Chromosomes

Overview

Chromosomes are highly compacted structures composed of linear DNA molecules wound around proteins. They carry the entire genetic blueprint of an organism, organised into thousands of individual genes. Human cells typically contain 46 chromosomes, which become visible under the microscope only during cell division. Understanding chromosome structure, classification, and function is fundamental to medical genetics, diagnosis of chromosomal disorders, and cell biology.

What Are They?

• Linear, DNA-protein complexes found in the nucleus
• Composed of DNA coiled around histone proteins, forming chromatin
• Contain many genes, each with coding and non-coding regions
• Become microscopically visible only during M phase (mitosis or meiosis)
• Human chromosomes vary in size from 50 to 250 million base pairs

Chromosome Number

• Humans have 46 chromosomes (23 pairs):
  • One set of 23 from each biological parent
  • 22 pairs = autosomes
  • 1 pair = sex chromosomes
• Chromosome number varies by species
  • e.g. Humans = 46, Chimpanzees = 48
  • Chromosome count does not correlate with species complexity

Autosomes vs XY

Sex Chromosomes

• Determine biological sex
• XX = Female (X from each parent)
• XY = Male (X from mother, Y from father)

Autosomes

• The remaining 44 chromosomes
• Not involved in sex determination
• Numbered from 1–22 by size and banding

Chromosome Structure and Banding

Banding Patterns

• Chromosomes show light and dark bands after staining (e.g. Giemsa)
• Reflect differences in gene activity:
  • Heterochromatin (dark bands):
    • Gene-poor
    • Densely packed
    • Inactive regions (genes switched off)
  • Euchromatin (light bands):
    • Gene-rich
    • Loosely packed
    • Actively transcribed genes

Chromosome Pairing

• Homologous chromosomes are paired by length and banding
• Paired chromosomes are numbered 1–22, plus the X and Y chromosomes

Chromosome Arms and Regions

• Short arm = ‘p’ arm
• Long arm = ‘q’ arm
• Arms are subdivided into:
  • Regions (two per arm)
  • Bands numbered outward from the centromere (Band 10)

Key Structures

• Centromere: Central region where sister chromatids join
• Telomere: Protective, repetitive DNA at the ends of each arm
  • Prevents degradation and fusion with other chromosomes

Chromosome Shapes

1. Metacentric – Centromere near the centre; equal arm lengths
2. Submetacentric – Centromere offset; one arm shorter
3. Acrocentric – Centromere near one end
4. Telocentric – Centromere at the terminal end (not found in humans)

Chromosomal Nomenclature

Normal Karyotypes

• 46, XX = Normal female
• 46, XY = Normal male

Abnormal Karyotypes

• 47, XX, +10 = Female with trisomy 10
• 45, XY, -22 = Male missing one copy of chromosome 22

Common Abbreviations

• ‘+’ / ‘–’ → Gain/loss of chromosome
• ‘cen’ → Centromere
• ‘del’ → Deletion
• ‘dup’ → Duplication
• ‘inv’ → Inversion
• ‘ins’ → Insertion
• ‘der’ → Derivative chromosome
• ‘mar’ → Marker chromosome
• ‘rob’ → Robertsonian translocation
• ‘mat’ / ‘pat’ → Maternal / Paternal origin

Karyotyping

1. Cell extraction (commonly white blood cells)
2. Centrifugation to isolate the cell pellet
3. Culture cells to amplify numbers
4. Mitotic arrest using spindle inhibitors (e.g. colchicine)
5. Protease digestion and Giemsa staining
6. Dropping cells onto a slide flattens the nucleus → spreads chromosomes
7. Image analysis pairs and aligns chromosomes by software
   → Final result: Human Karyotype for diagnostic or research use

Summary

Chromosomes are tightly packed DNA structures that organise and store genetic material in human cells. Each carries genes and regulatory regions essential to development, function, and heredity. Their structural features, banding patterns, and karyotyping help identify chromosomal abnormalities and genetic diseases. For a broader context, see our Genetics & Cancer Overview page.