Genetic code -
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The genetic code is the system by which nucleotide sequences in mRNA determine the amino acid sequences in proteins. The genetic code uses triplets of nucleotides called codons to specify which amino acid will be incorporated into the growing polypeptide chain. There are 64 possible codons but only 20 standard amino acids, so most amino acids have multiple codons. Three codons act as stop signals to end protein synthesis. The genetic code is nearly universal across all life due to its high degree of specificity and redundancy.
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Genetic code -
- 2. Genetic Code The information needed to direct the synthesis of protein is present in the mRNA in the form of a genetic code. The genetic code is the system of nucleotide sequences of mRNA that determines the sequence of amino acids in protein. Codons are a group of three adjacent bases that specify the amino acids of protein.
- 3. • Since there are 4 bases and 3 positions in each codon, there are 4 x 4 x 4 = 64 possible codons • 64 codons but only 20 amino acids, therefore most have more than 1 codon • 3 of the 64 codons are used as STOP signals; they are found at the end of every g ene and mark the end of the protein • One codon is used as a START signal: it is at the start of every protein
- 4. U C A G U UUU Phe UCU Ser UAU Tyr UGU Cyc U UUC UCC UAC UGC C UUA Leu UCA UAA Stop UGA Stop A UUG UCG UAG UGG Trp G C CUU CCU Pro CAU His CGU Arg U CUC CCC CAC CGC C CUA CCA CAA Gln CGA A CUG CCG CAG CGG G A AUU IIe ACU Thr AAU Asn AGU Ser U AUC ACC AAC AGC C AUA ACA AAA Lys AGA Arg A AUG Met ACG AAG AGG G G GUU Val GCU Ala GAU Asp GGU Gly U GUC GCC GAC GGC C GUA GCA GAA Glu GGA A GUG GCG GAG GGG G U C A G U UUU Phe UCU Ser UAU Tyr UGU Cyc U UUC UCC UAC UGC C UUA Leu UCA UAA Stop UGA Stop A UUG UCG UAG UGG Trp G C CUU CCU Pro CAU His CGU Arg U CUC CCC CAC CGC C CUA CCA CAA Gln CGA A CUG CCG CAG CGG G A AUU IIe ACU Thr AAU Asn AGU Ser U AUC ACC AAC AGC C AUA ACA AAA Lys AGA Arg A AUG Met ACG AAG AGG G G GUU Val GCU Ala GAU Asp GGU Gly U GUC GCC GAC GGC C GUA GCA GAA Glu GGA A GUG GCG GAG GGG G U C A G U UUU Phe UCU Ser UAU Tyr UGU Cyc U UUC UCC UAC UGC C UUA Leu UCA UAA Stop UGA Stop A UUG UCG UAG UGG Trp G C CUU CCU Pro CAU His CGU Arg U CUC CCC CAC CGC C CUA CCA CAA Gln CGA A CUG CCG CAG CGG G A AUU IIe ACU Thr AAU Asn AGU Ser U AUC ACC AAC AGC C AUA ACA AAA Lys AGA Arg A AUG Met ACG AAG AGG G G GUU Val GCU Ala GAU Asp GGU Gly U GUC GCC GAC GGC C GUA GCA GAA Glu GGA A GUG GCG GAG GGG G
- 6. Characteristics of Genetic Code Triplet Codons Number of codons Stop or termination or nonsense codons Degenerate Unambiguous Universal Non-overlapping and Nonpunctuation
- 7. Initiator codon AUG is the initiator codon in majority of proteins In a few cases GUG may be the initiator codon Methionine is the only amino acid specified by just one codon, AUG.
- 8. Specificity- Genetic code is specific (Unambiguous) A specific codon always codes for the same amino acid. e.g. UUU codes for Phenyl Alanine, it can not code for any other amino acid.
- 9. Universal In all living organism Genetic code is the same. The exception to universality is found in mitochondrial codons where, AUA methionine and UGA tryptophan, In Cytoplasmic codons AUA isoleucine and UGA termination codon, AGA and AGG code for Arginine in cytoplasm but in mitochondria they are termination codons.
- 10. Degenerate Genetic code is Redundant, also called Degenerate. Although each codon corresponds to a single amino acid but a single amino acid can have multiple codons. Except Tryptophan and Methionine each amino acid has multiple codons.
- 11. Non Overlapping & Non Punctuated All codons are independent sets of 3 bases. There is no overlapping , Codon is read from a fixed starting point as a continuous sequence of bases, taken three at a time. The starting point is extremely important and this is called Reading frame.
- 12. Non Sense Codons There are 3 codons out of 64 in genetic code which do not encode for any Amino Acid. These are called termination codons or stop codons or nonsense codons. The stop codons are UAA, UAG, and UGA. These codons does not code for any amino acids. The ribosome pauses and falls off the mRNA.
- 13. Wobble Hypothesis The genetic code assumes that each codon base pairs in antiparallel fashion with the anticodon of the tRNAs that are specific for the amino acid corresponding to the codons on mRNA. The rules of base pairing are relaxed at the third position, so that a base can pair with more than one complementary base. Some tRNA anticodons have Inosine at the third position. Inosine can pair with U, C, or A. This means that we don't need 61 different tRNA molecules, only half as many are required.
- 14. First two bases in Codon in mRNA (5’-3’) base pair traditionally with the 2 nd and 3rd base of the Anticodon in t RNA(5’-3’) Non traditional base pairing is observed between the third base of the codon and 1st base of anticodon. The reduced specificity between the third base of the codon and the complementary nucleotide in anticodon is responsible for wobbling.
- 16. Example Proline has 4 codons(5’-3’) CCU CCC CCA CCG The first three codons can be recognized by a single t RNA having Inosine at the first place.(IGG- 5’-3’)
- 17. Clinical Significance Mutations can be well explained using the genetic code. A) Point Mutations 1) Silent 2) Misense 3)Nonsense B) Frame shift mutations
- 18. Genetic Co deAshok K Thank you Ashok Katta Dept. of Biochemistry, Dhanalakshmi Srinivasan Medical College, Perambalur Contact no. - +917418831766 E mail - For more presentation visit -