Question

Genetic Code:

Using the standard genetic code, write a sequence encoding the peptide "MASTERMIX"

How many different sequences can encode this peptide?

How many genetic codes have been described? (Hint: search NCBI Genetic Codes)

How, in a general way, do these alternative codes tend to differ from the standard genetic code?

How is selenocysteine encoded?

Answer 1

Using the Standard Genetic Coe, the Peptide MASTERMIX has the mRNA sequence:

5'-AUGGCUUCUACUGAACGUAUGAUUUAA-3'

The DNA sequence for this peptide is:
5'-ATGGCTTCTACTGAACGTATGATTTAA-3'

The Amino acid M is coded for by 1 codon, A is coded for by 4 codons, S is coded for by 6 codons, T is coded for by 4 codons, E is coded for by 2 codons, R is coded for by 6 codons, I is coded for by 3 codons and X (Stop Codon) is encoded for by 3 codons.

So, the number of different sequences that can code for the peptide 'MASTERMIX' is 1 * 4 * 6 * 4 * 2 * 6 * 1 * 3 * 3 =10,368

The NCBI Genetics codes website described 25 Genetic codes, which primarily differ in their Stop codon usage and changed codon usage, where some Amino acids that are coded for by one codon in the Standard Genetic code, code for other Amino acids.

For example, in the Vertebrate Mitochondrial code, AGA is a stop codon, whereas AGA codes for Arginine in the Standard code. Also in the Vertebrate Mitochondrial code, UGA codes for Tryptophan, which is a Stop codon in the Standard Code.

Selenocysteine is coded for by UGA, which is normally a stop codon. However, the presence of a Selenocysteine Insertion Sequence (SECIS) signal following the UGA codon directs the insertion of Selenocysteine at loci where UGA occurs.