Question

Gene expression is determined by several factors, including
chemicals in the body
• the brain
• protein synthesis
senescence

Answer 1

Short answer: yes — those things can affect gene expression, but some of the items you listed are causes (signals that change which genes are transcribed) while others are consequences (protein synthesis is mainly an outcome of gene expression) or are complex processes that both affect and are affected by expression (senescence, brain activity). Below is a clearer breakdown.

What directly determines gene expression
- Transcription factors and other DNA‑binding proteins — they turn genes on or off by binding promoters/enhancers.
- Chromatin state and epigenetic marks — DNA methylation, histone modifications, and chromatin remodeling control access of the transcription machinery to DNA.
- Noncoding RNAs — microRNAs, lncRNAs and others regulate mRNA stability and translation or influence chromatin.
- Signaling pathways — extracellular signals (growth factors, cytokines, hormones) activate intracellular cascades that modify transcription factors and chromatin.

How your listed items fit in
- Chemicals in the body: yes. Hormones, metabolites, cytokines and drugs are classic regulators because they trigger signaling pathways that change transcription factor activity and epigenetic state. Example: cortisol alters transcription via the glucocorticoid receptor.
- The brain: yes, indirectly and directly. Neuronal activity and neurotransmitters induce activity‑dependent genes (e.g., c‑fos, BDNF) in neurons; the brain also controls systemic hormones (hypothalamic–pituitary axis) that change gene expression bodywide.
- Protein synthesis: mainly a downstream step (translation) of gene expression. However, proteins produced (transcription factors, signaling enzymes) feed back to regulate transcription, so protein levels do influence future gene expression.
- Senescence: yes. Cellular aging changes chromatin structure and epigenetic marks, activates specific transcription programs (e.g., p16INK4a, SASP cytokines) and thus alters gene expression patterns.

Other important determinants
- Cell type and developmental stage (different transcriptional programs).
- Environmental factors: diet, toxins, temperature, circadian cues.
- Genetic sequence and mutations (promoter/enhancer variants).
- Stochastic (random) fluctuations in molecular components.

Short summary: gene expression is controlled by transcriptional regulators, chromatin/epigenetic state, noncoding RNAs, and signaling inputs (including body chemicals and brain-derived signals). Protein synthesis is the output that can feedback into regulation, and senescence shifts the regulatory landscape to produce different expression patterns.

If you want, I can diagram the steps from signal → transcription factor → chromatin change → transcription → mRNA → translation, with examples for each step.