RNA differs from DNA: ribose sugar (2'-OH present), uracil replaces thymine, usually single-stranded, less stable (2'-OH makes it susceptible to alkaline hydrolysis).

mRNA (messenger RNA): Carries genetic code from DNA to ribosome. Codons (triplets of bases) specify amino acids. Read 5'→3'. Shortest-lived RNA type.

tRNA (transfer RNA): Cloverleaf shape with anticodon loop (binds to mRNA codon) and 3'-CCA acceptor stem (amino acid attaches here). ~75-90 nucleotides. At least one specific tRNA for each amino acid.

rRNA (ribosomal RNA): Structural and catalytic component of ribosomes (ribosome = rRNA + proteins). Most abundant RNA (~80% of total cellular RNA). The peptidyl transferase activity (forming peptide bonds) is catalyzed by rRNA — making it a ribozyme.

Central dogma: DNA → (transcription) → mRNA → (translation) → Protein. Reverse transcriptase (in retroviruses like HIV) can make DNA from RNA.