Final answer:
RNA, specifically messenger RNA (mRNA), is the molecule commonly measured to detect the level of gene expression, as it carries the genetic message from DNA and indicates which genes are actively being expressed in the cell.
Step-by-step explanation:
The molecule commonly measured to detect the level of gene expression is RNA, and more specifically, messenger RNA (mRNA). During the process of gene expression, DNA is transcribed into RNA, and then RNA is translated to form proteins. This transcription process makes an mRNA molecule that carries the genetic message from DNA to the cell's protein-synthesizing machinery. The use of mRNA is crucial because it directly represents the protein-coding genes that are being expressed in the cell at a given time. Measuring mRNA helps in understanding the transcriptional activity and the regulation of gene expression.
Techniques such as DNA microarrays and RNA analysis are employed to monitor mRNA levels for multiple genes simultaneously. Tools like gel electrophoresis are also used to study RNA sizes and abundance. Unlike DNA that remains in the nucleus of eukaryotic cells, mRNA molecules travel out of the nucleus, making them accessible for analysis and serving as indicators of active gene expression.