Transfer RNA

转移RNA

The ribonucleic acid (RNA) that is directly engaged in the translation of the sequence of nucleotides in messenger RNA to amino acid sequences for the construction of proteins is called transfer RNA or commonly tRNA. The manufacture of the tRNA itself is directed by the DNA in the cell that provides a pattern for the production of RNA by "transcription".

直接参与将信使RNA中核苷酸序列翻译成氨基酸序列以构建蛋白质的核糖核酸（RNA）称为转移RNA或简称tRNA。tRNA本身的制造由细胞中的DNA指导，通过‘转录’过程产生RNA。

Below is an illustration of the composition of an example of tRNA in what is commonly called its cloverleaf form. It is a two-dimensional projection of the form of the molecule. It's actual three-dimensional shape is more complex, but this is a common way to depict it to emphasize its function of binding an amino acid to one end corresponding to the anticodon on the opposite end. This anticodon will bind to a codon consisting of three nitrogenous bases which specify an amino acid according to the genetic code. This illustration is patterned after an example in Rana. This tRNA was investigated in yeast and its structure was the culmination of seven years of work by Robert Holley of Cornell University.

以下是对tRNA（通常称为叶形结构）的一个示例组成的示意图。这是该分子的二维投影。其实际的三维结构更为复杂，但这种表示方式是一种常见的描绘方法，以强调其功能：将一个氨基酸连接到一端，对应于位于另一端的反密码子。该反密码子将与由三个氮碱基组成的密码子结合，这些碱基根据遗传密码指定一种氨基酸。此示意图是根据Rana中的一个例子设计的。该tRNA在酵母中被研究，其结构是Cornell大学的Robert Holley七年研究工作的成果。

When the messenger RNA blueprint for a protein reaches a ribosome for the process of building a protein by translation of that blueprint, tRNA molecules with all the required amino acids must be present for the process to proceed. Since most proteins use all twenty amino acids, all must be available, attached to appropriate tRNA molecules. In this example, the tRNA has bonded to the codon GCC on the mRNA, GCC being one of the codons corresponding to alanine. This leads to the placement of alanine in the proper place in the growing polypeptide chain that is to become a protein.

当mRNA蓝图到达核糖体，准备通过翻译过程合成蛋白质时，必须存在携带所有所需氨基酸的tRNA分子，以便该过程能够进行。由于大多数蛋白质使用二十种氨基酸，因此所有氨基酸都必须可用，并且连接到适当的tRNA分子上。在这个例子中，tRNA已与mRNA上的密码子GCC结合，GCC是对应于丙氨酸的一种密码子。这导致丙氨酸被正确地放置到正在形成蛋白质的生长多肽链中的适当位置。

I	Inocine Inocine（专名或术语）
mI mI（专名或术语）	methylinosine 甲基肌醇
mG	methylguanosine 甲基鸟嘌呤
m₂G	dimethylguanosine 二甲基鸟苷
Psi Psi（专名或术语）	Pseudouridine 假尿苷
D	Dihydrouridine 二氢尿苷

The many forms of tRNA have roughly the same size and shape, varying from about 73 to 93 nucleotides. Besides the usual bases A, U, G, and C, all have a significant number of modified bases, which are thought to have been formed by modification after the transcription. Some of those modifications are listed in the table. All tRNAs have sequences of nucleotides that are complementary to other parts of the molecule and base-pair to form the five arms of the tRNA. Four of the arms are fairly consistent, but the variable arm can range from 4 to 21 nucleotides.

tRNA有许多不同的形式，它们的大小和形状大致相同，从约73到93个核苷酸不等。除了通常的碱基A、U、G和C外，所有tRNA都有大量修饰的碱基，这些修饰被认为是在转录后形成的。表中列出了一些这些修饰。所有tRNA都有与分子其他部分互补的核苷酸序列，并通过碱基配对形成tRNA的五个臂。其中四个臂相对一致，但可变臂的长度可以从4到21个核苷酸不等。

References:
Wiki tRNA

参考文献：Wiki tRNA

How are the amino acids correctly assigned to tRNA molecules?

氨基酸如何正确地分配到tRNA分子上？

The assignment of the correct amino acid to each of the forms of tRNA is crucially important to the translation process. This assignment is accomplished by twenty different enzymes called aminoacyl-tRNA synthetases (aaRS) for the twenty amino acids that are incorporated into proteins. There are thought to be 31 different tRNAs, but these 20 synthetases are capable of "charging" all of them with the correct amino acid.

将正确的氨基酸分配给每种tRNA形式对于翻译过程至关重要。这种分配是由二十种不同的酶完成的，这些酶被称为氨基酸-tRNA合成酶（aaRS），用于将二十种氨基酸纳入蛋白质中。人们认为存在31种不同的tRNA，但这些20种合成酶能够为它们全部正确地“装上”相应的氨基酸。

How many distinct tRNAs are required?

需要多少种不同的tRNAs？

In the genetic code, codons made of of three bases specify an amino acid. With three bases, there are 64 possible permutations. With three codons corresponding to STOP codons, this leaves 61 combinations that code for an amino acid. This would require 61 distinct tRNA species were it not for the "wobble base".

在遗传密码中，由三个碱基组成的密码子指定一种氨基酸。有三个碱基，共有64种可能的排列方式。考虑到三个密码子对应停止密码子，这剩下61种组合可以编码氨基酸。如果没有‘摇摆碱基’的存在，这将需要61种不同的tRNA物种。