Metaphase Karyotyping and Banding

Metaphase karyotyping is the classical cytogenetic technique in which cells are arrested in metaphase, their condensed chromosomes are stained to produce a reproducible pattern of light and dark bands, and the chromosomes are then arranged and examined as a karyotype. Banding makes each chromosome individually identifiable and allows numerical and large structural abnormalities to be recognised across the whole genome in a single test.

Definition

Metaphase karyotyping with banding is the microscopic analysis of chromosomes arrested at metaphase and stained to reveal a characteristic band pattern, used to determine chromosome number and to detect microscopically visible structural rearrangements.

Scope

This topic covers how metaphase chromosomes are prepared and stained, the principal banding methods (notably G-banding), the karyotype as a standardised representation, and what conventional karyotyping can and cannot detect. It is a methodological reference and does not provide clinical management guidance.

Core questions

• How are dividing cells arrested and processed to obtain analysable metaphase chromosomes?
• What produces the reproducible banding pattern, and how does G-banding work?
• What is the approximate resolution limit of conventional banding?
• Which abnormalities (e.g. balanced translocations, ploidy) can only karyotyping reveal?

Key concepts

• Metaphase arrest (mitotic spindle inhibition)
• Chromosome banding pattern
• G-banding (Giemsa)
• Karyotype and idiogram
• Band-level resolution
• Numerical versus structural abnormality
• Balanced rearrangement
• Mosaicism detection

Mechanisms

Dividing cells are arrested in metaphase by inhibiting spindle formation, then exposed to a hypotonic solution that swells them and a fixative, and dropped onto slides so that the condensed chromosomes spread. Staining produces a reproducible alternating pattern of bands; in the most widely used method, mild trypsin treatment followed by Giemsa stain (G-banding) yields dark and light bands reflecting differences in chromatin composition and condensation. The banded chromosomes are then paired and ordered into a karyotype, in which each chromosome is identified by its size, centromere position, and band pattern. Because the whole genome is examined microscopically, karyotyping detects gains or losses of whole chromosomes, large deletions and duplications, and uniquely both balanced rearrangements (such as reciprocal translocations and inversions) and many forms of mosaicism, although its resolution is limited to abnormalities of roughly several megabases.

Clinical relevance

Karyotyping has long been used in the evaluation of suspected chromosome disorders, recurrent pregnancy loss, and haematological malignancies, and it remains the reference method for detecting balanced rearrangements and ploidy that higher-resolution copy-number methods miss. This entry describes how karyotype findings are generated; it is not a basis for individual diagnostic or treatment decisions.

Evidence & guidelines

Karyotype results are reported using the International System for Human Cytogenomic Nomenclature (ISCN), which provides a standardised notation for describing normal and abnormal chromosome complements across laboratories.

History

The field became possible once Tjio and Levan established in 1956 that human cells carry 46 chromosomes. Caspersson and colleagues introduced quinacrine fluorescence banding in the late 1960s, demonstrating that chromosomes could be differentiated along their length, and Seabright's 1971 trypsin-Giemsa method gave a simple, durable G-banding technique that made routine identification of every human chromosome practical and underpinned clinical cytogenetics for decades.

Key figures

• Joe Hin Tjio
• Albert Levan
• Torbjörn Caspersson
• Lore Zech
• Marina Seabright

Seminal works

• tjio-levan-1956
• caspersson-1968
• seabright-1971

Frequently asked questions

Why must cells be in metaphase for karyotyping?

Chromosomes are maximally condensed and individually distinct at metaphase, so arresting cells at that stage and spreading them allows each chromosome to be counted and examined for structural change.

What can a karyotype detect that a microarray cannot?

A karyotype can reveal balanced rearrangements such as reciprocal translocations and inversions, as well as ploidy changes and many forms of mosaicism, because it visualises whole chromosomes rather than only measuring copy-number gains and losses.