Genome complexity: picking one X chromosome per cell (Introduction)

by David Turell , Monday, August 17, 2020, 22:59 (1346 days ago) @ David Turell

Two active x chromosomes in female cells is toxic. Only one can be active:

https://phys.org/news/2020-08-reveals-sex-chromosomes-female-embryo.html

"Researchers at Massachusetts General Hospital (MGH) have solved a mystery that has long puzzled scientists: How do the bodies of female humans and all other mammals decide which of the two X chromosomes it carries in each cell should be active and which one should be silent?

"...the MGH team discovered the role of a critical enzyme in the phenomenon known as X chromosome inactivation (XCI), which is essential for normal female development and also sets the stage for genetic disorders known as X-linked diseases (such as Rett Syndrome) to occur.

"Scientists have known for over a half century that female mammals undergo XCI during embryo formation. Females have two copies of the X chromosome, and each carries many genes. Having genes expressed on both X chromosomes would be toxic to the cell, as would having both X chromosomes inactivated. To avoid these fates, females evolved with a mechanism that inactivates, or silences, one of the chromosomes. (my bold)

***

"It was already known that, prior to pairing, both X chromosomes are identical, or "symmetrical," meaning that they express the same genes.

"Importantly, both express a form of noncoding RNA called Xist, which plays a vital role in inactivating the X chromosome. However, both X chromosomes also express another form of RNA, Tsix, which blocks Xist and prevents XCI.

"In the Nature Cell Biology paper, Lee and her team show that an enzyme called DCP1A randomly chooses one X chromosome to bind to, and in doing so it cuts off, or "decaps," Tsix's protective cover, making the RNA unstable. However, because DCP1A exists in tiny quantities, there is only enough to bind to one X chromosome. "DCP1A flips the switch that starts the entire cascade of X chromosome inactivation," says Lee.

"As a result, a protein called CTCF—the "glue" that holds X chromosomes together during pairing—binds to the unstable Tsix RNA and causes it to shut down permanently. Xist is then able to complete the silencing of that X chromosome.

"DCP1A allows the two X chromosomes to have a fateful 'conversation'," says Lee, noting that there are many other instances where the body must choose which copy of a gene to express in order to maintain a healthy state. "This discovery," says Lee, "will help scientists understand how other molecular conversations take place in the cell."

Comment: Both chromosomes are identical so either will do. When sexual reproduction appeared during the evolutionary process, this mechanism had to be in place for survival after reproduction to occur. My bold points out the Darwinist authors assuming natural evolution created this by chance. I don't believe it. DCP1A is a giant enzyme molecule specific for the process. Unguided nature is extremely unlikely to find it in a necessary form. Only design fits.