英汉对照分子生物学导论 教学课件 ppt 作者 [美]西尔维恩 W.勒潘 王勇 编著Chapter 6.ppt

OK, now, let’s move on to discuss section 6.4, about tRNA Structure and Wobble. Actually, tRNA plays very important roles in translation. It acts as a converter between codons and amino acids. To simplify our discussion of translation above, we did not talk about some important facts about tRNA. We are going to talk about them to in this section. 好了，现在我们转到第4节，关于tRNA结构与摇摆。实际上，tRNA在翻译过程中扮演着很重要的角色，它是密码子与氨基酸之间的转换器。在上面的讨论中，为简明起见我们忽略了一些关于tRNA的重要事项，本节对这些事项做一番探究。 Initiation begins when the 30S subunit alone binds to an mRNA. At this point, the ribosome must center itself at a start codon. There may be many AUG sequences in an mRNA; the AUG that acts as a start codon is designated by a conserved sequence called the Shine-Dalgarno sequence. The 16S rRNA in the 30S subunit of the ribosome recognizes this sequence by base pairing to it directly. 在转译开始的时候，30S亚基结合到一条mRNA上。这时，核糖体必须把自己放到起始密码子上。在一条mRNA上可能会有许多AUG；能作为起始密码子的AUG由称为SD序列的保守序列来标明。核糖体30S亚基中的16S rRNA通过与SD序列发生直接的碱基配对而识别这一序列。 Once the 30S subunit has attached near start codon, a specialized tRNA called initiator tRNA binds to the start codon. The collection of the 30S subunit, the mRNA, and the initiator tRNA is called the 30S initiation complex. 一旦30S亚基附着到了靠近起始密码子的地方，一种称为起始tRNA的特殊tRNA就会结合到起始密码子上。30S亚基、mRNA和起始tRNA的集合称为30S起始复合体。 Next, the 50S subunit binds to the 30S complex to form the 70S initiation complex. The 70S complex contains three tRNA binding sites: the A (aminoacyl) site, the P (peptidyl) site, and the E (exit) site. These sites allow tRNA to join the ribosome and bind directly to mRNA codons. Please note that the initiator tRNA enters P site because the bond between formyl group and methionine is like the peptide bond. 起始的下一个步骤是50S亚基与30S复合体结合形成70S起始复合体。70S复合体含有三个tRNA结合位点：A（氨酰基）位、P（肽基）位和E（退出）位。这些位点使tRNA进入到核糖体中并直接结合到mRNA密码子上。请注意：起始tRNA直接进入P位，因为甲酰基与甲硫氨酸之间的键类似于肽键。 Now, let’s examine the detailed steps of elongation process in translation. Elongation process can be divided into four steps. They are 1) tRNA e