
5. ATP hydrolysis and formation
Reaction Equation Enzyme Effect
Hydrolysis ATP + water → ADP + Pi + ATPase Terminal phosphate removed,
energy energy released
Formation ADP + Pi + energy → ATP + ATP synthase A phosphate is added, using
(phosphorylation) water energy from respiration or
photosynthesis
and Their Functions
Part 1: Nucleotides and ATP
1. What is a nucleotide?
Key definition
A nucleotide is the monomer of nucleic acids. Nucleotides join to form polynucleotides (DNA, RNA),
and single nucleotide derivatives include ATP and ADP.
• Each nucleotide has three parts, joined by condensation reactions: a phosphate
group, a pentose sugar, and a nitrogen-containing organic base.
The three parts
Part Details
Phosphate group Contains phosphorus and oxygen. Forms the sugar-phosphate backbone, is
involved in ATP energy transfer, removed by hydrolysis, added during
phosphorylation
Pentose sugar 5-carbon sugar. DNA = deoxyribose, RNA = ribose. Deoxyribose has one less
oxygen than ribose
Nitrogenous base Contains nitrogen; the base sequence stores genetic information. Bases: A,
C, G, T (DNA) and A, C, G, U (RNA). RNA has no thymine
2. Forming a polynucleotide
• The phosphate group of one nucleotide joins to the sugar of the next, in a
condensation reaction releasing water, forming a phosphodiester bond.
Bond Where it is found
Phosphodiester bond Between sugar and phosphate groups, within one strand
(the sugar-phosphate backbone)
, Hydrogen bond Between complementary bases on opposite strands
Say it like this in the exam
Nucleotides are joined by condensation reactions between the phosphate group of one nucleotide and
the pentose sugar of the next, forming phosphodiester bonds.
3. Purines and pyrimidines
Group Ring structure Bases
Purines Double ring Adenine, Guanine
Pyrimidines Single ring Cytosine, Thymine, Uracil
A purine always pairs with a pyrimidine, keeping the width of the DNA molecule constant.
4. ATP: adenosine triphosphate
Key definition
ATP is the universal energy currency: it provides immediate energy for metabolic reactions in all
living things.
• Contains: the base adenine, the sugar ribose, and three phosphate groups.
"Adenosine" = adenine + ribose; "triphosphate" = three phosphates.
• Structure shown as: adenine–ribose–P–P–P. ATP hydrolysis breaks the bond
between the middle and terminal phosphate.
5. ATP hydrolysis and formation
Reaction Equation Enzyme Effect
Hydrolysis ATP + water → ADP + Pi + ATPase Terminal phosphate removed,
energy energy released
Formation ADP + Pi + energy → ATP + ATP synthase A phosphate is added, using
(phosphorylation) water energy from respiration or
photosynthesis
