Our Understanding of Rules that Produce Lifes Genetic Code May Require a Revision
Get me a rewrite, reporters used to shout into a telephone when they called into a newsroom with changing information. Scientists may now want to yell the same thing but about how our existing genetic code came to be, according to a study in PNAS. That rewrite could shift our understanding of how life evolved on Earth from its simplest forms, and what it could look like on other planets.How the Genetic Code WorksThat code is both deceptively simple in appearance, but amazingly complicated in function. Its basis is four molecular letters that always pair in particular ways (code is different than sequence, which is the order all those letters are arranged). Next, three sets of those letters (called codons) work together to form one of 20 amino acids. Those, in turn, create proteins, which are essential building blocks of life.This code has created amazing diversity over millions of years. But it wasnt written all at once, likely changed over time, and may have left earlier versions behind in the developmental dust, according to the study.One of the most amazing things about the groups new look at the old code is how little many of its words changed over billions of years. That indicates that a complicated, lengthy process led to a robust code.There are things that have changed so little over 4 billion years that we can triangulate and figure out a little bit what life 4 billion years ago must have been like, says Joanna Masel, senior author of the paper and a professor at the University of Arizona. It blows my mind.Looking BackwardMasel led a team of scientists in retracing how and when new amino acid words emerged over time. They found that smaller, simpler amino acids likely came first, with larger more complex ones arriving later. They also discovered that amino acids that bind to metals joined the rulebook earlier than previously though. And, finally, they hypothesize that the existing genetic code may have arisen from other, now extinct chemical rulebooks.So, how did the team come to these conclusions?Masel, paradoxically came up with the analytical technique when she was studying how new genes emerge from random-seeming DNA. She was essentially looking forward. In doing so, she kept seeing amino acid commonalities, so she decided to see how far back they could be traced. The answer, it turns out, is billions of years.Her group analyzed amino acid sequences present in life in different time periods including a last universal common ancestor (LUCA). LUCA represents a theoretical population of organisms that lived four billion years ago and from which all subsequent life emerged.In doing so, they took a novel approach. Previous studies looked at the full set of amino acids that makes each protein. This group instead relied on shorter protein domains."If you think about the protein being a car, a domain is like a wheel," Sawsan Wehbi, a graduate student at the University of Arizona, said in a press release. "It's a part that can be used in many different cars, and wheels have been around much longer than cars."The team used statistical analysis to determine when each amino acid likely entered the genetic code. They employed the assumption that specific amino acids that show up more frequently in the oldest sequences did so because they were added to the codebook earlier, and vice versa. The team ultimately identified about 400 sequence families dating back to LUCA, with 100 or so likely emerging earlier than previously thought. Read More: Cracking the Genetic Code on Facial FeaturesChallenging AssumptionsThe authors argue that the current assumption of how the code evolved is flawed because it is partially based on misleading laboratory experiments. For example, the Urey-Miller experiment of 1952 tried to simulate the emergence of life on ancient Earth.It demonstrated that life could arise from nonliving matter, including amino acids, through chemical reactions. But those experiments didnt include sulfur, even though the element was plentiful on early Earth an omission Masel calls bizarre, because, in retrospect it seems obvious.Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.
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