Practise DNA structure. 14 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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DNA is a polymer. Give the letters of the four bases found in DNA.
A, C, G and T
The sequence of bases on one strand of a short section of DNA is: A T G C C T A G Write the sequence of bases on the complementary strand.
T A C G G A T C
The part of a gene that codes for a protein contains 1380 bases. Calculate the number of amino acids in the protein.
460
DNA carries the genetic code. How many strands make up one DNA molecule?
2 (two)
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 2Medium5 marks
A DNA molecule is made of two strands joined by pairs of complementary bases.
(a) The sequence of bases on one strand of a short section of DNA is: A T G C C T A G Write the sequence of bases on the complementary strand.[2]
(b) In a sample of DNA, 28% of the bases are A. Calculate the percentage of the bases that are G.[2]
(c) Explain why the number of A bases in a DNA molecule is always equal to the number of T bases.[1]
Show the answer and mark scheme
(a)Answer: T A C G G A T C
first four bases correct: T A C G
last four bases correct: G A T C
(b)Answer: 22%
T = 28% so G + C = 100 − 56 = 44 (%)
G = 22 (%)
(c)Answer: A always pairs with T on the opposite strand
A always pairs / links with T (on the opposite strand)
Question 3Hard8 marks
Genes carry the instructions for making proteins such as enzymes.
(a) Describe how a protein is made from the instructions in a gene, and explain how a change in the DNA of the gene could stop an enzyme from working.[6]
(b) Most mutations do not change the protein, or change it only slightly so that its function is not affected. Suggest two reasons why.[2]
Show the answer and mark scheme
(a)Answer: Level 3 answers describe protein synthesis in sequence and link a change in base sequence to a change in amino acid sequence, shape of the active site and loss of function
the gene is in the DNA in the nucleus
a copy of the gene / template (allow mRNA) is made and moves out of the nucleus to a ribosome
the ribosome uses the template: each sequence of three bases codes for one amino acid
carrier molecules bring specific amino acids to the ribosome
amino acids are joined in the order set by the bases to form a protein chain
the chain folds into a unique shape
a mutation changes the sequence of bases
so a different amino acid (or amino acids) is placed in the chain
the protein folds into a different shape, changing the shape of the active site
the substrate no longer fits the active site, so the enzyme cannot catalyse the reaction
Marked with levels of response: the full level descriptors are in the app.
(b)Answer: Many mutations happen in non-coding DNA; a changed triplet may still code for the same amino acid; a changed amino acid may not affect the shape of the protein
the mutation is in a non-coding part of the DNA
the new triplet codes for the same amino acid
the new amino acid does not change the shape / folding of the protein (or active site)