Translation involves the synthesis of proteins using the RNA template. Processed messenger RNA (mRNA) molecules are exported from the nucleus to the cytoplasm where the process of translation to polypeptide chains occurs (Fig. 1.11). This involves large RNA-protein complexes (ribosomes) with successive groups of three nucleotides (codons) coding particular amino acids. This specificity is achieved by transfer RNA (tRNA) molecules which have a particular trinucleotide sequence (anticodon) complementary to the RNA codon, and which bind a particular amino acid. The RNA sequence 'AUG' is recognized as the start or initiation codon when embedded in the initiation codon recognition sequence (for example GCCPuCCAUGG, where Pu is purine). The 'genetic code' relates codons in the RNA to amino acids in the protein. Each amino acid is specified on average by three different codons: overall there are 64 possible codons (43) and about 30 types of cytoplasmic tRNA (Fig. 1.11). This relates to degeneracy in codon-anticodon pairing most often involving the third base of the codon, such that either of two possible purine or pyrimi-dine bases will be recognized (for example AAA or AAG for lysine) or any of the four possible bases (ACA, ACG, ACC, ACU for threonine).
Genomic ^ sequence ^non template template strand <--- T G G ---> A C C
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