Do you know Down syndrome? It is a serious genetic disorder characterized by physiological and psychological retardation. As you might know, healthy person should have only two of every type of chromosome. One from mother and one from father. Sometimes, unfortunatelly, one of the parents gives 2 chromosomes (number 21) resulting in a child that have 3 of them. That is the genetic cause of the Down syndrome (see the Picture above). But what if I tell you that approximatelly 50% humans on Earth have one too many chromosome and it doesn't have any adverse effects?
One too many chromosome X
Below you can see a healthy man and a healthy woman karyotype. It is obvious they have the exact same types and number of chromosomes. With one exception – the sex chromosomes. That little poor crippled one in the case of a man is the Y chromosome. It has only few genes and majority of its DNA sequence is under huge degeneration. So if you think about that, men have to function properly with only one X chromosomes. But if you look on woman’s karyotype, the are two X chromosomes. Isn’t it weird? We know that in the case of Down syndrome, if there are more genes that it is supposed to be, it has a huge consequences. How is it that women aren’t retarded? (lol) The answer is quite simple: women can simply turn off one of the X chromosome on the very beginning of their development. How do they do it?
How to store 2 meters of DNA?
The DNA from one cell have approximatelly 2 meters. That is A LOT. In order to pack it inside a cell, DNA is wrapped around the proteins called histones. It is the very same logic as with the cable from the old telephones.
Now, the trick is that not all of your DNA is wrapped evenly. Some of the areas are wrapped more and some less. For example your brain cells need information from the DNA sequences A, B, C and do not need the sequences D, E, F that are needed in your pancreas. So in your brain cells the DNA that resembles to A,B,C sequences will be less wrapped so the cell can reach the information and the sequences D, E, F would be silenced by wrapping. Your pancreatic cells the same logic vice versa.
In the case of one X chromosome of women, the whole chromosome is turned off this way. In the early development special enzymes come to one chromosome X and wrap DNA very tightly. Inactivated X chromosome is also called Barr body and can by seen as black spot on the edge of nuclear membrane of the cell.
And that is, my friends, why the women aren’t retarded.
References:
Molecular Cell Biology, Eighth Edition (2016): Harvey Lodish (Author), Arnold Berk (Author), Chris A. Kaiser (Author), Monty Krieger (Author), Matthew P. Scott (Author), Anthony Bretscher (Author), Hidde Ploegh (Author), Paul Matsudaira (Author)
