How Are Chromosomes Identified When Making a Karyotype?

Your karyotype shows the number and structure of your chromosomes. Each chromosome consists of two identical strands of DNA, held together at a narrowed structure called the centromere. Humans have 23 pairs of chromosomes. Each pair can be distinguished based on its size, the position of the centromere, and its reaction to dyes or colored probes. Expert analysis of a karyotype can reveal if DNA was lost, duplicated, or grafted onto the wrong chromosome.

Chromosome Structure

• The structure or morphology of a chromosome is defined by its overall size and the position of the centromere, which pinches the chromosome into two sections called arms. The shorter arm is called "p" for petite and the longer arm is called "q". Chromosomes with a centromere roughly in the middle, so that the p and q arms are similar sized, are classified as metacentric. If the centromere lies somewhat off-center, the chromosome is classified as submetacentric; and if the centromere is very close to the end of an arm, the chromosome is classified as acrocentric. By convention, when chromosomes are arranged in a karyotype, the p arm always is on top.

  Chromosomes are numbered from largest (chromosome 1) to smallest (chromosome 22), except for the sex chromosomes, which are called X (female) and Y (male). The X chromosome is quite large, approximately the size of chromosome 7, while the Y is quite small, about the size of chromosome 22.

  Some chromosomes are easily identified by size and centromere position alone. For example, chromosome 1 is the largest, and it is metacentric, while chromosome 2 is slightly smaller, but sub-metacentric. Chromosomes 19 and 20 are small and metacentric, while chromosomes 21 and 22 are even smaller and acrocentric.

Giemsa Banding

• Giemsa staining is the traditional method for karyotyping DNA. Giemsa is a chemical that stains DNA darkly in regions rich with the DNA bases adenine and thymine and weakly in regions rich with the bases guanine and cytosine. Giemsa staining gives a pattern of alternating light and dark bands (G-bands) that is unique to each chromosome pair. For example, chromosome Y stains lightly on the p arm and the top half of the q arm, while the bottom half of the q arm stains darkly. Chromosome 19 stains darkly only in two thin bands, one just above the centromere and the other just below it. However, most of the G-band patterns are more difficult to distinguish. It can take an expert hours to determine if all the DNA is present and on the appropriate chromosomes.

Other Methods

• Newer methods of identifying chromosomes such as FISH (fluorescence in situ hybridization) rely on colored DNA probes that specifically bind to a particular chromosome. These methods simplify identification of the chromosomes, and they have sharper resolution than Giemsa staining.

  With spectral karyotyping (SKY), each chromosome binds a unique set of different colored fluorescent DNA probes. The blending of different amounts of each probe "paints" each chromosome with a unique color. Given a color-coded key, anyone can quickly identify the 23 pairs of chromosomes.