Before generating sequencing data, first researchers collect and preserve a biological sample. Typically, the material sequenced is living tissue that was surgically extracted in a biopsy or other procedure or after a dissection in the case of laboratory animal experiments. Collected samples (called fresh frozen) are preserved by freezing the sample in liquid nitrogen immediately after extraction, rapidly cooling the tissue before it can break down so that DNA or RNA remains intact. Without freezing or another preservation method, RNA starts to degrade in minutes, and DNA not long after.
Occasionally sequence samples are preserved by other techniques like formalin-fixed, paraffin-embedded (FFPE). FFPE is a standard laboratory tool that predates genomic sequencing, developed for studying samples under a microscope. Because FFPE samples store well for a long period of time at room temperature, they can be inexpensively transported and relatively large historical collections of rare samples exist that can be mined for sequences. However, since the FFPE technique damages the DNA and RNA, whenever possible, tissue samples will typically be frozen to capture the highest quality sequences. For more information about FFPE sample collection, please visit this excellent guide.