AQA · GCSE · Biology

    Course version: 8461 · Version from Sept 2018

    The long strands of DNA consist of alternating sugar and phosphate…

    Full specification wording

    DNA structure (biology only): The long strands of DNA consist of alternating sugar and phosphate sections. Attached to each sugar is one of the four bases.

    A DNA strand is a polymer in which sugar and phosphate groups alternate to form a backbone.

    Checked against the AQA 8461 specification (opens in a new tab) on .

    • Point 6 of 19
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    A DNA strand is a polymer in which sugar and phosphate groups alternate to form a backbone. Each sugar in the backbone is deoxyribose, and attached to each sugar is one of four bases: adenine (A), thymine (T), cytosine (C) or guanine (G). The bases project from the backbone and form hydrogen bonds with bases on the opposite strand, always A with T and C with G. This arrangement produces the double helix. For example, a short section of one strand might read 5′–phosphate–sugar(A)–phosphate–sugar(T)–phosphate–sugar(G)–3′, with the opposite strand running antiparallel and complementary. The sugar-phosphate backbone gives DNA strength and stability, while the base sequence stores genetic information.

    What earns the marks

    • A DNA strand has a backbone of alternating sugar and phosphate groups.
    • The sugar in DNA is deoxyribose.
    • Each sugar is attached to one of four bases: adenine, thymine, cytosine or guanine.
    • Bases on opposite strands pair specifically: A with T and C with G.
    • The two strands are held together by hydrogen bonds between complementary bases, forming a double helix.

    Common mistakes and how to put them right

    • Thinking that the bases form the backbone.

      Put it right: The backbone is made of alternating sugar and phosphate groups, with bases attached to the sugars.

    • Confusing the sugar in DNA with ribose.

      Put it right: DNA contains deoxyribose, while RNA contains ribose.

    • Mixing up base pairing, such as pairing A with G.

      Put it right: The correct pairs are A with T and C with G.

    Exam tips

    • When drawing or describing DNA, show the sugar-phosphate backbone as alternating units and label the bases attached to the sugars.
    • State the base pairing rules clearly: A pairs with T, and C pairs with G.
    • Use the term complementary to describe the relationship between the two strands.

    Try the lesson

    The start of the DNA structure (biology only): The long strands of DNA consist of alternating sugar and phosphate sections. Attached to each sugar is one of the four bases. lesson, made from checked teaching notes for this specification point.

    Have a go first

    In a DNA strand, the sugar and phosphate sections are joined together in a pattern. Which word describes this pattern?

    Listen

    A DNA strand is a polymer in which sugar and phosphate groups alternate to form a backbone. Each sugar in the backbone is deoxyribose, and attached to each sugar is one of four bases: adenine (A), thymine (T), cytosine (C) or guanine (G). The bases project from the backbone and form hydrogen bonds with bases on the opposite strand, always A with T and C with G. This arrangement produces the double helix. For example, a short section of one strand might read 5′–phosphate–sugar(A)–phosphate–sugar(T)–phosphate–sugar(G)–3′, with the opposite strand running antiparallel and complementary. The sugar-phosphate backbone gives DNA strength and stability, while the base sequence stores genetic information. A strong answer earns marks for these points. A DNA strand has a backbone of alternating sugar and phosphate groups. The sugar in DNA is deoxyribose. Each sugar is attached to one of four bases: adenine, thymine, cytosine or guanine. Bases on opposite strands pair specifically: A with T and C with G. Exam tip: When drawing or describing DNA, show the sugar-phosphate backbone as alternating units and label the bases attached to the sugars.

    See it

    A top-down flowchart. The first box states that the sugar-phosphate backbone consists of alternating sugar and phosphate. An arrow leads to a box saying a base is attached to each sugar, which branches into four boxes: adenine (A), thymine (T), cytosine (C) and guanine (G). The adenine and thymine boxes both lead to a box saying A pairs with T, and the cytosine and guanine boxes both lead to a box saying C pairs with G. Both pairing boxes lead to a box saying hydrogen bonds form between complementary bases, which leads to a final box labelled double helix.
    Shows the alternating sugar-phosphate backbone with a base attached to each sugar, the four bases, the complementary pairing rules and the resulting double helix.

    Key words

    Say what each word means before you turn the card.

    sugar-phosphate backbone

    Try answering aloud or in your head before turning the card.

    complementary base pairing

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    double helix

    Try answering aloud or in your head before turning the card.

    deoxyribose

    Try answering aloud or in your head before turning the card.

    See every definition
    sugar-phosphate backbone
    The long strands of DNA consist of alternating sugar and phosphate sections, and this backbone gives DNA strength and stability.
    complementary base pairing
    Bases on opposite strands pair specifically, always A with T and C with G.
    double helix
    The arrangement in which the bases project from the backbone and form hydrogen bonds with bases on the opposite strand produces the double helix.
    deoxyribose
    Each sugar in the backbone is deoxyribose, and attached to each sugar is one of four bases.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first box states that the sugar-phosphate backbone consists of alternating sugar and phosphate. An arrow leads to a box saying a base is attached to each sugar, which branches into four boxes: adenine (A), thymine (T), cytosine (C) and guanine (G). The adenine and thymine boxes both lead to a box saying A pairs with T, and the cytosine and guanine boxes both lead to a box saying C pairs with G. Both pairing boxes lead to a box saying hydrogen bonds form between complementary bases, which leads to a final box labelled double helix.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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