The transcriptome is a complete collection of RNA from an organism, a group of cells, or even a specific cell. RNA is synthesized from DNA through a process known as transcription. The RNA present in a cell and the body determines which genes will be expressed, and it changes over the course of an organism's life, unlike DNA, which remains static. Environmental pressure is one of the leading reasons for changes, as the body attempts to adapt to changing environmental situations to keep itself functioning.
The transcriptome is much smaller than an organism's genome, the complete collection of DNA in the organism. Put in simple terms, one could think of DNA as a cookbook, listing a complete collection of recipes which can be made. The transcriptome is the code which determines which recipes are going to be made, and when. The cookbook remains static throughout life, while the transcriptome changes in response to environmental factors.
One important part of the transcriptome is mRNA molecules. Messenger RNA is RNA which communicates with the ribosomes in the cells to direct the cells to make various proteins. These proteins are used for a wide variety of functions. Other parts may not directly code for protein production, but do things like determining cell structure and regulating gene expression. Together, the various components of the transcriptome keep an organism functioning and determine gene expression within that organism.
Within various cells in the body, the transcriptome can be very different. Liver cells, for example, have RNA which makes them liver cells, and ensures that the cells perform necessary functions. Skin cells, on the other hand, have a different transcriptome which activates the genes necessary to make a cell a skin cell. Both liver and skin cells have a complete copy of the genome, but their transcriptomes determine which parts of the genome are activated.
Just as with DNA, it is possible to sequence RNA. Transcriptome sequencing can be done to learn more about the actions of a cell and the host organism's DNA, and to identify the places where the expression of genetic information goes wrong. When a liver cell becomes malignant, for example, it is because the transcriptome is not operating as it should be, and genes for malignancy have become activated, or the cells lack regulation in some way, causing them to grow out of control. Sequencing the RNA in a malignant cell can help a researcher figure out which gene was activated or inactivated to cause the malignancy.