Genome complexity: opening 'origin DNA' (Introduction)

by David Turell @, Wednesday, December 14, 2016, 01:29 (925 days ago) @ David Turell

Great care is taken on opening and copying DNA. If it is not accurate life would not continue:

https://www.sciencedaily.com/releases/2016/12/161213093123.htm

"For years, scientists have puzzled over what prompts the intertwined double-helix DNA to open its two strands and then start replication. Knowing this could be the key to understanding how organisms -- from healthy cells to cancerous tumors -- replicate and multiply for their survival.

"A group of USC scientists believe they have solved the mystery. Replication is prompted by a ring of proteins that bond with the DNA at a special location known as "origin DNA." The ring tightens around the strands and melts them to open up the DNA, initiating replication.

***

"When the origin DNA melts, the double helix divides into separate strands, Chen explained. Those DNA strands then become the template for faithful duplication of other strands -- a Xerox copy of their parental DNA. As soon as replication is complete, one double helix DNA now becomes two exact copies of the same double helix.

"'DNA replication is critical for heredity and survival," said Chen, who also is affiliated with the Norris Comprehensive Cancer Center at the Keck School of Medicine at USC. "The origin DNA's opening is an essential step for DNA replication in our cells and for some tumor viral pathogens to replicate and spread."

"Why is origin DNA so special? Regular DNA sequences contain the A, T, G and C nucleotides, more or less in equal ratio. But origin DNA sequences contain more A and T nucleotides than usual.

"To prompt replication, the scientists used a helicase from a "Large Tumor Antigen" or Large T. The antigen comes from a virus, SV40, linked to human cancers such as brain and bone cancers, mesothelioma and lymphoma. The six proteins from Large T comprise a "helicase" that mimics the structure of the healthy cells' helicases.

"The scientists obtained a 3-D view of the atomic structure of the helicase using X-ray crystallography, a technique for examining nano-biomolecules and their structures at the atomic level that has been refined over centuries. Chen said the images revealed that the proteins which surrounded the DNA had attached to it, then tightened like a vice until the bonds between the two strands of the double helix broke -- or melted -- the origin DNA.

"Although the scientists used a cancerous virus to study replication, healthy cells replicate in a similar way, Chen said."

Comment: Note the giant size and complexity of the helicase mechanism. Its function is based on the sequence of amino acids and of the folding patterns. Such a set of six molecules had to be 'found' by evolution as it advanced from the initial cells. But think about it. It really had to present in the initial cells' DNA or copy errors would have ended life before it got very far. Only true solution. It had to be supplied at the beginning! Not by accident.


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