You can design primers that target the desired mRNA fragment of your desired fragment is found in a cell.Then PCR products will give you the copies of desired fragment. If not, you can synthesis the fragment.

but after to see what it's goingh on in my tube, so to see if I have my desired fragment my only option is to use the luciferase?

Both ways are applicable but it depends on your target and your labratory condition.

If you have the oraganism that you want to produce the mRNA from that, so you can just deaigned specific primers according to your target gene and priduced your related mRNA. Eventhough you do not have the related organism you might search about the sequence and synthesise that chemically. The second method is relatively expensive.

In order to know whats going on in your test tube you can use a variety of gene reporter such as luciferase, gfp, and even you can investigate the gene expression and mRNA production by applying real-time pcr.

thanks a lot for your help. I don't have jet the mRNA fragment so I have to decide in way to produce it;

can you tell me the benefit and th elimits to use PCR and the Plasmid? like time, cost, difficulty of the procedur ect ect