{"id":4533,"date":"2019-06-24T12:58:22","date_gmt":"2019-06-24T12:58:22","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/5-4-rna-3\/"},"modified":"2023-11-30T17:56:40","modified_gmt":"2023-11-30T17:56:40","slug":"5-4-rna-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/5-4-rna-3\/","title":{"raw":"5.5\u00a0RNA","rendered":"5.5\u00a0RNA"},"content":{"raw":" \r\n\r\n \r\n\r\n[caption id=\"attachment_2208\" align=\"alignnone\" width=\"409\"]\"Image Figure 5.5.1 Diagram of a basic overview of protein. <\/em>[\/caption]\r\n\r\nA Deceptively Simple Model<\/span>\r\n\r\nThis simple model sums up one of the most important ideas in biology, which is called the central dogma of molecular biology (you'll read more about it below). You probably recognize the spiral-shaped structure in the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary]. It represents a molecule of [pb_glossary id=\"277\"]DNA[\/pb_glossary], the biochemical molecule that stores genetic information in most living cells. The yellow chain represents a newly formed polypeptide \u2014 the beginning stage of creating a [pb_glossary id=\"5813\"]protein[\/pb_glossary]. Proteins are the class of biochemical molecules that carry out virtually all life processes. What is the structure in the center of the model? It appears to resemble DNA, but it is smaller and simpler. This molecule is the key to the central dogma, and it may have been the first type of biochemical molecule to evolve.\r\n
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Central Dogma\u00a0of Molecular Biology<\/h1>\r\n<\/div>\r\nDNA\u00a0is found in\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]. In [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary]\u00a0cells, chromosomes always remain in the\u00a0nucleus, but\u00a0proteins\u00a0are made at [pb_glossary id=\"5551\"]ribosomes[\/pb_glossary]\u00a0in the cytoplasm. How do the instructions in DNA get to the site of\u00a0[pb_glossary id=\"2210\"]protein synthesis[\/pb_glossary]\u00a0outside the nucleus?\r\n\r\nAnother type of\u00a0[pb_glossary id=\"5475\"]nucleic acid[\/pb_glossary]\u00a0is responsible. This nucleic acid is\u00a0[pb_glossary id=\"519\"]RNA[\/pb_glossary]<\/strong>, or ribonucleic\u00a0acid. RNA is a small molecule that can squeeze through pores in the nuclear membrane. It carries the information from\u00a0DNA\u00a0in the\u00a0nucleus\u00a0to a\u00a0ribosome\u00a0in the cytoplasm and then helps assemble the\u00a0protein. In short:\r\n

DNA <\/strong>\u2192<\/strong> RNA <\/strong>\u2192<\/strong>\u00a0<\/em><\/strong>Protein<\/strong><\/p>\r\nThis expresses in words what the diagram in Figure 5.5.1 shows. The genetic instructions encoded in DNA in the nucleus are transcribed to RNA. Then, RNA carries the instructions to a ribosome in the cytoplasm, where they are translated into a protein. Discovering this sequence of events was a major milestone in molecular biology. It's called the [pb_glossary id=\"2209\"]central dogma of molecular biology[\/pb_glossary]<\/strong>.\r\n

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Introducing RNA<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_2363\" align=\"alignright\" width=\"270\"]\"A Figure 5.5.2 RNA is a single strand of nucleotides, each containing the sugar ribose, a phosphate group, and one of four bases, A, C, G, or U.<\/em>[\/caption]\r\n\r\nDNA alone cannot \u201ctell\u201d your\u00a0cells\u00a0how to make\u00a0proteins. It needs the help of RNA, the other main player in the central dogma of molecular biology. Like DNA, RNA is a\u00a0nucleic acid, so it consists of repeating [pb_glossary id=\"518\"]nucleotides[\/pb_glossary] bonded together to form a polynucleotide chain. RNA differs from DNA in several ways: it exists as a single stranded molecule, contains the sugar [pb_glossary id=\"5943\"]ribose<\/strong>[\/pb_glossary] (as opposed to [pb_glossary id=\"5944\"]deoxyribose<\/strong>[\/pb_glossary]) and uses the base uracil instead of thymine.\r\n
