(三)多肽链的延长：
在多肽链上每增加一个氨基酸都需要经过进位，转肽和移位三个步骤。
(一)为密码子所特定的氨基酸tRNA结合到核蛋白体的A位，称为进位。氨基酰tRNA在进位前需要有三种延长因子的作用，即，热不稳定的EF(Unstable temperature,EF)EF-Tu,热稳定的EF(stable temperature EF,EF-Ts)以及依赖GTP的转位因子。EF-Tu首先与GTP结合，然后再与氨基酰tRNA结合成三元复合物，这样的三元复合物才能进入A位。此时GTP水解成GDP，EF-Tu和GDP与结合在A位上的氨基酰tRNA分离。
①核蛋白体“给位”上携甲酰蛋氨酰 基(或肽酰)的tRNA
②核蛋白体“受体”上新进入的氨基酰tRNA;
③失去甲酰蛋氨酰基(或肽酰)后，即将从核蛋白体脱落的tRNA;
④接受甲酰蛋氨酰基(或肽酰)后已增长一个氨基酸残基的肽键
(二)转肽–肽键的形成(peptide bond formation)
在70S起始复合物形成过程中，核糖核蛋白体的P位上已结合了起始型甲酰蛋氨酸tRNA，当进位后，P位和A位上各结合了一个氨基酰tRNA，两个氨基酸之间在核糖体转肽酶作用下，P位上的氨基酸提供α-COOH基，与A位上的氨基酸的α-NH2形成肽键，从而使P位上的氨基酸连接到A位氨基酸的氨基上，这就是转肽。转肽后，在A位上形成了一个二肽酰tRNA。
(三)移位(Translocation)
图18-14 Diagram of the elongation process in protein synthesis following initation.tRNAs are shown as lue lines;AA,aamicoacyl group.The positioning shown of the EF-Tu-GTP on the tRNA and on the ribosome are arbitraty .This diagram does not make evident why the ribosomal enzyme catalysing the peptide bond synthesis is called peptidyl transferase .However,if you do the next round of synthesis yourself(see text),you will see that in all subsequent rounds of synthesis it is a peptide that is transferred to the incoming aminoacyl tRNA-hence the name.
转肽作用发生后，氨基酸都位于A位，P位上无负荷氨基酸的tRNA就此脱落，核蛋白体沿着mRNA向3’端方向移动一组密码子，使得原来结合二肽酰tRNA的A位转变成了P位，而A位空出，可以接受下一个新的氨基酰tRNA进入，移位过程需要EF-2，GTP和Mg2+的参加(图18-14)。
以后，肽链上每增加一个氨基酸残基，即重复上述进位，转肽，移位的步骤，直至所需的长度，实验证明mRNA上的信息阅读是从5’端向3’端进行，而肽链的延伸是从氮基端到羧基端。所以多肽链合成的方向是N端到C端(图18-15)。
图18-15 The elongation phase of protein synthesis on a ribosome.The three-step cycle shown is repeated over and over during the synthesos of a protein chain.An aminoacyl-tRNA moleculie binds to the A-site on the rebosome is step l,a new peptide bond is formed in sted 2,and the ribosome moves a distance of three nucleotides along the mRNA chain in step 3,ejecting an old tRNA molecule and "resetting" the ribosome so that the next aminoacyl-tRNA molecule can bind.As indicated in Figure 6-21,the p-site is drawn on the left side of the ribosome,with the A-site on the right.
图18-16 The final phase of protein synthesis .The binding of release factor to a stop codon terminates translation.The completed polypeptide is released,and the ribosome dissociates into two separte subunits.