Genome complexity: getting X and Y together (Introduction)

by David Turell , Saturday, May 30, 2020, 00:12 (1421 days ago) @ David Turell

Y is tiny but active in the control of this process:

https://www.sciencedaily.com/releases/2020/05/200528160557.htm

"As chromosomes go, X and Y make an unlikely pair. The X is large and contains thousands of genes critical for life. The Y, by contrast, is little more than a nub. Its main purpose is to provide the instructions for initiating male development and making sperm. Yet these two very different chromosomes must work together if they are to meet and pair up properly during meiosis -- the special form of cell division that creates sperm and egg.

***

"Breaking is a theme of meiosis. During this process, every chromosome we got from our mothers lines up with every chromosome we got from our fathers and the two swap segments. Before this swapping can occur, the DNA in the chromosomes must be deliberately broken. The regions that are swapped are "homologous" -- they're found in the same place along the chromosome and contain the same genes (although the particular DNA sequence of each gene may be slightly different).

"Homologous recombination is vastly more challenging for males because most of the X chromosome has nothing to pair with. In fact, only a very tiny portion of the already tiny Y chromosome has any homology with the X. This region is called the pseudoautosomal region (PAR), and it's critical for making sure that X and Y find their way into different sperm cells.

"Scientists have known for a long time that the PAR undergoes breaking and swapping of segments at a level that far outpaces what one would expect, given its size.

"'On most chromosomes, DNA double-strand breaks typically occur once every 10 million base pairs," says Scott Keeney, a molecular biologist at SKI, who studies this phenomenon. "The PAR in mice is less than 1/10 that size but it still manages to undergo frequent double-strand breaks."

***

"The key to proper pairing of X and Y, they discovered, is a repeated sequence of DNA in the PAR that attracts several double-strand break-related proteins to this region. These protein clusters -- which Dr. Acquaviva dubbed "blobs" -- change the architecture of the chromosome in this region in such a way that the PAR becomes, as the authors put it, "the hottest area of double-strand break formation in the male mouse genome."

***

"'At first glance, the blobs just look like a mess you might see in the microscope if the experiment didn't work," Dr. Acquaviva says. "But they turned out to be completely predictable in number, timing, and location, so it became clear that in reality they are very complex structures that the cell builds on purpose." (my bold)

"In fact, he says, these blobs were key to understanding how the PAR DNA is tethered as short loops to the linear axis that is the structural backbone of the chromosome.

"Though the X chromosome also has this same repeated DNA sequence, the two X chromosomes in female meiosis typically do not recombine at this region. Why not? The SKI scientists show that it is because pairing between other regions of the X tends to happen first and directly opposes breakage at the PAR."

Comment: The X and Y chromosome are involved separately in purposeful activity (note the bold) but each does its own thing before the changes each created for itself produces the desired result. Not by chance. Must be designed. And as a result looks like intelligent activity. I can't imagine sex cells cooking this up on their own. The use of sexual reproduction evolved and produced a greater variety of diversity, but it obviously created such problems, only a designer could have produced it.