Chemistry of Down Syndrome
Chromosomes are present in every cell of the human body and carry genetic information necessary for cells to develop. A chromosome is a threadlike structure that consists of DNA and proteins. In a human being, there are 46 chromosomes in every cell, which are arranged in 23 pairs. Out of these, 22 pairs are alike in both males and females – these are called autosomes. The 23rd pair is called the sex chromosome (XX in females and XY in males).
Human cells divide in two different ways: the first one is mitosis. In this kind of cell division, one cell divides into two and the newly formed cells possess the same chromosome number as the parent cell. The second kind of cell division, called meiosis, occurs in reproductive cells such as those in the ovaries and testicles. In this kind of cell division, the daughter cell formed contains half the number of chromosomes of the parent cell.
Errors can occur during cell division. In meiotic division, the chromosome pair splits and goes to the opposite poles in the dividing cell. This event is called ‘disjunction.’ However, in some rare cases, the pair does not divide and the complete pair goes to one pole. As a result, in the resulting cells, one will contain 24 pairs and the other will contain 22 pairs of chromosomes. This event is called ‘nondisjunction.’ During the mating process, if a sperm or an egg containing an abnormal number of chromosomes (24 pairs) merges with a normal mate (23 pairs of chromosomes), the fertilized egg will have abnormal chromosome numbers. In Down’s syndrome, almost 95 percent of the cases occur due to this reason. One cell contains two 21st chromosomes instead of one and, therefore, the resulting fertilized eggs have three 21st chromosomes (also called Trisomy 21). Researchers have found that approximately 90 percent of the cases occur due to abnormal eggs.