There is sufficient reason to think that the two-nucleotide code evolved to form the current triplet genetic code. With this in mind, one can conclude that the number of amino acids was slightly less than the current 20 because the two nucleotide codes could constitute only 16 amino acids. Sixteen possible combinations of the four nucleotide bases of the DNA would give rise to the 16 amino acids. Therefore, it posits that the ancient primitive proteins constituted fewer amino acids encoded by the two-nucleotide code. Furthermore, deletion or insertion mutation experiments of one or two nucleotides into a gene formed no protein (Frank & Froese, 2018). However, the reading frame of the mRNA got restored once a third nucleotide was deleted or inserted. Nonetheless, the current triplet genetic code entails 64 codons encoding for the twenty amino acids.
The genetic code is termed degenerate, where a single amino acid can specify more than one codon. Each codon specifies a single amino acid, while the amino acids can have more than two codons. For instance, tryptophan and methionine have a single codon each except for UGA, UAG, and UAA termination codons. Isoleucine has three codons, valine, threonine, and glycine, proline, and alanine have four codons. Arginine, serine, and leucine have six codons, and the remaining nine amino acids have two codons (Frank & Froese, 2018). The genetic code’s degeneracy feature helps prove that in ancient times, there was a two-nucleotide code that translated into the 16 amino acids in primitive proteins. The first two bases of each codon have greater specificity than the third. The connection between the mRNA and tRNA anticodon of the first two bases is often stronger. The third base loosely pairs with its corresponding anticodon base.
The DNA sample with a G + C content of 70 percent will require more temperature than the G + C content of 45 percent to separate its strands. The DNA double helix stability that constitutes two complementary polynucleotide chains that interact via the hydrogen bonds depends on several factors. They include the length of the chains, sequence, base composition, and the presence of mismatches. A factor like alterations in temperature can further perturb this stability. A DNA sample with higher G + C content is more stable than the one with a low G + C content, as the G + C base pairs are more strongly bonded together (Piovesan et al., 2019). This increased stability raises its thermal melting temperature; thus, to reach such temperatures, more heat energy will be required during the denaturation process of separating the strands. This explains the high melting point of a high G + C content DNA.
When conducting polymerase chain reactions, the denaturation temperatures in the separation of DNA double strands are set at 94-950 C. The denaturation temperatures will be at their maximum peak for the DNA samples with a higher G + C content composition to enable faster strand separation. The set time between 0.5 to 2.0 minutes will also last to its maximum limits for the high G + C content DNA (Piovesan et al., 2019). These instances are attributed to the stability of the DNA due to the high G + C content base pairing hydrogen bonds. The high stability raises the melting temperatures of the high G + C content DNA that imminently requires higher temperatures for denaturation of the bonds and a long time to achieve the process effectively. Therefore, factors such as a high G + C content composition and longer polynucleotide chains require higher denaturation temperatures when compared to the low G + C content DNA samples.
References
Frank, A., & Froese, T. (2018). The standard genetic code can evolve from a two-letter GC code without information loss or costly reassignments.Origins of Life and Evolution of Biospheres, 48(2), 256-272. Web.
Piovesan, A., Pelleri, M. C., Antonaros, F., Strippoli, P., Caracausi, M., & Vitale, L. (2019). On the length, weight, and GC content of the human genome.BMC Research notes, 2(1), 1-7. Web.