Karyotyping

During mitosis, DNA condenses into visible chromosomes. At this stage, the chromosome number and structure of a cell can be observed. This complete set is called a karyotype.

Specific stains, such as Giemsa, reveal distinctive banding patterns that look like miniature barcodes.

These stained chromosomes are then assembled into a complete chromosome map, called a karyogram, and arranged based on size, centromere position, and banding pattern.

In humans, chromosomes are ordered from the largest, chromosome 1, to the smallest, chromosome 22, followed by the 23rd pair, the sex chromosomes.

This layout helps scientists detect chromosomal abnormalities, such as an extra copy of a chromosome.

Staining can also highlight different features along a chromosome's arms.

For example, specific regions, called cytogenetic bands, are labeled based on their position from the centromere to the telomere.

When counting outward from the centromere, the short arm is labeled p, which stands for petite. The longer arm is labeled q, which is the next letter in the alphabet after p.

This numbering scheme helps scientists pinpoint gene locations and study how changes in chromosome structure link to diseases and disorders.