The primary, tetiery and quadry

生物是个很豪的学科，为什么这么说呢？实验室高精密仪器随便一指就是一台特斯拉就不说了，研究蛋白质结构的蛋白结晶，一克拉的制作成本要比相同重量的一克拉钻石贵五十几倍。1 carat diamond costs about $1500, the formation of 1 carat crystalized protein costs $80000. 这是这学期一个course的final assignment，有关蛋白质结构机制review的一个选段。写作目的是让学生了解蛋白质的各级结构，在细胞中的作用机制及其研究手段。这段已做删减改写，相当容易了，看不看在你啦。

The primary structure of Streptococcus pyogenesCas9 endonuclease contains 1368 amino residues Fig. 1A (Nishimasu, et al. 2015; Nishimasu, et al. 2014). With the help of Cryo-electron microscopy (cryo-EM) and X-ray crystallography, Nishimasu and Jinek found that two functional lobes, the REC (recognition) and NUC (nuclease) lobe, were constructed by six tertiary domains. The REC lobe helps the enzyme to identify specific DNA location under the help of crRNA and tracrRNA hybrid, which composed of three tertiary domains: long a-helix domain, REC1 and REC2 domain Fig. 1B-1C (Jinek, et al. 2014; Nishimasu, et al. 2014). Following the recognition of target sequence, the NUC lob of Cas9 cut the binding site to induce a double strand break, which composed by RuvC III, HNH and PI domain.