不可缺少

Lake Tomyhoi in the background, just above the edge of the snowfield in the center of the photo.

远眺Tomyhoi湖，现在的位置正处于雪地的边缘我们应该称之为雪线

The zot gene encoding Zonula occludens toxin was amplified from classic Vibrio cholerae genomic DNA by PCR.

从霍乱疫苗菌中抽提基因组DNA ，用PCR的方法扩增zot基因。