describe the structure of a chromosome, limited to: DNA, histone proteins, sister chromatids, centromere, telomeres

Structure of a Chromosome – Cambridge IGCSE / A‑Level

Learning Objective

Describe the structure of a chromosome, covering:

  • DNA – nucleotide composition and double‑helix

  • Histone proteins and nucleosomes

  • Higher‑order chromatin fibres

  • Sister chromatids

  • Centromere (primary constriction) and kinetochore

  • Telomeres

1. DNA – Nucleotides & the Double Helix

  • Nucleotide structure: deoxyribose sugar + phosphate group + one of four bases (A, T, G, C).

  • Backbone: phosphodiester bonds link the 3’‑OH of one sugar to the 5’‑phosphate of the next.

  • Double helix:

    • Two antiparallel strands wound around each other.

    • Base‑pairing: A ↔ T (2 H‑bonds), G ↔ C (3 H‑bonds).

2. Histone Proteins & Nucleosomes

  • Core octamer: 2 × H2A, 2 × H2B, 2 × H3, 2 × H4.

  • Nucleosome: ≈ 147 bp of DNA wrap ~1.65 turns around the octamer.

  • Linker DNA (≈ 20‑80 bp) connects successive nucleosomes; histone H1 binds this linker and stabilises the structure.

3. Higher‑Order Chromatin Fibres

Term used in the syllabus to describe the progressive folding of nucleosome‑packed DNA.

  • 30 nm fibre (solenoid): nucleosomes coil together, aided by H1, forming a thicker fibre.

  • Looped domains: the 30 nm fibre forms loops anchored to a protein scaffold (condensin).

  • Condensed chromosome: during mitosis condensin further compacts the loops, producing the visible X‑shaped chromosome.

4. Anatomy of a Mitotic Chromosome

4.1 Sister Chromatids

  • Each chromosome consists of two identical DNA molecules produced during S‑phase.

  • Both chromatids contain a complete set of genes, ensuring genetic uniformity in the daughter cells.

  • Cohesin protein complexes hold the sister chromatids together along their length; cohesin is cleaved at the onset of anaphase, allowing separation.

4.2 Centromere (Primary Constriction) & Kinetochore

  • Centromere: a specialised DNA region (often rich in repetitive satellite sequences) that appears as the primary constriction on a mitotic chromosome.

  • Kinetochore: a protein complex that assembles on the centromere and provides the attachment site for spindle micro‑tubules.

  • Essential for accurate chromosome movement during mitosis and meiosis.

4.3 Telomeres

  • Short tandem repeats at each chromosome end (e.g., TTAGGG in vertebrates).

  • Protect chromosome ends from degradation and from fusing with other chromosomes.

  • Prevent loss of genetic material during DNA replication; repeats are replenished by the enzyme telomerase in many eukaryotes.

5. Summary Table

ComponentComposition / StructureKey Function
DNADeoxyribose‑phosphate backbone + A, T, G, C bases; antiparallel double helixStores the genetic information
Histone proteinsCore octamer (H2A, H2B, H3, H4) + linker H1Package DNA into nucleosomes; enable higher‑order folding
Higher‑order chromatin fibres30 nm solenoid → looped domains → condensed chromosome (condensin)Progressive compaction of DNA for mitosis; gives the X‑shaped chromosome
Sister chromatidsTwo identical DNA molecules joined by cohesinEach contains a complete set of genes; ensure each daughter cell receives a full genome
Centromere (primary constriction) & kinetochoreRepetitive DNA region + protein complex (kinetochore)Attachment point for spindle fibres; directs chromosome movement
TelomeresRepeating TTAGGG (vertebrates) sequencesProtect chromosome ends; prevent loss of genetic material during replication

6. Suggested Diagram

Linear illustration of a mitotic chromosome (X‑shaped). From left to right: telomeric caps, a stretch of nucleosome‑packed DNA (showing the 30 nm fibre), the primary constriction (centromere) with a labelled kinetochore, and two sister chromatids joined by cohesin rings. The diagram should also indicate looped domains that arise from the 30 nm fibre.