Genome complexity: slicing and dicing DNA for germ cells (Introduction)

by David Turell , Friday, August 13, 2021, 19:18 (987 days ago) @ David Turell

Sperm and egg must have DNA mixed up to to pass on proper mixed Mother and Dad inheritance:

https://phys.org/news/2021-08-brca2-protein-complex-important-dna.html

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"The initials BRCA2 may be best known for a gene associated with many cases of breast cancer, and the protein encoded by the BRCA2 gene is critical to repairing breaks in DNA.

"The breakdown of this interaction is a hallmark of many cancers. Now, U-M scientists have determined the structure of a complex of two proteins—BRCA2 together with MEILB2—that allows repairs to happen efficiently in cells undergoing cell-splitting, called meiosis. Their results, reported in Nature Structural and Molecular Biology, have major implications for cancer and infertility.

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"In germ cells—the cells that give rise to sperm or eggs—DNA breaks occur in every chromosome before the cells undergo meiosis. The breaks ensure mixing of genes to create genetic diversity rather than exact copies of the parents. In meiosis, each germ cell splits twice so that each egg or sperm ends up with only one copy of each chromosome. Then when egg meets sperm, the embryo has the right number of chromosome pairs.

"Before the first split occurs, the chromosomes in the germ cell pair up tightly and then each chromosome within a pair breaks and rejoins with pieces from its partner to exchange genes in a process called crossover. Then all these DNA breaks need to be rejoined quickly.

Think of a sandwich, Nandakumar explains. The "bun" is composed of four identical copies of a protein called MEILB2 on the top and bottom, with the two BRCA2 proteins between. The MEILB2 protein sandwich carries the BRCA2 protein precisely to the DNA break points.

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"'While we have known BRCA2 was necessary for DNA recombination in meiosis, we didn't know how it was able to do this critical job efficiently," Nandakumar said. "The MEILB2 that is part of this repair complex is only supposed to be present in cells that undergo meiosis but MEILB2 has also been found in several cancers. It may be that MEILB2 is very efficiently 'hijacking' the BRCA2 in cancer cells, preventing proper repair of the DNA."

"Without other factors usually found in meiotic cells, the BRCA2 in these MEILB2-positive cancers might not get to the DNA breakpoints."

Comment: another highly complex process in making egg and sperm that runs at extremely fast micro-second rates. Must be designed all at once not by any stepwise process. Shows how there must be a designer.