Emulous Universe

May 4, 2011

DNA is such a fragile molecule that some researchers don’t think it could have survived in the hydrothermal vents outside of a cell membrane where the earliest life was thought to develop. This is an enigma for researchers studying the origin of life. DNA consists of a double helix molecule that resembles a twisted ladder; the backbone of DNA are linked groups of phosphate chemicals These negatively charged strands run antiparallel to each other, meaning that the top of one strand runs parallel to the bottom of the second strand. Areosian life is truly alien because instead of the familiar phosphate, it uses arsenate ions as a backbone. Arsenate is a polyatomic ion with an arsenic atom in the center and four oxygen atoms bonded to the central arsenic. And Areosian DNA gets weirder still because it is a triple helix.

The ends of a DNA molecule are marked as 3’ (three-prime) on one end and 5’ (five-prime) on the other, so our antiparallel strands link a 3’ to a 5’ end and vice versa. This is important because DNA replication proceeds from an area called the origin of replication on both strands in the 5′-to-3′ direction, forming two replication forks where an enzyme called helicase unzips DNA into two strands. RNA resembles a single strand of DNA, but instead of thymine, RNA exclusively uses the base uracil, which binds to adenine.

In the process of RNA replication called transcription. RNA polymerase unzips a DNA molecule by breaking the hydrogen bonds between complimentary nucleotides. RNA nucleotides are paired with complementary DNA bases. RNA sugar-phosphate backbone forms with assistance from RNA polymerase. Hydrogen bonds of the untwisted RNA+DNA helix break, freeing the newly synthesized RNA strand. If the cell has a nucleus, the RNA is further processed in a reaction called methylation and then moves through the small nuclear pores to the cytoplasm.

Some scientists speculate that there was a time when life may have used RNA instead of DNA, but there is little evidence to support it. Some have cited that retroviruses based on RNA could indicate that DNA need not have always been the nucleic acid used by life, and that retroviruses may be a throwback from the time when an RNA-World existed. Retroviruses are viruses that run off of RNA-hardware, and Astrobiologist Peter Ward in his book Life As We Do Not Know It, proffers a title for these earlier life forms as members of his proposed kingdom of Ribosa, or RNA-based life.

DNA on Earth uses the same four nucleic acids. And these four nucleic acids come in two categories; pyramidines and purines. Pyramidines are aromatic hydrocarbons like uracil, cytosine and thymine with nitrogen in the 1,3 position of a six-member ring. Purines are organic compounds like adenine and guanine that consist of a pyrimidine ring fused to an alkaloid imidazole ring. Adenine and thymine pair together with 2 hydrogen bonds just like guanine and cytosine pair together with 3 hydrogen bonds. Areosian life uses entirely different purines and pyramidines, but these chemicals function in much the same way as DNA. For instance, Areiosan life features thiopurines, which incorporate sulfur in a purine’s pyramidine ring. Chemicals like mercaptopurine or tioguanine can be found Areiosan cells. Their pyridimine bases incorporate fluorine in their structure, with fluorouracil, floxouridine and gemcitabine as analogues of the purine bases in our cells. These analogues are called antimetabolites because some chemicals like fluorouracil are so chemically similar to uracil that they interfere with our metabolism. These analogues are used in chemotherapy because they interfere with the function of cancer cells. But in Areiosan cells, they function like the real thing and while they may be different from a chemically perspective, they are not fundamentally different in any other way. The machinery inside their cells works just like the machinery in our cells, but the parts are just made of a different material.

RNA synthesis separates the DNA strands and RNA polymerase builds RNA in the 5' to 3' direction, using one of the DNA strands as a template.


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