TRANSCRIPCIÓN | Biología Molecular 2/4 — Transcript

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1. 0:08Hello, today we are going to analyze
2. 0:09the process of cellular transcription.
3. 0:12To do this, we have to understand the
4. 0:13context in which this process occurs.
5. 0:17The central dogma of molecular biology
6. 0:20consists of DNA replication in order to
7. 0:23multiply cells. Transcription, which is
8. 0:27the ability to make RNA from a specific
9. 0:30fragment of that DNA, and translation,
10. 0:33the ability to generate proteins from
11. 0:36the different types of RNA generated
12. 0:39from DNA. Among these RNAs, after
13. 0:43transcription, we can have
14. 0:45transcriptional, messenger, and
15. 0:47ribosomal RNAs, among others. Remember
16. 0:51that RNA is formed by nucleotides, more
17. 0:55specifically ribonucleotides, formed by
18. 0:58a phosphoric acid, a ribose aldopentose
19. 1:01, and different nitrogenous bases. In
20. 1:05the case of RNA, it will be formed by
21. 1:08adenine, uracil, cytosine, and guanine.
22. 1:11Transcription consists of using a DNA
23. 1:14molecule as a template in a specific
24. 1:17sector, where the hydrogen bonds will
25. 1:20be broken. And we are going to use one
26. 1:22of those strands as a template strand.
27. 1:25This template strand has certain
28. 1:27characteristics; it is antisense and
29. 1:29non-coding. Why antisense? Because it
30. 1:33has the 3-prime to 5-prime direction.
31. 1:36This strand will be used as a guide to
32. 1:38synthesize a new strand in RNA format,
33. 1:40which will be the transcript strand,
34. 1:42which will have sense, which will code.
35. 1:46To do this, different transcription
36. 1:48factors and enzymes will interact. Now
37. 1:52we are going to observe the
38. 1:53transcription process in a little more
39. 1:55depth. We will observe that in a
40. 1:58specific region of the DNA, the
41. 2:00hydrogen bonds will break and both
42. 2:02strands will separate. One of these
43. 2:04strands will be our template strand,
44. 2:07the one that has the 3-prime to 5-prime
45. 2:10direction, which will be used as a
46. 2:12guide for the synthesis of that new RNA
47. 2:15. The process consists of different
48. 2:17stages. First, we will have what is the
49. 2:19promoter. The promoter, as we see here,
50. 2:22is a specific region of the DNA, which
51. 2:25has a specific sequence. For example,
52. 2:28here we have the sequence guanine,
53. 2:31cytosine, guanine, thymine, adenine,
54. 2:33thymine, adenine, guanine. In other
55. 2:37words, this specific region of the DNA,
56. 2:39which we call the promoter, will
57. 2:42indicate to me that from this sector
58. 2:44onwards a specific RNA will be encoded,
59. 2:47whether it be messenger,
60. 2:49transcriptional, or transfer. In this
61. 2:52specific case, we have the messenger
62. 2:55promoter. What follows is the binding
63. 2:59of transcription factors to this
64. 3:02specific promoter, which will then
65. 3:04stimulate the binding of the different
66. 3:07polymerases to these transcription
67. 3:10factors and begin the synthesis of that
68. 3:13new strand in RNA format, as we see
69. 3:16here in green. Let's remember that what
70. 3:21the polymerases do is place the
71. 3:23nucleotides in their corresponding
72. 3:26regions. They will then rely on
73. 3:28Chargaff's rules of base
74. 3:29complementarity. Adenines will
75. 3:31complement with uracil and vice versa,
76. 3:34and guanines with cytosine and vice
77. 3:36versa. Thus, after this polymerization,
78. 3:39there will be certain inhibitory
79. 3:41sectors that will terminate that
80. 3:44transcription, and the result will be
81. 3:46an immature RNA molecule. Immediately
82. 3:50after RNA transcription ends, the
83. 3:53hydrogen bonds reassemble and the DNA
84. 3:55double helix structure reforms without
85. 3:58any modifications to it. After the
86. 4:02transcription of an RNA, what we have
87. 4:04is simply a strand of ribonucleotides.
