Question

Gene expression is determined by several factors, including
Responses

the brain
the brain - no response given

protein synthesis
protein synthesis - no response given

chemicals in the body
chemicals in the body - no response given

senescence

Answer 1

Gene expression is influenced by many things. From your list:

- Chemicals in the body — Yes. Hormones, growth factors, nutrients, metabolites and other signaling molecules directly alter transcription factor activity, signaling pathways and epigenetic states to change gene expression.
- Senescence — Yes. Aging changes chromatin structure, epigenetic marks and regulatory protein levels, causing widespread shifts in gene expression.
- Protein synthesis — Not really a determinant; it is a primary outcome of gene expression. (That said, some proteins feed back to regulate transcription, so there is an indirect role.)
- The brain — Not as a direct determinant, but neural signals and brain-controlled hormones (e.g., hypothalamic/pituitary hormones) can influence gene expression in many tissues.

More generally, gene expression is controlled by DNA sequence, transcription factors, chromatin/epigenetic modifications, noncoding RNAs, cell type and developmental stage, internal chemical signals, and environmental cues.