In a simple model, gene expression is directly controlled by the rate of transcription.
In response to a stress, a gene (coding for a specific protein) that is not constitutive (not always expressed)
will be transcribed. As such, the level of messenger RNA (mRNA) will vary depending on the stress level.
Here, the level of mRNA increases immediately upon stress (even so this stress is transient (noise)).
To filter noise, more advanced types of regulation exist. Let us just assume that (i) a transcript (black) is always produced and that this transcript is non coding (ii) the mRNA (coding for a specific protein) is not constitutive and (iii) this mRNA and the non-coding transcript are not stable (in other words, the mRNA and the non-coding transcript form a structure that is degraded). In that case, the level of mRNA will start to increase only after a certain time. This mechanism allows a fine tuning of gene expression as no protein will be produced if the stress is only transient.
Using single molecule techniques (e.g. Magnetic Tweezers), we study relevant processes (initiation of transcription, initiation of replication) and their implication in the control of gene expression.