DNA e RNA (introduzione) || F.A.Q. l’evoluzione


Hi warm-blooded craniates! In the last months you’ve asked me a lot of questions worth of a reply, so I decided to introduce on the channel this new series of video, in which I hope to be able to explain as many doubts as possible. As you’ll have got from the title, today we’re talking about DNA and RNA. I get the chance to thank the person that asked this question and to remind you that if you have any, you can write them in the comments, little by little I’ll try to reply to everybody! Besides, since the topic of this video is actually quite extended, probably there will be be other videos to complete and deepen this one, so if you have anything about this topic that you’d like me to talk about, feel free to ask! So, let’s start talking about DNA and RNA and, as always, I think it’s good to start from the beginning and clear an essential point: what are DNA and RNA? These two names are actually abbreviations that stands respectively for deoxyribonucleic acid and ribonucleic acid. Apart from being interesting tongue-twisters that, if said out loud at the age of 7 can make you feel like the most amazing genius in the room, they give us a hint on on the biochemical nature of our protagonists; both are organic compounds formed by monomers (or units) called nucleotides, in turn formed by three parts: a nitrogenous base, a 5-carbon sugar and a phosphate group. Despite the differences between these two compounds (in general, DNA is double stranded while RNA is single stranded; for the DNA, as sugar, we have the deoxyribose, while for the RNA we have the ribose; 3 nitrogenous bases out of 4, adenine, cytosine and guanine, are the same, while the fourth is the thymine for the DNA and the uracil for the RNA), in general their structures are very similar; this has resulted in having in common being some sort of codes that carry information, other than being at the base of life and evolution. In fact, we can find DNA and RNA in every living beings…and not only: also viruses have one or another; however, viruses are a separate and quite considerable discussion that makes biologists argue, so I’ll not go into detail in this video because it’s a topic worth of a separated video. Going back to the living beings surely considered to be such, we can find the DNA in almost every cell of an organism (there are exceptions, such as mammals red blood cells, that secondarily lose their nucleus and so they don’t have DNA), but the position changes according to the organism we are considering: Eukaryotes (like Plants, Fungi and Animals) have in the cytoplasm the nucleus that contains the DNA, while Prokaryotes (like Bacteria) don’t have a nucleus, so their DNA is scattered all over the cytoplasm; the RNA is found in the cytoplasm of both types of organism. This crucial difference can help us understand the evolutionary history of all living beings: the ancestral condition of the position of the DNA is the one of the Prokaryotes, so they’ve evolved in a more conservative way than Eukaryotes. To make it simple, prokaryotes look more like the hypothetical common ancestor of all the living beings, LUCA for friends. To explain how LUCA appeared (and so how life appeared) various hypothesis have been made, of which the primordial broth one is the most accredited now: according to this hypothesis, in the beginning the Earth’s atmosphere would have had a different composition compared to the current one, with the oxygen totally absent; in this context, because of the physical agents like UV rays and lightnings (so electric discharges), a series of chemical-physical reactions would have happened in water, taking to the formation of the first simple molecules that, with the passing of time, would have formed, following other chemical-physical reactions, molecules more and more complex. This condition has been recreated in lab with Miller’s experiment (and others that take inspiration from this); if we “let the system go”, after a bit we can notice the presence of the first amino acids, which are the bricks for the proteins, and nitrogenous bases that, as said before, can be part of DNA and RNA. What might have happened, that has brought to the development of the first life forms, is that this nitrogenous bases might have started to assemble, giving origin to various sequences similar to DNA and RNA; these would have not deteriorate easily because of the composition of the atmosphere we were talking about before: the absence of oxygen, in fact, wouldn’t have allowed the occurrence of of oxidation reactions, that are those which contribute to the degradation of organic matter. Of the various sequences formed in this way, it might have happened that one or more with the ability of replicating themselves using the material scattered in the broth assembled casually; this might have brought to an increased number of copies of the sequence, that might have evolved differently because of the mutations of their structures. One of these, for example, might have developed the ability to bind with another sequence with different features, or maybe the ability to cut in a lot of pieces other sequences, or even it might have developed the capacity to attract molecules scattered in the broth as a shell. Precisely these would have been the first protocells, that with the proceeding of evolution would have further modified, giving origin to our dear LUCA. As said before, this is one of the many hypotheses formulated on the origin of life, but it’s not the only one! In case you’re interested, it might be the topic of future videos. Going back to today’s topic, as in this video, it’s common to talk about DNA and RNA together, implying a bound between the two; we’ve seen some aspects they have in common, of which maybe the origin, but what kind of bond there is actually? If in high school you’ve done a bit of biology, it’s highly probable that you’ve heard of the central dogma of molecular biology, that tells us how information flows when we talk about DNA and RNA: DNA, which is what is replicated in new copies, is transcripted in RNA, which is in turn translated into proteins in a one-way flow that goes, so to speak, from the information to the action. Well, it would be awesome if the matter worked like this ‘cause it would make simpler studies on the genome, for example, but actually the issue is a bit more complicated. In fact, in a lot of years of studies what emerged is that there are a ton of different mechanism that can modify this one-way flow: from a single DNA sequence, various RNA can be transcripted according to how the sequence is read during transcription, that in turn turn can give various proteins according to how they’re translated; there are proteins capable of modifying not only RNA (it’s one of the way that makes it possible for RNA to be translated in different proteins), but even DNA; some proteins do not modify the DNA but they can interact with the proteins that transcript it, or they can interfere with the protein that fold the DNA to make it fit into the nucleus, interfering with various factors. And, getting back to viruses, there’s a category of them called “retroviruses” that are able to, once they take over the transcriptional apparatus of the host, to transcript DNA from their RNA (one of them is HIV). Basically, between us, it’s a big mess. As you may have gotten from this video, the topic is huge and we have but scratch the surface, but fear not! We’ll keep talking about it in future videos. In case you have questions or topics you’d like me to talk about, don’t hesitate and write them in the comments! If you liked the video and you’re happy of having met your great-great-great- very great-grandfather LUCA, leave a “like” and share it where and with whom you prefer. I also invite you to subscribe to the channel to be always updated! Thank you so much for watching, see ya!

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