Describe chromosome structure
A replicated chromosome contains DNA associated with histone proteins and two sister chromatids joined at a centromere, with telomeres at the ends.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain cell-cycle concepts and image interpretation. Written practice is self-checked; it does not demonstrate practical microscopy competence or replace teacher-reviewed assessment.
Before you begin
- Recognise DNA as genetic material and use the terms cell, nucleus and chromosome accurately.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What joins sister chromatids in a replicated chromosome?
What you will learn
- Describe chromosome structure using DNA, histone proteins, sister chromatids, a centromere and telomeres
- Distinguish a chromosome from one of its sister chromatids
DNA is packaged with proteins
In eukaryotic chromosomes, DNA is associated with histone proteins. Packaging allows a long DNA molecule to be organised within the nucleus. When a cell is not dividing, this material is less condensed; before nuclear division, chromosomes condense and become easier to distinguish.
A chromosome is not made only of protein and is not the same thing as a gene. Genes are sections of DNA; a chromosome contains one long DNA molecule and associated proteins.
Replication produces sister chromatids
After DNA replication, a duplicated chromosome consists of two sister chromatids. Each chromatid contains one DNA molecule associated with histones. The sister chromatids carry the same genetic information unless a copying error has occurred.
The centromere is the region joining the sister chromatids and is where spindle attachment is organised. Telomeres are protective regions at the ends of chromosomes. A chromosome’s shape in a diagram depends on whether it has replicated and condensed.
Pause and explain
Where are telomeres found?
Worked example
A diagram shows two identical arms joined at a constricted region, with protective caps at each end. Label the sister chromatids, centromere and telomeres.
Show the worked solution
- The two arms are sister chromatids.
- The constriction joins them at the centromere.
- The end regions are telomeres.
Answer The two identical arms are sister chromatids. They are joined at the centromere, and the protective end regions are telomeres. Each chromatid contains DNA associated with histone proteins.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Name five structures or components in the syllabus description of a chromosome.
Give me a hint
Include the genetic material, associated protein and replicated-chromosome features.
Compare my reasoning
- DNA is associated with histone proteins.
- A replicated chromosome has sister chromatids joined at a centromere, with telomeres at the ends.
The syllabus description includes DNA, histone proteins, sister chromatids, a centromere and telomeres at chromosome ends.
Look for these in your work
- All five named
- Chromosome ends and joining region distinguished
A student labels each arm of an X-shaped chromosome as a chromosome. Correct the description.
Give me a hint
Describe the whole replicated structure and its two copies.
Compare my reasoning
- The whole X-shaped structure is one replicated chromosome.
- Its two arms are sister chromatids joined at a centromere.
The whole X-shaped structure is one replicated chromosome. Its two arms are sister chromatids joined at the centromere; telomeres are at their ends.
Look for these in your work
- Whole chromosome identified
- Chromatids and centromere distinguished
Explain how the terms DNA, histone and chromatid relate in a replicated chromosome.
Give me a hint
State what each term refers to.
Compare my reasoning
- Each chromatid contains a DNA molecule.
- DNA is associated with histone proteins; sister chromatids are joined at a centromere.
Each chromatid contains DNA associated with histone proteins. In a replicated chromosome, two sister chromatids are joined at a centromere.
Look for these in your work
- DNA and histone relation stated
- Two chromatids and centromere stated
Common mistakes
- Calling one chromatid a complete pair of sister chromatids
- Calling the centromere the chromosome end
- Treating telomeres as genes or as structures that join the chromatids
What can you explain now?
What joins sister chromatids in a replicated chromosome?
Compare with the explanation
The centromere
The centromere region joins the sister chromatids.
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Make it stick
What does each term describe: DNA, histone, sister chromatid, centromere and telomere?
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