A centromere is a constricted region of a chromosome that separates it into a short arm (p) and a long arm (q). During cell division, the chromosomes first replicate so that each daughter cell receives a complete set of chromosomes. Following DNA replication, the chromosome consists of two identical structures called sister chromatids, which are joined at the centromere.
The centromere is a very specific part of the chromosome. When you look at the chromosomes, there's a part that is not always right in the middle, but it's somewhere between one-third and two-thirds of the way down the chromosome. It's called the centromere. That's the part where the cell's chromosomes are constricted, and they're a little bit tighter, and it almost looks like a little ball in the middle of two sticks. The centromere is what separates the chromosome into what we call, for human chromosomes, the P and Q arm. And these P and Q arms are a part of what we use when we do cytogenetics to say how many chromosomes are present in a cell and what chromosome number they are. That's based on the banding pattern of the cell, but a lot of that is based on how big the P arm is relative to the Q arm. So it's always an important consideration for us to know where the centromere is. That's visually how we use it for some genetic tests, but it's also important that the centromere has a very important function during cell division. During cell division, this is the place where the chromosomes, when they're undergoing replication, that they're held together so that the chromosomes don't lose their sister chromatid during the cell division process.Julie A. Segre, Ph.D.