- The letters A, G, T and C correspond to the nucleotides found in DNA. They are organized into codons.
- The collection of codons is called Genetic code.
- For 20 amino acids there should be 20 codons.
- Each codon should have 3 nucleotides to impart specificity to each of the amino acid for a specific codon
- 1 Nucleotide- 4 combinations
- 2 Nucleotides- 16 combinations
- 3 Nucleotides- 64 combinations ( Most suited for 20 amino acids).
- Genetic code is a dictionary that corresponds with sequence of nucleotides and sequence of Amino Acids.
- Words in dictionary are in the form of codons
- Each codon is a triplet of nucleotides
- 64 codons in total and three out of these are Non Sense codons (Figure-1)
- 61 codons for 20 amino acids.
Figure-1- Genetic code is a dictionary that corresponds with sequence of nucleotides and sequence of Amino Acids.
Genetic Code-Characteristics (Table-1)
- Genetic code is specific (Unambiguous)
- A specific codon always codes for the same amino acid.
- e.g. UUU codes for Phenyl Alanine, it cannot code for any other amino acid.
- In all living organism Genetic code is the same.
- The exception to universality is found in mitochondrial codons-
- where AUA codes for Methionine and UGA for tryptophan, instead of termination codon respectively of cytoplasmic protein synthesizing machinery.
- AGA and AGG code for Arginine in cytoplasm but in mitochondria they are termination codons.
- 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.
4) Non Overlapping and 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.
5) 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.
- They encode no amino acid.
- The ribosome pauses and falls off the mRNA.
6) 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.
Table-1- Characteristics of Genetic code (Summary)
|1.||Specific/ Unambiguous||Given a specific codon, only a single amino acid is indicated.|
|2.||Universal||In all living organism Genetic code is the same (Except mitochondrial codons)|
|3.||Redundant/ Degenerate||Multiple codons can decode the same amino acid|
|4.||Non Overlapping||The reading of the genetic code during the process of protein synthesis does not involve any overlap of codons|
|5.||Non Punctuated||Once the reading is commenced at a specific codon, there is no punctuation between codons, and the message is read in a continuing sequence of nucleotide triplets until a translation stop codon is reached.|
- The rules of base pairing are relaxed at the third position, so that a base can pair with more than one complementary base (Table-2)
- 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 (Table-2)
- First two bases in Codon in m RNA(5’-3’) base pair traditionally with the 2 nd and 3rd base of the Anticodon in t RNA(5’-3’)
- Nontraditional 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 (Table-2)
- 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’)
Table-2- Showing traditional and nontraditional base pairing between codon and anticodon.
|t RNA (first base) Anticodon||m RNA (Third base) Codon||Base pairing|
Clinical Significance- Mutations can be well explained using the genetic code.
A) Point Mutations
1) Silent- Single nucleotide change-A to G (Figure-2) same amino acid is incorporated. Mutation goes unnoticed.
Figure-2- Silent mutation – same amino acid is incorporate due to degeneracy of the genetic code
2) Missense-Single nucleotide change A to C – different amino acid incorporated (Figure-3). Loss of functional capacity of protein.
Figure-3- Single nucleotide change leads to incorporation of different amino acid with the resultant synthesis of faulty protein.
3) Nonsense–Single nucleotide change from C to T, stop codon is generated (In m RNA represented by UAG) (Figure-4) , there is premature termination of chain, may be incompatible with life.
Figure-4- Single nucleotide change (C to T), leads to generation of stop codon (UAG), causing premature termination of growing peptide chain.
B) Frame shift mutations-Insertion or removal of a base/bases can alter the reading frame with the resultant incorporation of different amino acids (Figure-5)
Figure-5- Insertion or removal of nucleotides can alter the reading frame with the resultant incorporation of different amino acids.
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