Women have two X chromosomes (XX), however men have only one. So one X chromosome is enough to create two very different worlds. X chromosome has many significant functions. What is the secret of X chromosome?
Human females have two X chromosomes, while human males have just one. A single X chromosome is sufficient to create two completely different living things. For humans, the genetic information that determines our physical traits is stored in 23 pairs of chromosomes in the nucleus of the very cell. These chromosomes are made up of proteins and long, coiled strands of DNA. Segments of DNA, called genes, tell the cell to build specific proteins, which control its identity and function. For every chromosome pair, one comes from each biological parent; one set from mother, and one set from father. The first 22 are so-called autosomes. In 22 of these pairs, the chromosomes contain the same set of genes but may have different versions of those genes. The result of having two versions of each gene is that you display a combination of your biological parents’ traits.
The secret of X Chromosome
All the human autosomes are designated by a number. The 23rd pair is unique and it makes boys and girls different. This pair called the X and Y chromosomes influences your biological sex. The X and Y chromosomes are the only two that have a letter designation. Most women have two X chromosomes while most men have one X and one Y. The Y chromosome contains genes for male development and fertility. On the other hand, the X chromosome contains important genes for things other than sex determination or reproduction, like skeletal muscle function and the receptors in the eyes that detect green light.
For the first time, Hermann Henking, a German biologist, noted that the X chromosome was specific in 1890. The idea that the X chromosome is named because it resembles the letter of X is incorrect. Karl Heinrich Henking designated the 12th chromosome the “X element” using the mathematician’s symbol for an unknown quantity while studying wasp sperm cell mitosis. He prognosticate on its possible role in sex determination in insects.
The X chromosome is very large. It accounts for about 5% of the human genome, and it has approximately 1000 genes. By contrast, the Y chromosome is much, much smaller, and only contains a fraction of genes. The level of gene activity produced by a single X chromosome is the regular “dosage” for a human. Due to men only have one X chromosome, they have this dosage. Women have two X chromosomes (XX), so women are expected to have twice the regular dosage. However, they have the same dosage for a different reason: X-inactivation.
What is X inactivation?
X inactivation is an important process in which one of a woman’s X chromosomes gets shut off. This process happens in the embryonic stage. The inactive X chromosome is compact in appearance whereas the active X occupies a much greater volume despite the two chromosomes having the same gene content. The active X chromosome has a dispersed or open appearance. Often described as resembling beads on a string. Molecules involved in gene transcription can access open DNA.
Genes tell the body what and how much of a protein to make. If there were no X inactivation, women would make twice as much of all proteins encoded by X chromosome genes. That is, X chromosome inactivation equalizes the protein dosage difference between male and female mammals.
The probability of both X chromosomes being inactivated is equal. If one of the two X chromosomes is inactivated, the one X which is inactivated does not produce protein. In females, one X set comes from the father and the other set from the mother (XX). This means that one of the inactivated X chromosomes is either the X from the mother or the father. As a result, each female is a mosaic of cells in which either the maternally or the paternally inherited.
Ugur Yilbasi is a Junior Research Fellow at European Institute of Policy Research and Human Rights. Ugur is a senior student at Department of Molecular Biology and Genetics, Istanbul Kültür University.