Final answer:
The regulation of gene expression typically involves transcription factors that can either activate or repress genes, as well as epigenetic changes that modify the accessibility of genes for transcription. In complex organisms, gene expression can also be regulated by hormones and other signaling molecules in response to environmental changes or developmental processes.
Step-by-step explanation:
What can be decisive in switching on or off a gene or group of genes? For organisms, especially single-celled like bacteria, conserving energy by not producing unnecessary proteins is crucial. Transcription factors are key proteins that bind to specific genes on the DNA molecule to promote or inhibit their transcription, thus regulating gene expression. Moreover, genes can be positively regulated by stimulatory proteins/compounds or negatively regulated by repressor proteins/compounds. For example, a steroid hormone acts as a positive modulator, enhancing gene expression until the hormone is degraded.
Epigenetic regulation is another important mechanism that involves temporary modifications to the histone proteins and DNA, influencing chromosomal structure and thereby gene accessibility for transcription. In multicellular organisms, signals such as hormones can trigger transcription control, which could respond to nutrient levels or developmental cues in body fluids.