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Exam questions · Biology · What Happens in Cells

The Structure of DNA

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

    State the shape of a DNA molecule and the number of strands it has.

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    Model answer

    A double helix, with two strands.

    Mark scheme

    • Double helix — 1 mark
    • Two strands — 1 mark
  2. 2 Name [3 marks]

    Figure 1 shows a DNA molecule. Name the parts shown at A, B and C.

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

    A is a strand (of DNA). B is a pair of bases. C is a gene.

    Mark scheme

    • A: strand — 1 mark
    • B: pair of bases — 1 mark
    • C: gene — 1 mark
  3. 3 Describe [3 marks]

    Describe the structure of a DNA molecule.

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    Model answer

    DNA is a polymer. It is made of two strands. The strands are twisted together into a double helix and are joined by pairs of bases.

    Mark scheme

    • DNA is a polymer — 1 mark
    • Two strands — 1 mark
    • Twisted into a double helix, joined by base pairs — 1 mark
  4. 4 Describe [2 marks]

    Describe the difference between a gene and a chromosome.

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    Model answer

    A gene is a section of DNA that codes for a protein. A chromosome is a long DNA molecule that carries many genes.

    Mark scheme

    • Gene: a section of DNA that codes for a protein — 1 mark
    • Chromosome: a long DNA molecule carrying many genes — 1 mark
  5. 5 Compare [3 marks]

    Compare the position of the DNA in a bacterial cell with the position of the DNA in an animal cell.

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    Model answer

    In an animal cell the DNA is in chromosomes inside the nucleus. In a bacterial cell there is no nucleus, and the DNA is a loop in the cytoplasm. A bacterial cell may also have plasmids.

    Mark scheme

    • Animal: chromosomes in the nucleus — 1 mark
    • Bacterial: no nucleus, loop of DNA in the cytoplasm — 1 mark
    • Bacteria may also have plasmids — 1 mark
  6. 6 Describe [4 marks]

    Describe how DNA can be extracted from strawberries.

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    Model answer

    Mash the strawberries to break up the tissue. Add detergent and salt to break open the cells. Filter the mixture. Add ice-cold ethanol carefully, and the DNA appears as a white stringy solid that can be lifted out.

    Mark scheme

    • Mash the fruit — 1 mark
    • Add detergent and salt to break open the cells — 1 mark
    • Filter — 1 mark
    • Add cold ethanol so the DNA appears as a white solid — 1 mark
  7. 7 Explain [3 marks]

    Explain why the DNA in a human cell is able to fit inside the nucleus even though it is very long.

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    Model answer

    The DNA is a very long thin molecule. It is twisted into a double helix, and the molecules are coiled tightly into chromosomes, so they take up very little space.

    Mark scheme

    • The DNA molecule is very thin — 1 mark
    • It is twisted into a double helix — 1 mark
    • It is coiled up tightly into chromosomes — 1 mark

Quick check

  1. 1

    What is the shape of a DNA molecule?

    1. AA single straight chain
    2. BA double helix
    3. CA flat sheet
    4. DA hollow sphere
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    B: A double helix

    DNA is a double helix, which is a twisted ladder.

  2. 2

    How many strands are in a DNA molecule?

    1. ATwo
    2. BOne
    3. CThree
    4. DFour
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    A: Two

    DNA has two strands wound round each other.

  3. 3

    What is a gene?

    1. AA whole chromosome
    2. BA type of protein
    3. CA section of DNA that codes for a protein
    4. DThe nucleus
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    C: A section of DNA that codes for a protein

    A gene is a section of DNA that codes for a protein.

  4. 4

    Where is DNA found in a bacterial cell?

    1. AIn the nucleus
    2. BIn the mitochondria
    3. CIn the cell wall
    4. DIn the cytoplasm as a loop, and in plasmids
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    D: In the cytoplasm as a loop, and in plasmids

    A bacterial cell has a loop of DNA in the cytoplasm, and may also have plasmids.

  5. 5

    What is a polymer?

    1. AA single small molecule
    2. BA large molecule made of many smaller units
    3. CA type of enzyme
    4. DA part of a cell
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    B: A large molecule made of many smaller units

    A polymer is a large molecule made of many smaller units joined together.

  6. 6

    What do genes code for?

    1. AProteins
    2. BSugars
    3. CLipids
    4. DWater
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    A: Proteins

    Genes are sections of DNA that code for proteins.

  7. 7

    What is the genome?

    1. AA single gene
    2. BA type of cell
    3. CA protein
    4. DAll of the genetic material of an organism
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    D: All of the genetic material of an organism

    The genome is all of the genetic material of an organism.

  8. 8

    Why is cold ethanol used in the DNA extraction?

    1. ATo break down the cell membranes
    2. BTo filter the mixture
    3. CDNA is not soluble in it
    4. DTo warm the mixture
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    C: DNA is not soluble in it

    DNA does not dissolve in ethanol, so it appears as a white solid.