Building RNA and DNA codes (Introduction)

by David Turell , Thursday, January 21, 2016, 01:13 (3012 days ago)

How the enzyme works has been shown. It builds both RNA and DNA. Remember that DNA is RNA without the oxygen which is replaced by a hydrogen:- http://www.evolutionnews.org/2016/01/an_exquisitely102401.html-"Cell survival depends on having a plentiful and balanced pool of the four chemical building blocks that make up DNA -- the deoxyribonucleosides deoxyadenosine, deoxyguanosine, deoxycytidine, and thymidine, often abbreviated as A, G, C, and T. However, if too many of these components pile up, or if their usual ratio is disrupted, that can be deadly for the cell.-"A new study from MIT chemists sheds light on a longstanding puzzle: how a single enzyme known as ribonucleotide reductase (RNR) generates all four of these building blocks and maintains the correct balance among them. [Emphasis added.]-"The image shows the complex machine with active sites that precisely fit the four different building blocks. Its job is to take the ribonucleotides that make up RNA and turn them into the deoxyribonucleotides that make up DNA. -"There's no other enzyme that really can do that chemistry," she says. "It's the only one, and it's very different than most enzymes and has a lot of really unusual features."-***-"This four-in-one machine takes on four different shapes depending on whether the RNA nucleotide is A, G, C, or U. It sends out DNA's four, A, G, C, and T with the appropriate sugar deoxyribose instead of ribose (replacing an OH radical with a hydrogen atom, a "reduction" reaction). But that's not all this amazing machine does:-The effectors can also shut off production completely, by binding to a completely different site on the enzyme, if the pool of building blocks is getting too big.-***-"Deoxyribonucleotides are generated from ribonucleotides, which are the building blocks for RNAs -- molecules that perform many important roles in gene expression. RNR, which catalyzes the conversion of ribonucleotides to deoxyribonucleotides, is an evolutionarily ancient enzyme that may have been responsible for the conversion of the earliest life forms, which were based on RNA, into DNA-based organisms, Drennan says.-***-" RNR is an enzyme made of protein. Did a world of floating RNA fragments somehow build this complex, multi-part protein machine before DNA-based organisms existed? That makes no sense. A ribozyme with enough "code" for RNR would itself be impossibly complex to imagine forming by chance. It would have no way to translate that code into a polypeptide without a ribosome, also made of RNA and protein. Finally, even if by multiple miracles a primitive RNR appeared with its effectors and started cranking out product, the "RNA world" would have no idea what to do with a bunch of DNA building blocks floating around. "-Comment: Last paragraph contains my thoughts exactly. Look at the picture in the article. THAT appeared at the start of life when RNA and DNA were first used!