<P> In 2012, a group of American scientists led by Floyd Romesberg, a chemical biologist at the Scripps Research Institute in San Diego, California, published that his team designed an unnatural base pair (UBP). The two new artificial nucleotides or Unnatural Base Pair (UBP) were named d5SICS and dNaM . More technically, these artificial nucleotides bearing hydrophobic nucleobases, feature two fused aromatic rings that form a (d5SICS--dNaM) complex or base pair in DNA . His team designed a variety of in vitro or "test tube" templates containing the unnatural base pair and they confirmed that it was efficiently replicated with high fidelity in virtually all sequence contexts using the modern standard in vitro techniques, namely PCR amplification of DNA and PCR - based applications . Their results show that for PCR and PCR - based applications, the d5SICS--dNaM unnatural base pair is functionally equivalent to a natural base pair, and when combined with the other two natural base pairs used by all organisms, A--T and G--C, they provide a fully functional and expanded six - letter "genetic alphabet". </P> <P> In 2014 the same team from the Scripps Research Institute reported that they synthesized a stretch of circular DNA known as a plasmid containing natural T-A and C-G base pairs along with the best - performing UBP Romesberg's laboratory had designed, and inserted it into cells of the common bacterium E. coli that successfully replicated the unnatural base pairs through multiple generations . The transfection did not hamper the growth of the E. coli cells, and showed no sign of losing its unnatural base pairs to its natural DNA repair mechanisms . This is the first known example of a living organism passing along an expanded genetic code to subsequent generations . Romesberg said he and his colleagues created 300 variants to refine the design of nucleotides that would be stable enough and would be replicated as easily as the natural ones when the cells divide . This was in part achieved by the addition of a supportive algal gene that expresses a nucleotide triphosphate transporter which efficiently imports the triphosphates of both d5SICSTP and dNaMTP into E. coli bacteria . Then, the natural bacterial replication pathways use them to accurately replicate a plasmid containing d5SICS--dNaM . Other researchers were surprised that the bacteria replicated these human - made DNA subunits . </P> <P> The successful incorporation of a third base pair is a significant breakthrough toward the goal of greatly expanding the number of amino acids which can be encoded by DNA, from the existing 20 amino acids to a theoretically possible 172, thereby expanding the potential for living organisms to produce novel proteins . The artificial strings of DNA do not encode for anything yet, but scientists speculate they could be designed to manufacture new proteins which could have industrial or pharmaceutical uses . Experts said the synthetic DNA incorporating the unnatural base pair raises the possibility of life forms based on a different DNA code . </P> <P> The following abbreviations are commonly used to describe the length of a D / R NA molecule: </P>

Which one of the following nucleotide pair bonds