Cationic liposomes are traditionally used for the delivery of genetic materials such as various types of DNA (pDNA, cDNA, CpG DNA, oligonucleotide, antisense oligonucleotide, etc.), various types of RNA such as (siRNA, mRNA, etc.) and nucleic acid mimics (NAMs). The encapsulation of DNA into the conventional neutral charged PC based liposomes can be a technical problem mainly due to the plasmid size. Due to this problem in late 80s, the liposomes composed of cationic lipids and PE have been developed. The idea was to neutralize the negative charge of pDNA with positive charge of cationic lipids in order to capture more plasmid efficiently mainly due to electrostatic interaction and deliver them into the cells. Generally, the procedure is simply based on mixing the cationic liposomes with DNA or RNA and adding them to the cells. This results in the formulations of aggregates.
In order to design a proper cationic lipid for gene delivery, two approaches have been used for the cationic lipid synthesis: 1) cholesterol-based design such as DC-Cholesterol and GL-67 lipids, and 2) non-cholesterol-based designs such as DOTAB, DDAB and DOTMA. To successfully transfer the gene in vitro using liposomes, some consideration should be taken into account: i) the ability of binding and packing DNA/RNA in liposomes; ii) the interaction of the packaged DNA/RNA to the cell surface; iii) the efficiency of the internalization of DNA/RNA; iv) the intracellular DNA-release from endosomes in case of endocytosis involvement; v) the transgenic expression level in cell nuclei. pH-sensitive liposomes have been designed based on their tendency to release their content in the acidic condition. The primary concept was based on viruses that fuse with the endosomal membrane by means of a protein at pH 5-6, delivering their genetic material to the cytosol before reaching the lysosomes. Typically, a pH-sensitive liposome consists of dioleoylphosphatidylethanolamine (DOPE). Since phosphatidylethanolamine (PE) changes in acidic conditions, it is believed to act as a membrane fusion promoter. The effectiveness of the interaction between liposomes and cells is highly dependent on the liposome compositions. Liposomes are captured by various endocytosic processes, and the efficiency depends on the cell type and liposome size. Liposomes of various sizes and charges can attach to the macrophages and neutrophils through active phagocytosis. After attachment of the liposome to the cell surface, the internalization into the endosomes occurs due to a more acidic pH (6.50) at early endosomes. The liposomes are transferred to the last endosome with more acidic pH (5.5-6.0) by maturation or vesicular fusion, which takes 10-15 min. Twenty minutes (or more) after uptake, the contents are delivered to the lysosome with pH 5.0 or less. Lysosomes are the main degrading and last endocytotic section in the endocytotic pathway, in where pH-insensitive liposomes are accumulated and degraded. However, after penetration of pH-sensitive liposomes into cells, the accumulation and degradation do not occur.
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短截就是把枝条剪短,主要作用是促使其抽生新梢,增加分枝数目,以保证树势健状和正常结果。短截常用于骨干枝组修剪,结果枝组修剪,和树体局部更新复状。
短截按其长度可分为:
① 中短截:在一年生枝的中部短截,剪后萌发的顶端枝条,长势强,下部枝条长势弱。
② 重短截:剪去一年生枝的2/3。剪后萌发出的枝条较强状,一般用于主侧枝延长头修剪。
③ 重剪:剪去一年生枝的3/4-4/5,剪后萌发出的枝条长势强状,常用于发育枝作延长枝头和徒长果枝,中果枝的修剪。
④极重短截:剪去一年生枝的4/5以上,萌发后的枝条中庸偏状,常用于将发育枝和徒长枝培养结果枝组。
⑤留基部2芽剪:剪后萌发枝条较旺盛,常用于预备枝的修剪。对于幼龄树,树势较旺,以培养良好而牢固的树形结构,提早结果为主要目的,以轻短截,少疏间为主,从始果期到盛果期,主要使桃树多结果,并形成好的树形。
如果PCR产物不是很纯,或者PCR扩增条带比较小,PCR产物前面又有较多引物二聚体时,用胶回收,其余用PCR产物纯化试剂盒。
pcr纯化试剂盒和胶回收试剂盒的区别:
PCR纯化试剂盒:是直接水溶解的PAC产物就可以回收,回收效率高,但是只适合单一条带需要纯化测序的时候使用。
PCR凝胶试剂盒:是在PCR产物是混合物,有多条杂带的情况下,先跑胶将杂带分离,然后在将所要的条带位置的胶切下回收,后者的回收效率低,但是很纯净。
胶回收试剂盒操作步骤:
配制琼脂糖EB凝胶,电泳以分离DNA片段。任何类型或等级的琼脂糖都可以使用。
电泳足够时间后,在紫外灯下小心地把所需的DNA的片段切下来。并尽量去除多余的凝胶。
称取空离心管的重量,切下带目的片段的凝胶装在1.5ml离心管中并称其重量,求出凝胶块的重量,近似地确定其体积。一般情况下,凝胶的密度为1g/ml,于是凝胶的体积与重量的关系可按下面换算:凝胶薄片的重量为0.2g 则其体积为0.2ml;加入等倍凝胶体积的Binding Buffer,把混合物置于55℃~65℃水浴中温浴7min至凝胶完全融化,其间每隔2-3分钟混匀一次;
转移700μl的DNA-琼脂糖溶液到一个HiBindTM DNA柱子,并把柱子装在一个干净的2ml收集管内,室温下,10,000×g离心1min,弃去液体。
将柱子重新套回收集管中,加300μl Binding Buffer至HiBind DNA 柱子中;室温下,10,000×g离心 1分钟,去弃滤出液;这一步相当关键,不要忽略此步。
将柱子重新套回收集管中,加入700μl SPW Wash buffer至HiBind DNA柱子中,室温下,10,000×g离心1分钟,去弃滤出液;注:SPW Wash buffer在使用前必须按瓶子标鉴要求用无水乙醇进行稀释。
将柱子重新套回收集管中,重复加入700μl SPW Wash buffer至HiBind DNA柱子中,室温下,10,000×g离心1分钟,去弃滤出液;
弃去液体,将空柱子重新套回收集管中,10,000×g离心1min以甩干柱基质残余的液体。
这步可以去除柱子基质上残余的乙醇,不要省略此步―――对得到好的DNA产量是十分重要的。
把柱子装在一个干净的1.5ml离心管上,加入30~50μl洗脱液或灭菌水上柱子膜上,10,000×g离心1分钟,离心管中的溶液就是纯化的DNA产物,保存于-20度。
如果用组织DNA提取试剂盒,提出来的就是组织细胞的DNA了
过表达慢病毒
>10^8 PFU/ml
干扰慢病毒
>10^8 PFU/ml