EVOLUTIONARY RELATIONSHIPS THROUGH NUCLEIC ACID SIMILARITIES.

Abstract

The sequence of nucleotides in the genetic material, DNA, determines the amino-acid sequences of the cell proteins. This process is mediated by generally called messenger RNA, which moves from the nucleus to the cytoplasm where it specifies the amino-acid sequence of the proteins. If we could lay bare the entire DNA nucleotide of an organism and make assumptions about the size of genes, and further assume that all DNA is involved with protein synthesis, and use the code which equates nucleotide triplets with an amino acid, then we might predict the amino-acid sequences of all cell proteins. A comparison between the DNA sequences of different organisms might lead to an understanding of the evolutionary relationships between these, since it could be assumed that the phenotype of the organism bears some relationship to the proteins present. The prediction of the composition of proteins will depend on the phenomenon of degeneracy, among other things, and also must take account of certain cases of cytoplasmic directed processes which have an effect on the phenotype. These cytoplasmic phenomena, often grouped as cases of cytoplasmic inheritance, may not involve nucleic acid sequences (Sonneborn, 1963). Comparison of the genetic material of organisms which cannot interbreed can be made using the technique of nucleic acid hybridization. The method has been used to detect nucleic acid homologies in viruses, bacteria, protozoans and vertebrates (Lacy and Green, 1965; McCarthy and Bolton, 1963; Gibson, 1966; Hoyer et al., 1964). It can be used to test similarities in DNA and RNA nucleotide sequences in different tissues of the same organism or of different organisms (Whiteley et al., 1966; McCarthy and Hoyer, 1964). We have used it here to examine similarities between DNA and RNA molecules at different stages of development of some invertebrates and vertebrates.