Describe nucleotide structure and ATP
A nucleotide contains a pentose sugar, phosphate group and nitrogenous base; ATP is a phosphorylated nucleotide.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain DNA, RNA and protein synthesis. Written practice is self-checked; it does not replace teacher review or practical assessment.
Before you begin
- Recognise that DNA stores genetic information and that proteins are made from amino acids.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which three components make up a nucleotide?
What you will learn
- Describe the three components of a nucleotide
- Recognise ATP as a phosphorylated nucleotide without drawing structural formulae
Build a nucleotide from three components
A nucleotide contains a pentose sugar, a phosphate group and a nitrogenous base. A DNA nucleotide contains deoxyribose; an RNA nucleotide contains ribose. Nitrogenous bases vary among nucleotides.
Nucleotides are monomers of nucleic-acid chains. In a diagram, label the sugar, phosphate and base separately; the base is not part of the sugar–phosphate backbone.
ATP is also a nucleotide
ATP contains adenine, ribose and three phosphate groups, so it is a phosphorylated nucleotide. The syllabus asks candidates to recognise ATP in this context; structural formulae are not required.
Do not confuse the three phosphate groups in ATP with the single-phosphate component of a nucleotide in a DNA or RNA chain.
Pause and explain
Which sugar occurs in an RNA nucleotide?
Worked example
A diagram labels a pentose, a phosphate and a nitrogenous base. Identify the molecule represented and state which sugar occurs in an RNA nucleotide.
Show the worked solution
- The three parts are a nucleotide.
- RNA nucleotides contain ribose.
Answer The diagram represents a nucleotide. An RNA nucleotide contains ribose, a phosphate group and a nitrogenous base.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the structure of one nucleotide.
Give me a hint
Name all three components.
Compare my reasoning
- A nucleotide has a pentose sugar and phosphate.
- It also has a nitrogenous base.
A nucleotide consists of a pentose sugar, a phosphate group and a nitrogenous base.
Look for these in your work
- All three components named
- Components distinguished
State how an RNA nucleotide differs from a DNA nucleotide in its sugar.
Give me a hint
Recall the two pentose names.
Compare my reasoning
- RNA contains ribose.
- DNA contains deoxyribose.
An RNA nucleotide contains ribose, whereas a DNA nucleotide contains deoxyribose.
Look for these in your work
- RNA sugar correct
- DNA contrast correct
A student calls ATP a nitrogenous base because it contains adenine. Correct the classification.
Give me a hint
Identify ATP’s complete nucleotide structure.
Compare my reasoning
- ATP contains adenine and ribose.
- It also has three phosphate groups.
ATP is a phosphorylated nucleotide: it contains adenine, ribose and three phosphate groups. Adenine is its nitrogenous base, not the whole ATP molecule.
Look for these in your work
- ATP correctly classified
- Adenine role distinguished
Common mistakes
- Leaving out the phosphate group
- Calling a nitrogenous base a sugar
- Drawing ATP with one phosphate when the question asks for ATP
What can you explain now?
Which three components make up a nucleotide?
Compare with the explanation
Pentose sugar, phosphate and nitrogenous base
These are the three components.
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Make it stick
What three parts form a nucleotide, and how does the sugar distinguish DNA from RNA nucleotides?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.