A **gene** is a unique sequence of nucleotides that codes for a functional product — usually a **protein**, but sometimes a functional RNA (tRNA, rRNA, microRNA). Turning a gene into a protein takes two steps, together called the **central dogma**. ## Exons and introns In eukaryotes a gene contains coding **exons** and non-coding **introns**. Both are transcribed, but the introns are spliced out — **only the exons are translated** into the polypeptide. (Most prokaryotes have no introns.) ## Transcription — making mRNA In the **nucleus**, **RNA polymerase** reads the **template strand** of DNA and builds a complementary **mRNA** copy. RNA uses **uracil (U)** in place of thymine, so the mRNA has the same sequence as the **coding strand** (with U for T). DNA is read $5'\to3'$ by convention. ## Translation — making protein At the **ribosome** (made of rRNA + protein), mRNA is read in **codons** — triplets of bases. Each codon is matched by the complementary **anticodon** of a **tRNA**, which delivers one specific **amino acid**. The amino acids join into a polypeptide that folds into the finished protein. > Codon (mRNA) ↔ anticodon (tRNA) by complementary base pairing — each triplet specifies one amino acid.

Genes & Protein Synthesis