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Exam questions · Biology · DNA and the Genome

The Structure of DNA

  • 7 exam questions
  • 20 marks
  • 8 quick checks
  1. 1 Name [2 marks]

    Name the four bases found in DNA.

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    Model answer

    A, C, G and T (adenine, cytosine, guanine and thymine).

    Mark scheme

    • Any two bases correct — 1 mark
    • All four bases correct — 1 mark
  2. 2 Describe [3 marks]

    Describe the structure of one nucleotide of DNA.

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    Model answer

    A nucleotide has a sugar and a phosphate group. One of four bases is attached to the sugar.

    Mark scheme

    • A sugar — 1 mark
    • A phosphate group — 1 mark
    • One of four bases attached to the sugar — 1 mark
  3. 3 Name [3 marks]

    The diagram shows part of a DNA molecule. Name the parts labelled A, B and C.

    A DNA ladder with three parts marked A, B and C.
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    Model answer

    A is a phosphate group. B is a sugar. C is a base.

    Mark scheme

    • A: phosphate (group) — 1 mark
    • B: sugar — 1 mark
    • C: base — 1 mark
  4. 4 Calculate [2 marks]

    A gene has 1 800 bases. Every three bases code for one amino acid. Calculate the number of amino acids in the protein.

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    Model answer

    600 amino acids

    Mark scheme

    • \(1800 \div 3\) — 1 mark
    • 600 — 1 mark
  5. 5 Explain [3 marks]

    Explain how the order of bases in a gene controls the protein that is made.

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    Model answer

    A sequence of three bases is the code for one amino acid. The order of the bases therefore controls the order in which the amino acids are assembled. A different order of amino acids makes a different protein.

    Mark scheme

    • Three bases code for one amino acid — 1 mark
    • The order of the bases controls the order of the amino acids — 1 mark
    • The order of the amino acids decides which protein is made — 1 mark
  6. 6 Describe [4 marks]

    Describe the structure of the DNA polymer.

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    Model answer

    DNA is a polymer made of repeating nucleotide units. Each nucleotide has a sugar, a phosphate group and one of four bases attached to the sugar. The long strands have alternating sugar and phosphate sections, with a base attached to each sugar. There are four different bases, A, C, G and T.

    Mark scheme

    • A polymer made from repeating nucleotides — 1 mark
    • Each nucleotide is a sugar, phosphate and base — 1 mark
    • Alternating sugar and phosphate sections in the strands — 1 mark
    • Four different bases A, C, G and T — 1 mark
  7. 7 Give [3 marks]

    One strand of a DNA molecule has the base sequence T T A G C. Give the base sequence of the complementary strand and state the two pairing rules you used.

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    Model answer

    The complementary strand is A A T C G. A is always linked to T, and C is always linked to G.

    Mark scheme

    • A A T C G — 1 mark
    • T is linked to A — 1 mark
    • C is linked to G — 1 mark

Quick check

  1. 1

    How many different bases are found in DNA?

    1. ATwo
    2. BThree
    3. CFour
    4. DFive
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    C: Four

    DNA contains four bases: A, C, G and T.

  2. 2

    Which two parts alternate along the backbone of a DNA strand?

    1. ABases and sugars
    2. BSugars and phosphate groups
    3. CPhosphate groups and bases
    4. DAmino acids and sugars
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    B: Sugars and phosphate groups

    The long strands consist of alternating sugar and phosphate sections.

  3. 3

    To which part of a nucleotide is the base attached?

    1. AThe phosphate group
    2. BThe amino acid
    3. CThe next base
    4. DThe sugar
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    D: The sugar

    Each nucleotide has a sugar and a phosphate group, and the base is attached to the sugar.

  4. 4

    How many bases code for one amino acid?

    1. AThree
    2. BOne
    3. CTwo
    4. DFour
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    A: Three

    A sequence of three bases is the code for a particular amino acid.

  5. 5

    A gene is 1 500 bases long. About how many amino acids will its protein have?

    1. A50
    2. B150
    3. C500
    4. D4 500
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    C: 500

    Three bases code for one amino acid, so 1 500 divided by 3 is 500.

  6. 6

    What do the four different nucleotides in DNA differ in?

    1. ATheir sugar
    2. BTheir base
    3. CTheir phosphate group
    4. DTheir length
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    B: Their base

    They have the same sugar and phosphate group, and differ only in the base.

  7. 7

    In the complementary strands of DNA, which base is always linked to C?

    1. AG
    2. BA
    3. CT
    4. DC
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    A: G

    C is always linked to G on the opposite strand.

  8. 8

    One strand of DNA reads A C C G T. What is the complementary strand?

    1. AA C C G T
    2. BT C C G A
    3. CG T T C A
    4. DT G G C A
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    D: T G G C A

    A pairs with T, C with G, C with G, G with C and T with A, which gives T G G C A.