AQA · A-Level · Biology

    Course version: 7402 · Version from Sept 2016

    Together a DNA molecule and its associated proteins form a chromosome.

    Full specification wording

    DNA, genes and chromosomes: Together a DNA molecule and its associated proteins form a chromosome.

    A chromosome consists of one long, linear DNA molecule wrapped around histone proteins.

    Checked against the AQA 7402 specification (opens in a new tab) on .

    • Point 3 of 11
    • 2 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    A chromosome consists of one long, linear DNA molecule wrapped around histone proteins. During most of the cell cycle, it exists as extended chromatin and cannot be resolved with a light microscope. In G₁, each chromosome is a single structure. After DNA replication in S phase, it consists of two identical sister chromatids joined at a centromere. During prophase, chromatin supercoils and condenses, making chromosomes visible. Therefore, seeing condensed chromosomes in a micrograph confirms the cell is dividing. Homologous chromosomes are pairs carrying the same genes at the same loci, one from each parent, though alleles may differ and sex chromosomes may differ in size.

    What earns the marks

    • Stating a eukaryotic chromosome consists of DNA and associated histone proteins.
    • Explaining that after S phase, a chromosome consists of two identical sister chromatids.
    • Identifying that sister chromatids are joined at a centromere.
    • Defining homologous chromosomes as a pair carrying the same genes at the same loci.

    Common mistakes and how to put them right

    • Saying a chromosome is made only of genes or DNA, omitting the essential histone proteins.

    • Assuming all visible chromosomes always have two chromatids.

      Put it right: During anaphase, sister chromatids separate to become individual chromosomes.

    • Confusing a homologous pair (one maternal, one paternal) with a pair of identical sister chromatids.

    Exam tips

    • A complete definition of a eukaryotic chromosome must include both DNA and histone proteins.
    • When counting chromosomes, count the centromeres; a chromosome may consist of one or two chromatids depending on the cell cycle stage.

    Try the lesson

    The start of the DNA, genes and chromosomes: Together a DNA molecule and its associated proteins form a chromosome. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which combination of molecules forms a chromosome?

    Listen

    A chromosome consists of one long, linear DNA molecule wrapped around histone proteins. During most of the cell cycle, it exists as extended chromatin and cannot be resolved with a light microscope. In G₁, each chromosome is a single structure. After DNA replication in S phase, it consists of two identical sister chromatids joined at a centromere. During prophase, chromatin supercoils and condenses, making chromosomes visible. Therefore, seeing condensed chromosomes in a micrograph confirms the cell is dividing. Homologous chromosomes are pairs carrying the same genes at the same loci, one from each parent, though alleles may differ and sex chromosomes may differ in size. A strong answer earns marks for these points. stating a eukaryotic chromosome consists of DNA and associated histone proteins. explaining that after S phase, a chromosome consists of two identical sister chromatids. identifying that sister chromatids are joined at a centromere. defining homologous chromosomes as a pair carrying the same genes at the same loci. Exam tip: A complete definition of a eukaryotic chromosome must include both DNA and histone proteins.

    See it

    A flowchart running from top to bottom. The first box, labelled DNA molecule, points to a box labelled DNA plus associated histone proteins, which points to a box labelled Chromosome. From the Chromosome box three branches lead out. One branch goes to a box labelled G1: single structure. A second branch goes to a box labelled After S phase: two identical sister chromatids, which then points to a box labelled Joined at a centromere. A third branch goes to a box labelled Prophase: chromatin supercoils and condenses, which then points to a box labelled Chromosomes visible under a light microscope.
    It shows how DNA and histone proteins form a chromosome, and how that chromosome appears as a single structure in G1 or as two sister chromatids joined at a centromere after S phase, becoming visible when chromatin condenses in prophase.

    Key words

    Say what each word means before you turn the card.

    sister chromatids

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

    homologous pairs

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

    See every definition
    sister chromatids
    After DNA replication in S phase, a chromosome consists of two identical sister chromatids joined at a centromere.
    homologous pairs
    Pairs carrying the same genes at the same loci, one from each parent, though alleles may differ and sex chromosomes may differ in size.

    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 flowchart running from top to bottom. The first box, labelled DNA molecule, points to a box labelled DNA plus associated histone proteins, which points to a box labelled Chromosome. From the Chromosome box three branches lead out. One branch goes to a box labelled G1: single structure. A second branch goes to a box labelled After S phase: two identical sister chromatids, which then points to a box labelled Joined at a centromere. A third branch goes to a box labelled Prophase: chromatin supercoils and condenses, which then points to a box labelled Chromosomes visible under a light microscope.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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