Centromere
The primary constriction where sister chromatids join and the kinetochore forms. Its position defines metacentric (central), submetacentric (off-centre) and acrocentric (near one end) chromosomes.
Core concepts
What makes each chromosome distinguishable, and how cytogeneticists describe it.
Telomere
TTAGGG repeat caps at pter and qter that protect chromosome ends. Subtelomeric deletions are often invisible on routine banding.
p and q arms
p = short arm, q = long arm. Bands are numbered outward from the centromere: region, band, sub-band (e.g. 5p15.2).
Homologs
One maternal and one paternal copy of each chromosome. Homologs match in length, centromere position and banding — differences prompt closer review.
G-banding
Trypsin + Giemsa produces dark (AT-rich, gene-poor, late-replicating) and light (GC-rich, gene-rich) bands. Each chromosome has a unique pattern; resolution is quoted as band level (e.g. 400–550).
Acrocentric chromosomes
13, 14, 15, 21, 22 carry satellites and stalks (nucleolar organiser regions) on tiny p arms; Y is acrocentric without satellites. They participate in Robertsonian translocations.
Centromere types compared
Denver groups A–G
| Group | Chromosomes | Features |
|---|---|---|
| A | 1–3 | Largest; metacentric (1, 3) and submetacentric (2). |
| B | 4–5 | Large submetacentric with short p arms. |
| C | 6–12, X | Medium-sized submetacentric; X resembles 7 in size. |
| D | 13–15 | Medium acrocentric with satellites and stalks (NORs). |
| E | 16–18 | Small; 16 metacentric, 17–18 submetacentric. |
| F | 19–20 | Small metacentric. |
| G | 21–22, Y | Smallest acrocentric; Y lacks satellites. |
ISCN essentials
International System for Human Cytogenomic Nomenclature — order: count, sex chromosomes, then abnormalities (sex chromosome abnormalities first, then autosomes in numerical order).
46,XX / 46,XYTotal count, comma, sex chromosomes47,XY,+21Gain listed with +; sex chromosomes before autosomes45,XLoss of a sex chromosome shown by omission46,XX,del(5)(p15.2)Terminal deletion with breakpoint46,XY,inv(9)(p12q13)Pericentric inversion: breakpoints in both arms46,XX,t(11;22)(q23;q11.2)Lower-numbered chromosome first; semicolon separates45,X[12]/46,XX[18]Mosaicism: cell counts in brackets