What molecule carries genetic information in cells?

DNA (deoxyribonucleic acid) is the genetic material in all known living organisms. Structure: (1) Double helix — two strands wound around each other; (2) Nucleotides — building blocks; each contains a sugar (deoxyribose), phosphate group, and one of four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G); (3) Base pairing — A always pairs with T (2 hydrogen bonds); C always pairs with G (3 hydrogen bonds); this complementary pairing allows accurate replication; (4) Sugar-phosphate backbone — sides of the 'ladder'; bases are the 'rungs'. DNA structure discovered by Watson, Crick, Franklin, Wilkins (1953). DNA → genes → chromosomes: a gene is a DNA sequence coding for a protein; genes are organized into chromosomes; humans have 46 chromosomes (23 pairs). The Central Dogma: DNA → RNA → protein. (1) Transcription: DNA is copied into messenger RNA (mRNA) in the nucleus; (2) Translation: mRNA is read by ribosomes; transfer RNAs (tRNAs) bring amino acids; amino acids link to form proteins. Codons: three-letter mRNA sequences each coding for one amino acid (e.g., AUG = methionine/start). DNA replication: occurs before cell division; semi-conservative (each new molecule has one old and one new strand); enzymes include helicase (unwinds), DNA polymerase (adds nucleotides). Mutations: changes in DNA sequence (point mutations, insertions, deletions); can be harmful, neutral, or beneficial.