Exceptions to the Universal Genetic Code

E. coli the bacterium After the original genetic code of E. coli was completed in 1968, the Escherichia coli genetic code was subsequently determined for many other organisms ranging from bacteria to mammals, including humans. The codons were found to be the same for all organisms, leading to the idea that the genetic code is "universal." Furthermore, it also suggested that life on Earth had a single evolutionary origin, otherwise there would have been numerous genetic codes. The code was established during evolution, probably by chance, as there are no compelling reasons one codon should prevail over another. After it was established, any subsequent changes in the code would prove to be lethal, for if one codon changed, then all similar codons in the entire organism's genome would have to change simultaneously— a highly unlikely possibility.

Thus, it was surprising to find that there are, in fact, a few rare excep-endosymbiotic a type tions to the universal code. These exceptions are listed in Table 2. Most of these exceptions are found in the mitochondrial genome. The mitochondrion is thought to have evolved from an endosymbiotic bacterium at the time when the eukaryotic cell first arose. The mitochondrial genome is genetic material in a small, and most of the genes of the original endosymbiont have migrated to cell or organism the nucleus.

of symbiosis in which one partner lives within the other genome the total

EXCEPTIONS TO THE UNIVERSAL GENETIC CODE

Organism

Normal codon

Usual meaning

New meaning

Mammalian

AGA, AGG

Arginine

Stop codon

mitochondria

AUA

Isoleucine

Methionine

UGA

Stop codon

Tryptophan

Drosophila

AGA, AGG

Arginine

Serine

mitochondria

AUA

Isoleucine

Methionine

UGA

Stop codon

Tryptophan

Yeast

AUA

Isoleucine

Methionine

mitochondria

UGA

Stop codon

Tryptophan

CUA, CUC, CUG, CUU

Leucine

Threonine

Higher plant

UGA

Stop codon

Tryptophan

mitochondria

CGG

Arginine

Tryptophan

Protozoan nuclei

UAA, UAG

Stop codons

Glutamine

Mycoplasma capricolum

UGA

Stop codon

Tryptophan

bacteria

Table 2.

Genetic Counseling

Over the last half-century, our understanding of genetic disorders has increased spectacularly. When facts about inherited disorders first came to light, health professionals began to inform families about probable inheritance patterns and recurrence risks (the likelihood that offspring or other relatives might also inherit the disease).

In examining the exceptions to the universal genetic code in Table 2, you can see that there are only a few changes, most notably the use of a standard "stop" codon to encode an amino acid. For example, UGA normally is a stop codon. But in the mitochondria of the fruit fly Drosophila melanogaster, it encodes the amino acid tryptophan.

A few additional exceptions to the universal genetic code have also been identified. These include the nuclear genome of a few protozoan species and also in the bacterium Mycoplasma capricolum. These exceptions, however, do not imply multiple evolutionary origins of life. What is most striking is that the "exceptional" meanings of most of the codons are identical across all the organisms in which they are found, not different. Had there been multiple origins, we would expect to see drastically different genetic codes in these exceptional organisms. see also Crick, Francis; Escherichia coli (E. coli bacterium); Nucleotide; Reading Frame; Ribosome; Transcription; Translation.

Ralph R. Meyer

Bibliography

"The Genetic Code." Cold Spring Harbor Symposia on Quantitative Biology, vol. 31. Cold Spring Harbor, NY: Cold Spring Harbor Press, 1966.

Kay, Lily E. Who Wrote the Book of Life? A History of the Genetic Code. Stanford, CA: Stanford University Press, 2000.

Nirenberg, M. W., and J. H. Matthaei. "The Dependence of Cell-Free Protein Synthesis in E. coli upon Naturally Occurring or Synthetic Polyribonucleotides." Proceedings of the National Academy of Sciences 47 (1961): 1588-1602.

A couple receives genetic counseling during their sixth month of pregnancy. Genetic counselors are specially trained to provide support to couples whose families have histories of genetic diseases, or who are pregnant and carrying an affected child.

autosomal describes a chromosome other than the X and Y sex-determining chromosomes dominant controlling the phenotype when one allele is present recessive requiring the presence of two alleles to control the phenotype

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