Codons & Genetic code

Codons & Genetic code


CODON &GENETIC CODE

  • Codon- is a sequence of three adjacent bases that corresponds to one of the amino acid.
  • There are 64 possible codon of sequence.
  • Four nucleotide bases A, G, C and U.
  • If 4 bases 44 = 256 amino acids.
  • Methionine has only 1 codon.
  • Genetic code is the system of nucleotide sequences of mRNA that determines the sequence of amino acids in protein.
  • Characteristics of genetic codes-

1.  Triplet codon– each amino acid has triplet sequence.

2. Degenerate (Redundant)

  • A given amino acid may have more than one codon.
  • Degeneracy of the codon lies in the 3rd base.

3. Universal– a specific codon represent a specific amino acid in all the  species.

  • Genetic coder are found in human mitochondria, code is-
  1. AUA codes for methionine instead of isoleucine.
  2. AGA and AGG serve as as stop codon.
  3. UGA also codes for Selenocysteine, a mechanism called translational recording.

4.Unambiguous/ Specific– a particular codon always codes for the same amino acid

5. Non overlapping and nonpuntate (comma less)– reading of genetic code does not involve overlapping sequence.

  • E.g.- AUGCUA GACUUU reads as AUG/CUA/GAC/UUU without punctuation (comma) between codons.

6. Stop or termination or nonsense codons

  • The three nucleotide triplets do not code for any amino acid are- UAA (amber), UAG (ochre), UGA (opal) called as nonsense codons that normally signal termination of polypeptide chains.
  • Wobble Hypothesis- states that a single tRNA can recognise more than one codon.
  • Base pairing of 3rd base of codon (at 31 end) often fails to recognize the specific complementary base codon (at 51 end at tRNA)
  • Wobble explains the degeneracy of genetic code.
  • A minimum of 31 tRNAs are required to translate all 61 different codons for the amino acids.
  • Gene- is the smallest functional unit of genome. 2 types
  1. Inducible gene
  2. Constitutive gene (housekeeping genes)- genes whose expression is not regulated
  • Cistron- is the smallest unit of genetic expression.
  • The codons that designate the same amino acid are called synonyms.

 Exam Important

  • Information for synthesis of protein is contained in the mRNA.
  • Thymine is not involved in codons.
  • 64 (43) possible codon sequences.
  • tRNA acts as the adapter molecule between the codon and specific amino acid.
  • UUU is the codon for phenylalanine.
  • Degeneracy of the codon lies in the 3rd base.
  • Amino acid with maximum number codons are Serine, Arginine, Leucine.
  • Monocistronic- e.g. eukaryotic mRNA
  • Polycistronic- e.g. Prokaryotic mRNA
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