88. 4:07We call that immature. Why? Specific
89. 4:10post-transcriptional modifications must
90. 4:12occur for that type of RNA, which give
91. 4:15it function on one hand and protection
92. 4:17on the other so that it can leave the
93. 4:20nucleus and perform its function in the
94. 4:22cytoplasm. Now we are going to analyze
95. 4:24how the different types of RNA mature.
96. 4:27Conceptually, we have three major RNAs:
97. 4:30transfer, ribosomal, and messenger. The
98. 4:34messenger has the code, the guide to be
99. 4:36able to carry out that protein
100. 4:38synthesis. The ribosomal provides the
101. 4:42site, like the factory, the place where
102. 4:45that protein synthesis will be produced
103. 4:47. And the transfer RNA is the one that
104. 4:50allows the different amino acids to be
105. 4:53brought to the site to be able to join
106. 4:56them and make the proteins. Let's
107. 4:59analyze what the transcription of
108. 5:01messenger RNA is like. The messenger
109. 5:03RNA is what will carry the information,
110. 5:06the guide of the steps on how the amino
111. 5:08acids are located. in order to generate
112. 5:11that new protein. Messenger RNA
113. 5:14represents 5%of the total cellular RNA,
114. 5:16quite a small amount; it is located in
115. 5:18the nucleus and will fulfill its
116. 5:20function in the cytoplasm. For that,
117. 5:23the transcription of messenger RNA will
118. 5:26be located at the TATA box. Why TATA
119. 5:29box? Because the promoter is a DNA
120. 5:32sequence that is formed by several
121. 5:34thymines and adenines flanked by
122. 5:36guanines and cytosines. In the example
123. 5:39we saw previously, transcription
124. 5:41factors will bind to this promoter,
125. 5:44which will cause polymerase 2 to
126. 5:46activate. That polymerase will give
127. 5:48rise to the synthesis of the RNA. There
128. 5:52are three important modifications that
129. 5:54will be made to the messenger RNA in
130. 5:56order to mature it. On one hand, the
131. 5:59formation of the cap. The cap located
132. 6:02at the 5-prime end is nothing more than
133. 6:05a GTP, a triphosphate that has a methyl
134. 6:09group in its structure. This makes it
135. 6:12hydrophobic and allows it to repel, for
136. 6:15example, enzymes like ribozymes that
137. 6:17could bind to it and hydrolyze this
138. 6:20molecule once it exits the nucleus. In
139. 6:23other words, the cap has a protective
140. 6:24function. At the other end, we have
141. 6:28what is known as the poly-A tail. It is
142. 6:31simply a long sequence of adenines that
143. 6:34also has a protective function. And
144. 6:38finally, we have the splicing process.
145. 6:41Splicing, literally translated, would
146. 6:44be joining and rearranging. Splicing is
147. 6:48basically based on removing those
148. 6:51regions of immature messenger RNA that
149. 6:54do not code for the protein in question
150. 6:58, which we call introns, and keeping
151. 7:01only those regions that do code for
152. 7:04this final protein, which we call exons
153. 7:07. Ribonucleoproteins are used for
154. 7:10splicing. These riboproteins are
155. 7:14enzymes that have a protein part and a
156. 7:16non-protein part formed by
157. 7:17ribonucleotides. These proteins bind to
158. 7:20different sectors and will remove those
159. 7:23that do not code, leaving us with those
160. 7:26that do code. This way, we will have
161. 7:28all the exons in our mature messenger
162. 7:31RNA and a series of ribonucleotides
163. 7:33that will be degraded, to be used to
164. 7:36generate other types of RNA in the
165. 7:38future. Now we are going to analyze the
166. 7:41transcription of transfer RNA. The tRNA
167. 7:44is in charge of carrying and placing
168. 7:46the amino acid in question at the
169. 7:49correct site to build the protein
170. 7:51sequence. Transfer RNA represents
171. 7:56approximately 15%of the total cellular
172. 7:59RNA. It is generated inside the nucleus
173. 8:04and will perform its function in the
174. 8:06cytosol, binding to free amino acids
175. 8:09and taking them to the ribosome. The
176. 8:13transcription of transfer RNA consists
177. 8:15of a promoter in the DNA known as the A
