Suppose that you discover a new life form on Mars. These organisms have DNA cont

Question

Suppose that you discover a new life form on Mars. These organisms have DNA containing the same four nucleotides as Earth DNA. They also make proteins using a triplet code, so they use the same 64 codons that earth organisms do. Martian proteins are also similar to earth proteins, but it turns out that Martian proteins have 22 different amino acids instead of just 20. How might you expect the Martian code to differ from the Earth code? To put it another way, what changes in the Earth code would be necessary to accommodate 2 additional amino acids?  

Explanation / Answer

Genetic code is universal for both Earth and Martian proteins.

*The genetic code is the set of rules by which information encoded within genetic material (DNA or mRNA sequences) is translated into proteins by living cells.

*The code defines how sequences of these nucleotide triplets, called codons, specify which amino acid will be added next during protein synthesis.

In the most general case of x bases and y bases per codon, the total number of possible codons is equal to xy .  

For life on Earth, x = 4 and y = 3; thus the number of codons is 43, or 64. Because there are only 20 amino acids, there is a lot of redundancy in the code (there are several codons for each amino acid).

On Earth of the total of 64 codons, 61 encode for 20 essential amino acids and 3 specify termination of translation or stop codons.

Martian 64 possible three letter codons for the 22 essential amino acids plus ? stop codons.

The degeneracy of the genetic code refers to the fact that most amino acids are specified by more than one codon.  The exceptions are methionine (AUG) and tryptophan (UGG). The degeneracy is found primarily the third position.  Consequently, single nucleotide substitutions at the third position may not lead to a change in the amino acid encoded.  These are called silent or synonymous nucleotide substitutions.

There are still no ambiguous codons binding alternate tRNA-(amino acid) only synonymous codons binding the same tRNA-(amino acid).

Martian proteins are made using triplet code. In order to encode 22 amino acids and one stop codon 64 codons are required by bacteria. But Martian code will be less redundant than Earth code. Possibly 2 additional amino acids are coded by only one code each.

To accommodate these changes Earth code should become less redundant. To do this, a novel tRNA-codon pair and an aminoacyl–tRNA synthetase (aaRS) need to be generated that uniquely incorporate an unnatural amino acid.

Related Questions

Navigate

• Browse (All)
• Browse S
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.