1.1 The cell

Your body is ~30 trillion cells. Each is a bag (membrane) of watery fluid (cytoplasm) holding machinery. The membrane is a double layer of fat (lipid bilayer) — this matters because fat-soluble molecules (vitamin D, testosterone, curcumin) pass straight through it, while water-soluble ones (peptides, most drugs) need a door (a receptor or transporter). This single fact explains half of why some compounds are pills and others are injections.

1.2 The central dogma: DNA → RNA → protein

This is the most important idea in the whole wiki. Learn it once and every "gene target" link makes sense.

• DNA is the master blueprint, stored in the nucleus. A gene is one instruction in that blueprint — the recipe for one protein.
• To use a gene, the cell copies it into messenger RNA (mRNA) — a disposable working copy (this step is transcription).
• The mRNA travels to a ribosome, which reads it and strings together amino acids into a protein (this step is translation).
• A protein is a chain of amino acids that folds into a 3D shape. The shape is the function.

Worked example — a protein chain you can actually see: BPC-157 is a 15-amino-acid chain:

Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val (written in one-letter code: GEPPPGKPADDAGLV). Each three-letter block is one amino acid; the bonds between them are peptide bonds (which is why short chains are called peptides and long ones proteins). That chain folds into a specific shape that lets it interact with the VEGFR2 receptor. Visualize it: [PubChem 3D structure] shows the molecule; for full proteins, the Mol\* viewer at RCSB PDB renders the real 3D fold in your browser.

Why this matters for drugs: some compounds work by changing which genes get transcribed (testosterone, vitamin D, rapamycin — they turn recipes on or off). Others never touch DNA and just flip a switch on a protein (caffeine, GLP-1 drugs). When an entry says "upregulates X," it means "makes the cell transcribe more of gene X."

1.3 The four kinds of protein target (this covers ~all of the wiki)

Almost every compound acts on one of four protein types. Learn these four and you can classify any drug:

1. Receptors — proteins that receive a signal. A molecule that fits a receptor is a ligand. Two big families:

- G-protein-coupled receptors (GPCRs) — thread through the membrane 7 times; when a ligand binds outside, they trigger a chemical relay inside (e.g., adenosine receptor / caffeine, GLP-1 receptor / semaglutide, adrenergic receptors / clenbuterol). The largest drug-target family in medicine.

- Nuclear receptors — live inside the cell; when a fat-soluble hormone binds, the pair travels to the DNA and switches genes on/off (e.g., androgen receptor / testosterone, vitamin D receptor, PPARs). This is why steroids are slow but powerful — they rewrite gene expression.

1. Enzymes — proteins that speed up a chemical reaction. Drugs usually inhibit them (statins block HMG-CoA reductase; finasteride blocks 5-alpha-reductase; SSRIs block the serotonin transporter's function).
2. Transporters — doors that move things across the membrane (SLC6A8 pulls creatine into muscle; SGLT2 reabsorbs glucose; the dopamine transporter DAT recycles dopamine — blocked by stimulants).
3. Ion channels — gated pores for charged particles (nicotinic receptors, GABA-A channels, the potassium channels minoxidil opens).

1.4 The signaling relay (how one molecule outside becomes a big effect inside)

When a ligand hits a GPCR, the receptor activates a second messenger (often cAMP), which activates enzymes called kinases. A kinase attaches a phosphate group to another protein — this is phosphorylation, the cell's universal on/off switch. A cascade of phosphorylations amplifies a tiny signal into a big cellular response. Example you'll see repeatedly: ligand → receptor → cAMP → PKA → hormone-sensitive lipase → fat released. Once you recognize this shape, clenbuterol, caffeine, and ephedrine are obviously the same story.

1.5 The messengers: hormones vs neurotransmitters

• Hormones travel in blood to distant targets (testosterone, insulin, thyroid, GLP-1). Slow, body-wide.
• Neurotransmitters cross the tiny gap between two nerve cells (dopamine, serotonin, GABA, acetylcholine, glutamate). Fast, local. Most "focus/mood/sleep" compounds tune one of these five.