Which of the following is not a characteristic of genetic code?
A. It is a triplet code
B. It is non overlapping and without punctuation
C. It is nearly universal
D. It is degenerate – one code may code for more than one amino acid
E. It is unambiguous
Answer- The correct answer is D.
Genetic code is degenerate, but that does not mean, a given code may code for more than one amino acids.
In fact, multiple codons can decode the same amino acid, but vice versa is not true (figure)
Genetic Code- Characteristics
Specificity- Genetic code is specific (unambiguous)
- A specific codon always codes for the same amino acid (figure) e.g. UUU codes for Phenyl Alanine, it cannot code for any other amino acid.
Universal- 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 Isoleucine and termination codon respectively of cytoplasmic protein synthesizing machinery.
- AGA and AGG code for Arginine in cytoplasm but in mitochondria they are termination codons.
Redundant- 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 (figure)
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.
Non Sense Codons
- There are 3 codons out of 64 in genetic code which do not encode for any Amino Acid (figure)
- 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.
- 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.
Figure- Genetic code tablePlease help Biochemistry for Medics by "CLICKING ON THE ADVERTISEMENTS" every time you visit us. Thank you!