To "read" a set of human chromosomes, scientists first use three key features to identify their similarities and differences:

1. Size. This is the easiest way to tell two different chromosomes apart.
2. Banding pattern. The size and location of Giemsa bands on chromosomes make each chromosome pair unique.
3. Centromere position. Centromeres are regions in chromosomes that appear as a constriction. They have a special role in the separation of chromosomes into daughter cells during mitosis cell division (mitosis and meiosis).

Using these key features, scientists match up the 23 pairs -- one set from the mother and one set from the father.

What are centromeres for?

Centromeres are required for chromosome separation during cell division. The centromeres are attached to microtubules, which are proteins that can pull chromosomes toward opposite ends of each cell (the cell poles) before the cell divides. This ensures that each daughter cell will have a full set of chromosomes.

Normally, each chromosome has only one centromere.

The position of the centromere relative to the end of the chromosome helps scientists tell chromosomes apart. Centromere position can be described three ways: metacentric, submetacentric or acrocentric.

In metacentric (pronounced met-uh-CEN-trick) chromosomes, the centromere lies near the center of the chromosome.

Submetacentric (pronounced SUB-met-uh-CEN-trick) chromosomes have a centromere that is off-center, so that one chromosome arm is longer than the other. When chromosomes are aligned, they are oriented so that the short arm, designated "p" (for petite), is at the top, and the long arm, designated "q" (simply for what follows the letter "p"), is at the bottom.

In acrocentric (pronounced ACK-ro-CEN-trick) chromosomes, the centromere resides very near one end.

Making a Karyotype