Recombinant DNA Technology
Some genetic diseases cannot be diagnosed by changes in restriction sites. Some of these can be detected by allele-specific oligonucleotide probes. These are

short sequences that will hybridize only to a specific base sequence
mutagenized copies of a gene
PCR-amplified variable numbers of tandem repeats (VNTRs)
copies of the gene with an altered sequence so that a restriction site is inserted

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Recombinant DNA Technology
The order for the construction of a cDNA fragment from mRNA is to

bind oligo-dT, treat with reverse transcriptase, digest with RNase, add G residues to the 3' end, bind oligo-dC, treat with DNA polymerase
treat with reverse transcriptase, digest with RNase, add G residues to the 3' end, bind oligo-dC, treat with DNA polymerase and bind oligo-dT
bind oligo-dC, treat with reverse transcriptase, digest with RNase, add G residues to the 3' end, bind oligo-dT and treat with DNA polymerase
digest with RNase, add G residues to the 3' end, treat with reverse transcriptase, add G residues to the 3' end and treat with DNA polymerase

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