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
Component
Composition / Structure
Key Function
DNA
Deoxyribose‑phosphate backbone + A, T, G, C bases; antiparallel double helix
Stores the genetic information
Histone proteins
Core octamer (H2A, H2B, H3, H4) + linker H1
Package DNA into nucleosomes; enable higher‑order folding
Higher‑order chromatin fibres
30 nm solenoid → looped domains → condensed chromosome (condensin)
Progressive compaction of DNA for mitosis; gives the X‑shaped chromosome
Sister chromatids
Two identical DNA molecules joined by cohesin
Each contains a complete set of genes; ensure each daughter cell receives a full genome
Centromere (primary constriction) & kinetochore
Repetitive DNA region + protein complex (kinetochore)
Attachment point for spindle fibres; directs chromosome movement
Telomeres
Repeating TTAGGG (vertebrates) sequences
Protect 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.