5.10 Genetic Code · Part 2

example, the mRNA (UUU) codon codes for phenylalanine in all cells of all organisms. Some exceptions are reported in prokaryotic, mitochondrial and chloroplast genomes. However similarities are more common than differences. • A non-overlapping codon means that the same letter is not used for two different codons. For instance, the nucleotide sequence GUU GUC represents only two codons. • It is comma less, which means that the message would be read directly from one end to the other i.e., no punctuation are needed between two codes. • A degenerate code means that more than one triplet codon could code for a specific amino acid. For example, codons GUU, GUC, GUA and GUG code for valine. • Non-ambiguous code means that one codon will code for one amino acid. • The code is always read in a fixed direction i.e. from '→ ' direction called polarity. • AUG has dual functions. It acts as a initiator codon and also codes for the amino acid methionine. XII Std Zoology Chapter XII Std Zoology Chapter Molecular Genetics • UAA, UAG and UGA) codons are designated as termination (stop) codons and also are known as “non-sense” codons.