When you say you want to isolate 'a particular mRNA set' what do you mean exactly?  Genes of the same family? splice variants?

If we need to isolate mRNA  from a perticular cells  (eg-mature mRNA of a, DMD gene, encoding the dystrophin protein) of a person , the same cells contain variety  of other mRNA is it,....   

Therefore how to modify cDNA for that perticular mature mRNA 

Are you looking for full length transcripts/complete coding sequences or merely looking for the presence of a pArticular transcript for expression studies?

For Perticular gene identification 

You need to explain in a little more detail  exactly what the aims of your experiment are for anyone to offer the correct help.  If you are just looking for the presence of a pArticular mRNA then all you need do is design primers to amplify short (150-300 bp) fragments from 2 - 3 regions along your gene of interest and perform regular PCR on the cDNA.  Did you generated your cDNA library using poly-T primers or random hexamers?  If Poly T then amplificatin of fragments closer to the 3 ' end of the gene  are more likely to work.

 Yes Sir you are correct, I'm just clarify that many independant transcription events can (and do) occur simultaneously.

Many genes are in the process of transcription or translation or both all at the same

time within the same cell for survival is it....

There can be a bunch of mRNA in a cell at a time 

All the mRNA have poly A tail therefore all the mRNA will produce complementary  DNA while add the poly T primer is it correct sir.