To check the expression of the genes by real time PCR you may need to make the standard plasmid.
In a way, yes. You first need to clone the gene of interest into a vector. By definition, expression means DNA-->mRNA-->protein. Since real-time PCR only deals with DNA/RNA, then you need to detect the transcription product from DNA, ie. mRNA. So, essentially, it's the detection and quantification of mRNA-->real-time RT-PCR(rRT-PCR). In another word, plasmid (DNA) is not an appropriate standard for the process. Instead, you need to use RNA as the standard. To do this, you can use the cloned gene plasmid (preferably has built-in promoter, such as T7 or SP6) as template for in vitro transcription (Promega or Qiagen both have kits for this) to produce ssRNA. You can then measure the concentration of your RNA and do serial dilution to build standard curve for you rRT-PCR reactions.
The way to test your gene expression strongly depends on your experimental design. In the most common case when you need to check the differences of the expression of a gene in two different samples (control and treated, or different tissues) the best way is to normalize the Ct values of your gene(s) of interest with the Ct values of reference genes.
If you want to know almost everything about qPCR normalization I strongly suggest you to check this link.
http://www.gene-quantification.de/hkg.html
It is possible to check gene expression with a standard plasmid, using the plasmid as an internal control so to say. That requires that you pipet the exact amount of template in each well for qPCR. Depending on your experimental design, I strongly recommend to use a reference housekeeping gene, that doesn't change its expression in any of your experimental treatments / conditions. There are excellent publications on how to find a good housekeeping gene available online. Contact me, if you need to find something about plant housekeeping genes. Housekeeping gene = reference gene, that Matteo mentioned above.
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