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Lecture 6

Cell Biology - Lecture 6 -Video 1.1 - Notes

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Boston University
CAS BI 203
Martin Steffen

Lesson 6 – Video 1a [00:00:00.00] 59 [00:00:00.85] PROFESSOR: Hi. In this video, we're going 2132 to be discussing the process of 2133 transcription, which is the act of copying DNAto RNA in a process which is basically turning 2134 genes on. DNAis the repository for all information. RNArepresents those pieces of information 2135 that you're using now. 2136 [00:00:22.06] We'll talk about some of the molecular differences between DNAand RNA. We'll 2137 talk about several types of RNA. We'll show you the general machinery, which converts DNAto 2138 RNA. That's primarily the RNA polymerase.And we'll end with talking about a few differences 2139 between bacteria and eukaryotes. 2140 [00:00:45.10] On this slide, we see a number of processes that are involved with the concept of 2141 gene expression-- which, gene expression is taking a portion of the DNA, making RNA from 2142 that, exporting the RNAfrom the nucleus to the cytosol, taking that RNAand making a protein 2143 with the protein properly folded and then active. Gene expression is sometimes used 2144 synonymously, somewhat loosely, with the process of making RNA. But gene expression is 2145 more properly concerned with all these processes. 2146 [00:01:22.30] Levels of RNA depend not only on making RNA, but the rate at which it's 2147 degraded. Protein levels depend on the amount of protein that is made and the rate at which it is 2148 degraded. Once again, a misconception here is sometimes that levels of gene expression are only 2149 associated with making RNA. But you know that the level of anything is a balance between the 2150 rates that it's made and the rates at which it's degraded. 2151 [00:01:51.26] Part of the reason for the bias is we know a lot more about how RNAis made and 2152 how proteins are made then how they're degraded.And that's in part because they've been 2153 studied much more. But nevertheless, they are equally important. 2154 [00:02:09.71] So here we see a bit of a schematic. Remembering our central dogma, DNAmakes 2155 RNAmakes protein. We've studied, previously, the process of DNAreplication, which is DNA 2156 making DNA. Today we'll talk about transcription, DNAgoing to RNA.And a little later, we'll 2157 talk about translation, RNAgoing into protein. 2158 [00:02:33.83] There's really no way around memorizing these terms. You can maybe try to 2159 remember that translation is converting a nucleic acid code of four bases to an amino acid code 2160 of twenty.And that might help you remember that. But whatever works for you. 2161 [00:02:51.89] OK, a little bit of nomenclature when it comes from transcription.And that is there 2162 is a coding strand and a template strand. So the RNAis a duplicate of the coding strand. 2163 However, again, because of the way things are synthesized, it's going to use the template in order 2164 to produce the transcribed mRNA. 2165 [00:03:22.17] If I were to give you a problem question and say, here is a piece of DNA, and just 2166 give you a single-stranded amount an tell you that it's the coding strand. You would write down 2167 the exact same sequence, but converting the T's-- which is in DNA-- to U's, which are used in 2168 RNA. 60 [00:03:45.91] Now, we'll frequently use the expression "turning 2169 genes on" when we're talking 2170 about gene transcription. I think it makes intuitive sense.And it's a reminder that you have 2171 somewhere between 20,000 and 25,000 genes in your genome. Every cell in your body has that 2172 same set of genes. 2173 [00:04:09.19] But different parts of your body use different subsets of those. For instance, in the 2174 lung, the lung genes get turned on. There might be 10,000 or 15,000 of those. In the stomach, 2175 stomach genes will get turned on.And it'll be a different subset of 10,000 or 15,000. There'll be a 2176 lot of overlap for sure. But then there will also be genes that are uniquely expressed in one of 2177 those two organs. But remember, the stomach and the lung have the same set of genes. It's just 2178 going to express different genes to help it establish its unique character. 2179 [00:04:43.84] Now, here we have an example of two genes geneAand gene B. They are 2180 transcribed at different levels and then they're translated at different levels.Abiologist would say 2181 that this gene is highly expressed, and this gene is weakly expressed. 2182 [00:05:00.01] There's not a linear relationship in terms of when a gene is turned on, how many 2183 copies of the RNAand it will make, and how many copies of protein it will make for each copy 2184 of the RNA. But I think a typical number for eukaryotes would be about 1,000. That is, when a 2185 gene is getting turned on, it will make at least about 1,000 copies of mRNA. 2186 [00:05:24.29] And then each of those mRNA will produce about 1,000 copies of a protein. So 2187 you would have a total of about a million copies of protein per cell. I realize now that number's a 2188 little high. But that serves as an example. 2189 [00:05:39.95] There is a large degree of variability of the amount of amplification from going to 2190 the RNAstep to the protein step.And that is just something
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