178. 8:17and B box, which binds to transcription
179. 8:19factors that stimulate polymerase three
180. 8:21. Once that immature transfer RNA
181. 8:25strand is synthesized, it undergoes
182. 8:27three post-transcriptional
183. 8:29modifications. One of them is the
184. 8:32insertion of the ribozyme at the 3-
185. 8:34prime end. The ribozyme is an enzyme
186. 8:37that has an extra phosphate group. This
187. 8:40makes it inactive. So it positions
188. 8:42itself on its substrates, but since it
189. 8:44is inactive, it does not hydrolyze them
190. 8:46. By being occupied there at its active
191. 8:49site, it prevents other enzymes from
192. 8:50being able to bind there. Therefore, it
193. 8:52has a protective function very similar
194. 8:54to the cap. At the 3-prime end, a CCA
195. 8:57sequence is inserted: cytosine,
196. 9:00cytosine, adenine with a hydroxyl. This
197. 9:03has two functions. On one hand,
198. 9:05protection, and on the other hand, the
199. 9:08binding site for the specific amino
200. 9:10acid. We have different pairings in
201. 9:14these sectors that will allow the
202. 9:16transfer RNA to fold in this way due to
203. 9:18the attraction in these regions of
204. 9:21hydrophobic groups, thanks to the
205. 9:23methyls that will move away from the
206. 9:25exterior, which is hydrophilic. This
207. 9:30will be key to determining the
208. 9:32three-leaf clover shape so
209. 9:34characteristic of transfer RNA. Let us
210. 9:38remember, then, that at this
211. 9:40three-prime end, the binding site for
212. 9:42the future amino acid will be located.
213. 9:45What amino acid binds in this sector
214. 9:47will be determined by this region,
215. 9:50which is where the anticodon is located
216. 9:52. The anticodon consists of three
217. 9:55nitrogenous bases that interact with
218. 9:57the messenger, with a sector of the
219. 10:00messenger, and allow for the specific
220. 10:02placement of the amino acid in a
221. 10:04position. Finally, we are going to
222. 10:06analyze the transcription of ribosomal
223. 10:08RNA. Ribosomal RNA makes up 80%of the
224. 10:12total RNA that a cell has. This
225. 10:16percentage is entirely logical, since
226. 10:18we can observe with a scanning electron
227. 10:21microscope the ribosomes, whether free
228. 10:23or attached to the endoplasmic
229. 10:25reticulum, while messenger or
230. 10:27transcription RNA is impossible to see
231. 10:30with a microscope. Ribosomal RNA will
232. 10:33form the ribosomes that will provide a
233. 10:35site for the messenger and the
234. 10:37transcription RNA to be able to carry
235. 10:39out protein synthesis. There will be a
236. 10:42large number of promoters that
237. 10:44determine different ribosomal RNA
238. 10:47chains for me. The different ribosomal
239. 10:51RNAs will associate with proteins and
240. 10:54form what we observe in histology as a
241. 10:56ribosome with a large subunit and a
242. 10:59small subunit. The ribosome will be the
243. 11:04site of protein synthesis, both in
244. 11:06eukaryotic and prokaryotic cells. In
245. 11:08both, there is a large unit and a small
246. 11:10unit, but we will observe that the size
247. 11:12differs between one cell and another.
248. 11:15We will observe, then, that eukaryotic
249. 11:18cells have a large 60S unit and a small
250. 11:2040S unit, while prokaryotic cells have
251. 11:23a large 50S unit and a small 30S unit.
252. 11:27The S is a unit that stands for
253. 11:29sedimentation, which is practically
254. 11:32talking about the mass size at a very
255. 11:35small level of these two structures.
256. 11:38Remembering this size difference
257. 11:40between eukaryotic and prokaryotic
258. 11:42cells at the level of the ribosomes is
259. 11:45key to being able to understand the
260. 11:47mechanism of action of different
261. 11:49antibiotics. Some antibiotics will
262. 11:52inhibit protein synthesis in bacteria
263. 11:55by inhibiting either the 50S or 30S
264. 11:57structure, having no contact with our
265. 12:00eukaryotic ribosomes. This is as far as
266. 12:04we go with the transcription processes.
267. 12:05I hope it has been helpful to you.