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Functions of RNA<\/h2>\r\n<\/div>\r\nThe main function of RNA is to help make proteins. There are three main types of RNA involved in\u00a0protein synthesis:\r\n
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  1. \r\n\r\n[caption id=\"attachment_2217\" align=\"alignright\" width=\"200\"]\"Image Figure 5.5.3 The three types of RNA take very different forms.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"2211\"]Messenger RNA[\/pb_glossary] ([pb_glossary id=\"2212\"]mRNA[\/pb_glossary])<\/strong>\u00a0copies (or transcribes) the genetic instructions from DNA in the nucleus and carries them to the cytoplasm.<\/li>\r\n \t
  2. [pb_glossary id=\"2213\"]Ribosomal RNA[\/pb_glossary] ([pb_glossary id=\"2214\"]rRNA[\/pb_glossary])<\/strong>\u00a0helps form ribosomes, where proteins are assembled. Ribosomes also contain proteins.<\/li>\r\n \t
  3. [pb_glossary id=\"2215\"]Transfer RNA (tRNA)[\/pb_glossary]<\/strong>\u00a0brings\u00a0amino acids\u00a0to ribosomes, where rRNA catalyzes the formation of\u00a0chemical bonds\u00a0between them to form a protein.<\/li>\r\n<\/ol>\r\nIn section 5.7 Protein Synthesis<\/a>, you can read in detail about how these three types of RNA build primary structure of proteins.\r\n\r\nRNA is a very versatile molecule which plays multiple roles in living things. In addition to helping to make proteins, for example, there are RNA molecules that regulate the expression of genes, and RNA molecules that catalyze other [pb_glossary id=\"5673\"]biochemical reactions[\/pb_glossary] needed to sustain life. Because of the diversity of roles that RNA molecules play, they have been called the Swiss Army knives of the cellular world.\r\n
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    It's an RNA World<\/h3>\r\n<\/div>\r\nThe function of DNA is to store genetic information inside cells. It does this job well, but that's about\u00a0all<\/em> it can do. DNA can't act as an enzyme, for example, to catalyze biochemical reactions that are needed to keep us alive. Proteins are needed for this and many other life functions. Proteins work exceptionally well to keep us alive, but they are unable to store genetic information. Proteins need DNA for that. Without DNA, proteins could not exist. On the other hand, without proteins, DNA could not survive. This poses a chicken-and-egg sort of problem: Which evolved first? DNA or proteins?\r\n\r\nSome scientists think that the answer is neither. They speculate instead that RNA was the first biochemical to evolve. The reason? RNA can do more than one job. It can store information as DNA does, but it can also perform various jobs (such as catalysis) to keep cells alive, as proteins do. The idea that RNA was the first biochemical to evolve, predating both DNA and proteins, is called the RNA world<\/strong>\u00a0<\/strong>hypothesis<\/strong><\/a>. According to this\u00a0hypothesis, billions of years ago, RNA molecules evolved that could both survive and make copies of themselves. According to the hypothesis, early RNA molecules eventually evolved the ability to make proteins, and at some point RNA mutated to form DNA.\r\n
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    Feature: Reliable Sources<\/h3>\r\n<\/div>\r\nThe RNA world hypothesis has not gained enough support in the scientific community to be accepted as a scientific theory. In fact, there are probably as many detractors as supporters of the hypothesis. Do a web search to learn more about the RNA world hypothesis and the evidence and arguments for and against it. When weighing the information you gather, consider the likely reliability of the different websites you visit. Based on what you determine are the most reliable sources and the most convincing arguments, form your own opinion about the hypothesis. You may decide to accept or reject the hypothesis. Alternatively, you may decide to reserve judgement until \u2014 or if<\/em> \u2014 more evidence or arguments are forthcoming.\r\n
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    5.5 Summary<\/span><\/h1>\r\n<\/header>\r\n
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