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northern blotting the extension of the southern blotting method.   However, northern blotting method is mostly used to identify RNA and not DNA. you have to use southern blotting method for DNA fragments identification.

 sure.The basic principle behind southern blotting method is: in this technique the property of a   radio-labelled probe is exploited using a single stranded DNA. however,   If you want to determine the presence of a particular sequence in mixed DNA sample then the probe   must be designed accordingly which have complementary sequence to the target sequence.   To know more about this technique visit https://www.khanacademy.org/test-prep/mcat/biomolecules/dna-technology/v/southern-blot.

In southern blotting method, DNA from the sample must be cleaved into restric­tion fragments using a restriction endonuclease.  These fragments are then spread apart via gel elec­trophoresis. The double-stranded helix of the DNA fragment must be denatured into single strands using the pH of gel basic.  The is then blotted with nitrocellu­lose, thereby transferring few DNA strands to the nitrocellulose sheet. Next, a probe including purified, single-stranded DNA relevant to specific gene is transformed into the sheet.  The DNA fragment that has a nucleotide sequence will hybridise with the probe. If the probe if is 32P, will be radioactive,  and the nitrocellulose sheet will display a band of radioactivity where the probe will be hybridised with fragment spe­cific gene.