Before using biological samples in downstream molecular assays (such as PCR or gel electrophoresis), they must be extracted to isolate and purify the nucleic acids you are interested in studying. For example, DNA extraction is the prerequisite to genotyping and sequencing experiments. RNA extraction is required before use in gene expression analyses such as real time RT-PCR.
If you are extracting a sample other than blood or bodily fluid, tissue homogenization must be performed prior to the extraction protocol. Then, extraction is performed using either an “in-house” protocol or an extraction kit. Kits are generally “idiot-proof” and easier to use compared to in-house protocols; however, they are almost always more expensive. Kits are highly recommended for RNA extraction, because RNA is more sensitive to contamination and degradation.
Once your DNA is extracted, it is relatively stable and can be stored frozen for years. RNA samples are less stable and must be carefully stored to avoid freeze-thaw cycles until they are reverse-transcribed to produce cDNA.