To see first method click here

        Second method requires the isolation of the mRNA transcribed from a specific gene.If sufficiend amounts of the specific mRNA can be purified,the information in the sequence of bases in  the mRNA can be converted into a DNA copy by using the enzyme reverse transcriptase.Once the DNA has been synthesized ,it can be inserted into a plasmid,which is then cloned in the usual manner.

       Sometimes  it is not even possible to isolate the particular mRNA  of interest from the other mRNAs in cell.Often,however the polypeptide,or protein product can be isolated .If sufficient quantities of the protein can be purified, the complete amino acid sequence of the protein can b determined by chemical techniques. Using the genetic code, it is possible to determine the bases in the DNA that code for the particular amino acid sequence. Once the sequence of bases has been determined from the genetic code,the gene is chemically synthesized and cloned in the usual manner.