Chemosynthesis and protein biosynthesis

In replication enzymes separate the strands of the DNA double helix and build a new complementary strand on each, producing two identical DNA molecules, each with one old and one new strand. In transcription a complementary mRNA is made in the nucleus on one strand of DNA as a template (uracil instead of thymine: A–U, T–A, G–C, C–G). The genetic code is the writing of 20 amino acids with 4 kinds of nucleotides: each amino acid is specified by three nucleotides in mRNA, a triplet (codon). There are 4 × 4 × 4 = 64 codons: 61 code for amino acids (many amino acids by several codons) and 3 (UAA, UAG, UGA) stop synthesis; AUG marks the start and codes for methionine. The code is universal: almost the same from bacteria to humans. In translation the ribosome moves along the mRNA, a tRNA with an anticodon complementary to each codon brings the matching amino acid, and the amino acids are joined by peptide bonds. A gene is a segment of DNA carrying information about one protein (or RNA). Short table: AUG – Met, UUU – Phe, GGC – Gly, GAA – Glu, UGG – Trp, AAA – Lys, CCU – Pro, GCU – Ala.