AQA GCSE Biology Foundation (8461), Foundation tier · Inheritance, variation and evolution › Reproduction
Practise DNA structure. 2 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
DNA is a polymer of nucleotides, and the order of its bases codes for the order of amino acids in a protein. This is in GCSE Biology only, not in Combined Science.
Key facts
Nucleotide = sugar + phosphate + one base
Four bases: A, C, G, T
DNA strand: alternating sugar and phosphate, with a base on each sugar
3 bases = the code for 1 amino acid
Notes
DNA is a polymer of nucleotides
diagram
DNA is a polymer made from repeating units called nucleotides.
Each nucleotide is a common sugar and a phosphate group, with one of four bases attached to the sugar.
The four bases are A, C, G and T. You do not need to know the structures of the bases.
Each long strand has alternating sugar and phosphate sections, with a base attached to each sugar.
DNA is a polymer of nucleotides. Every three bases code for one amino acid.
The genetic code
A sequence of three bases is the code for one particular amino acid.
The order of bases controls the order in which amino acids are assembled to make a particular protein.
How to answer each type of question
Describe the structure of DNA
3 to 4 marksGrade 5
Say DNA is a polymer of repeating nucleotides.
Describe a nucleotide: sugar, phosphate and one of four bases (A, C, G, T).
Describe the strand: alternating sugar and phosphate, with a base on each sugar.
Example. Describe the structure of a DNA molecule. [4 marks]
Show the model answerHide the model answer
DNA is a polymer made of repeating nucleotides (1). Each nucleotide contains a sugar, a phosphate group and a base (1). There are four different bases: A, C, G and T (1). Each strand has alternating sugar and phosphate sections with a base attached to each sugar (1).
Don’t lose marks
Saying each base codes for an amino acid. It takes a sequence of three bases.
Saying a nucleotide contains amino acids. A nucleotide is a sugar, a phosphate and a base.
More tips
Memory tricks
Number of amino acids = number of bases ÷ 3.
Exam technique
Write 'a sequence of three bases codes for one amino acid', not 'three bases code for a protein'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
Name the four bases in DNAThe four bases are A, C, G and T.
Grade 5
Describe the structure of a nucleotideA common sugar and a phosphate group, with one of four bases attached to the sugar.
Grade 5
Describe DNA as a polymer of nucleotidesLong strands of alternating sugar and phosphate sections, with a base attached to each sugar.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
DNA is a polymer. Give the letters of the four bases found in DNA.
A, C, G and T
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
DNA is a polymer.
(a) What are the repeating units that make up DNA called? Tick (✓) one box.[1]
Amino acids
Fatty acids
Glucose molecules
Nucleotides
(b) What does each of these repeating units contain? Tick (✓) one box.[1]
A phosphate group and an enzyme
A sugar, a phosphate group and one of four bases
An amino acid and a base
Glucose and a fatty acid
(c) Give the letters of the four bases found in DNA.[1]
(d) How many bases code for one amino acid?[1]
(e) What makes up the long strands of a DNA molecule?[1]
Show the answer and mark scheme
(a)Answer: Nucleotides
(b)Answer: A sugar, a phosphate group and one of four bases
(c)Answer: A, C, G and T
A, C, G, T (all four)
(d)Answer: 3
3 / three
(e)Answer: Alternating sugar and phosphate sections
alternating sugar and phosphate (sections / groups)
Question 2Medium6 marks
{fig} shows part of one strand of a DNA molecule.
[object Object]
(a) Name the parts labelled X, Y and Z in {fig}.[3]
(b) What is the unit inside the dashed box labelled W called?[1]
(c) Describe how the nucleotides are joined together to make one strand of DNA.[2]
Show the answer and mark scheme
(a)Answer: X: phosphate (group); Y: sugar; Z: base
X: phosphate
Y: sugar
Z: base
(b)Answer: A nucleotide
nucleotide
(c)Answer: The sugar of one nucleotide is joined to the phosphate of the next, forming an alternating sugar–phosphate strand with a base attached to each sugar
sugar of one nucleotide joins to the phosphate of the next / alternating sugar and phosphate