Question

What are the important chacteristics features of the triplet codon?

Answer 1

CHARACTERISTICS OF TRIPLET CODON :

The genetic dictionary of mRNA codons reveals the following important features of triplet codons:

1. Degeneracy:

• The code contains many synonyms, in that almost all amino acids are represented by more than one codon.
• For example, the three amino acids-arginine, serine and leucine each have six synonymous codons.
• However, for many of the synonymous codons specifying the same amino acid the first two bases of the triplet are constant, whereas the third can vary; for example, all codons starting with CC specify proline (CCU, CCC, CCA and CCG and all codons starting with AC specify threonine.
• This flexibility the third nucleotide of a codon may well help to minimize the consequences of errors.

2. Non-overlapping:

• The code is non-overlapping, meaning that no single base can take part in the formation of more than one codon.

3. Ambiguity:

• The genetic code is ambiguous, that is, same codon may specifies more than one amino acid.
• For example, UUL codon usually code for phenylalanine but in the presence of streptomycin, may also code for isoleucine, leucine or serine.

4. Commaless:

• The genetic code is commaless, which means
  that no codon is reserved for punctuations.

5. Starting codons :

• AUG codon is called starting or chain initiation codon, because, it initiates, the synthesis of polypeptide chain.
  
  6. Non-sense codons:
• The UAA (also called ochre), UAG (also called amber ) and UGA codons do not specify any amino acid, are called non-sense codons.
• They are also called termination codons, because, these codons are used by the cell to signal the natural end of translation of a particular polypeptide.
• However, their inclusion in any mRNA results in the abrupt termination of the message at the point of their location even though the polypeptide chain has not been-completed.

7. Universality:

• The genetic code has been found to be universal, because, same code applies in all kinds of living systems.
• That the genetic codes are indeed universal can be demonstrated quite directly by presenting an E coli in vitro protein-synthesizing system with, for example, purified mRNA from polio virus (which is normally translated by human cells)) and observing the synthesis of virus protein.
• Further, perhaps the most dramatic in vivo demonstration of the universality of the code has recently come from J. Gardon's laboratory (see Goodenough and Levine, 1974).
• It was shown that purified mRNA from rabbit reticulocytes, which specialize almost exclusively in the synthesis of haemoglobin, can be injected into oocytes of a frog and that stable rabbit haemoglobin will be synthesized by the translation machinery of the frog.

Triplet codon:

The series of 3 nucleotides which determine a single amino acid is called a Triplet codon.