Biological complexity: diurnal rhythms (Introduction)

by David Turell , Thursday, March 12, 2020, 14:24 (1717 days ago) @ David Turell

Controlled by specific molecular reactions:

http://nautil.us/issue/83/intelligence/how-your-body-knows-what-time-it-is?utm_source=N...

"Many organisms perform best at certain hours of the day. The slug species Arion subfuscus, living in almost total darkness, knowing nothing about the Gregorian calendar, lays its eggs between the last week of August and the first week of September.1 Bees forage for nectar, knowing the best times to visit the best fields and the exact timing of nectar secretions for individual species of flowers.

"In the mid-20th century, the Austrian Nobel laureate Karl von Frisch provided enormous insights on honeybee communication and foraging time. He discovered that bees have internal clocks that tell them not only where the nectar is to be found but also precisely when that food will be ready. “I know of no other living creature,” he wrote in his book on bee language, “that learns so easily as the bee when, according to its ‘internal clock,’ to come to the table.”

***

"So, without getting into the full description of the differences between protein functions of humans and flies, we can interpret the Drosophila model of the circadian clock as simply a feedback loop that operates by a specific gene expression with a relatively short half-life. In essence and in generality, the loop simply behaves like this: The quantity of A molecules increases, reaching a threshold that creates B molecules (with a relatively short half-life), which in turn shut down production of the A molecules.

"Unlike fruit flies, humans have strong temperaments and wills that permit defiance of their weaker, yet persistent, biochemical controls. The model circadian rhythm for a person who rises at roughly 6 a.m. and runs through each day with a regularity tuned to the sun is broadly illustrated here.

Go tp site to see diagram

"All living things learn to manage daily environmental changes, especially the atmospheric lightness and darkness caused by the 24-hour cycle of earth rotation. A human’s hereditary information includes the biochemical mechanisms of proteins gotten from the routines of his or her ancestral life. And although millions of cells in a person’s body have specialized functions, every one of them contains the same code of hereditary information.

"This led to the idea that a circadian gene’s instructions are responsible for per mRNA cycling following a feedback loop with return response instructions. The magic here is that the per gene located on the X chromosome of the fly cell contains the information for the mRNA (that has a relatively short half-life), which instructs the ribosomes to produce proteins connected with the per gene (called PER molecules, capitalized to avoid confusion with the per gene) that effectively travel back to the cell nucleus to turn off the activity of the per gene. Morning light would then destroy PER molecules. With PER molecules gone, the per gene would renew the process of encoding mRNA to complete a 24-hour feedback loop. In effect, it is the fruit fly’s molecular clock hand encapsulated in a single cell; moreover, it has since been discovered that the biological clock in most mammals works by the same feedback loop, though in mammals it takes a whole group of per genes for the process to continue. It could be that this Drosophila melanogaster per gene model is the result of organic evolutionary adaptation of the Earth’s circadian environment to maximize survival and well-being on a planet where life existence is governed by the alternation of light and darkness.

***

"Here is how the circadian oscillator of Drosophila melanogaster works. The per gene in the nucleus of the cell transcribes mRNA molecules that migrate to the cytoplasm to give information and green light for ribosomes (the protein workshop) to build both stable and unstable protein molecules. The stable protein accumulates in the cytoplasm. As night goes on, protein levels accumulate to reach a threshold by roughly the middle of the night, when they enter the nucleus and begin to repress transcription from per gene instructions and soon after completely turn protein building off. In the morning, as the sun rises, the proteins decay and after several hours vanish. With all protein gone from the nucleus, the per gene turns on to restart transcription, and so the approximate 24-hour-cycle loop begins again. On and on it goes, indefinitely.

***

"... the full body-clock system involves both the SCN in the brain and the trillions of peripheral clocks embedded in almost every cell of the body. We are a bundle of clocks that are synchronized to the environment by zeitgebers, of which changes in light and darkness are but just one.

"With exception to cells in the eye, mammalian cells have no photoreceptors, so only the SCN
[suprachiasmatic nucleus in the brain ] can indirectly sense light by way of neural tract signals coming from the retina, and therefore, we tend to be awake mostly when light signals tell us to be."

Comment: this system is controlled by light and depends on specific molecular feedback loops. Only design can create this system.