Genetic Code PDF

# What Is Genetic Code? - Genetic code is the genetic information carried by living cells. - Term given by *Goerge Gamow*. ## Codon - It is a set of 3 nitrogenous bases in *mRNA* which provide genetic information for amino acid during synthesis of polypeptide chain. # THE GENETIC CODE - RNA con...

# What Is Genetic Code? - Genetic code is the genetic information carried by living cells. - Term given by *Goerge Gamow*. ## Codon - It is a set of 3 nitrogenous bases in *mRNA* which provide genetic information for amino acid during synthesis of polypeptide chain. # THE GENETIC CODE - RNA contains 4 different bases: **A**, **G**, **C**, **U**. - The genetic code is read *three letters* at a time - Each *“word”* of the coded message is *three* base pairs long - These *“words”* are called **codons** # Important features of the genetic code - Each codon consists of three bases (triplet) - There are 64 codons. They are all written in the 5’ to 3’ direction. - 61 codons code for amino acids. - The other three (UAA, UGA, UAG) are stop codons (or nonsense codons) that terminate translation. - There is one start codon (initiation codon), AUG, coding for methionine. - Protein synthesis begins with methionine (Met) in eukaryotes, and formylmethionine (fmet) in prokaryotes. - The code is unambiguous. - Each codon specifies no more than one amino acid. # Genetic Code - Table | **1st letter** | **U** | **C** | **A** | **G** | **3rd letter** | |:---|:---|:---|:---|:---|:---| | **U** | UUU Phe | UCU UCC Ser | UAU Tyr | UGU Cys | **U** | | | UUC | UCG | UAC | UGC | **C** | | | UUA Leu | UCA | UAA Stop | UGA Stop | **A** | | | UUG | UCG | UAG Stop | UGG Trp | **G** | | **C** | CUU | CCU | CAU His | CGU | **U** | | | CUC Leu | CCC Pro | CAC | CGC Arg | **C** | | | CUA | CCA | CAA Gln | CGA | **A** | | | CUG | CCG | CAG | CGG | **G** | | **A** | AUU Ile | ACU | AAU Asn | AGU Ser | **U** | | | AUC | ACC Thr | AAC | AGC | **C** | | | AUA | ACA | AAA Lys | AGA Arg | **A** | | | AUG Met | ACG | AAG | AGG | **G** | | **G** | GUU | GCU | GAU Asp | GGU | **U** | | | GUC Val | GCC Ala | GAC | GGC Gly | **C** | | | GUA | GCA | GAA Glu | GGA | **A** | | | GUG | GCG | GAG | GGG | **G** | Biochemistry For Medics # Salient Features of Genetic Code - **1. Triplet Codons:** - Each codon is a consecutive sequence of three bases on the mRNA. - e.g. UUU codes for phenylalanine - **2. Non overlapping:** - The codes are consecutive. - Therefore, the starting point is extremely important. - The codes are read one after another in a continuous manner, e.g. AUG, CAU, CAU, GCA, etc. - **3. Non punctuated:** - There is no punctuation between the codons. It is consecutive or continuous. - **4. Degenerate:** - When an amino acid has more than codon, this called degeneracy of the code. - E.g. *serine* has 6 codons while *glycine* has 4 codons. - **5. Unambiguous:** - Through the codons are degenerate, they are unambiguous: or without any doubtful meaning. - That is, one codon codes only one amino acid. - **6. Universal:** - The codons are the same for the same amino acid in all species; the same for *“Elephant and E.coli”*. - The genetic code has been highly preserved during evolution. - **7. Terminator codons:** - There are three codons which do not code for any particular amino acids. - They are “nonsense codons”, more correctly termed as punctuator codons or terminator codons. - They put “full stop” to the protein synthesis. - These three codons are UAA, UAG, and UGA. - **8. Initiator codon:** - In most of the cases, AUG acts as the initiator codon.