Unicellular Organisms

Nucleotides

			Nucleotide - The basic unit for DNA and RNA is the nucleotides, which consist of three components: the nitrogen bases, the ribose sugars, and the phosphates. The nitrogen bases include the two purines, adenine (A) and guanine (G); and the two pyrimidines, cytosine (C) and thymine(T). RNA contains the same bases,
Figure 11-13 Bases [vli]	Figure 11-14 DNA Sugars and Phosphate [view large image]	Figure 11-15 Nucleotide [view large image]	except thymine is replaced by uracil (U) (Figure 11-13).

In RNA, the sugar is ribose while in DNA, the sugar is deoxyribose (no oxygen is bonded in the 2' carbon) (Figure 11-14). And finally, there is the phosphate which forms part of the backbone (of the helix). The combination of the base and sugar is called nucleoside with the correponding products called (deoxy)adenosine, (deoxy)guanosine, (deoxy)cytidine, dexoythymidine and uridine. The product is called nucleotide with the additional element of phosphate (Figure 11-15); the naming convention is to add "5'-monophosphate" (5' indicates the 5^th carbon) at the end, e.g., "adenosine 5'-monophosphate". The abbreviations are (d)AMP, (d)GMP, (d)CMP, (d)TMP, and UMP. Any of the nucleotide such as AMP can bond to additional phosphate groups. For example, adding another phosphate to AMP gives ADP (adenosine 5'-diphosphate) and ATP (adenosine 5'-triphosphate) when there are a total of three phosphates. ATP is a nucleotide that is used as a carrier of energy in cells. Energy is released when ATP is broken

down to ADP and phosphate. As it will be explained further later, the energy package stored in the ATP serves to weld together the amino acid units in proteins and the nucleotide units in DNA and RNA, as well as the units in sugar and phospholipid molecules that abound in cells. The cAMP (c for cyclic) used by slime mould as molecular signal is a compound made from ATP. It is still used by more complex organisms for the same purpose. cAMP is widespread in animal cells as a second messenger in many biochemical reactions induced by hormones. Upon reaching their target cells, the hormones activate adenylate cyclase, the enzyme that catalyses cyclic AMP production. Cyclic AMP activates a cascade of enzymes, which results in a thousand-fold response just from the binding of a single hormone molecule to a receptor on the cell membrane. Cyclic AMP is also involved in controlling gene expression, cell division, immune responses, and nervous transmission.

[2022 Update] Exotic (Non-canonical) Nucleotide

DNA and RNA also commonly include non-canonical nucleotides, which are modified versions of the canonical ones and lacking a phosphate group. Among their key cellular roles, these modified nucleotides participate in translation by stabilizing and diversifying the tertiary (3D) structures of tRNAs, and by coordinating base pairing of tRNAs with mRNA. For example, the non-canonical nucleotide N⁶-threonylcarbamoyladenosine (t⁶A) is an essential and universally evolutionarily conserved nucleotide responsible for decoding codons whose first nucleotide contains adenine (see Figure 11-15b).

Figure 11-15b Exotic (Non-canonical) Nucleotide [view large image]

See "The Methyl Group of theN6-Methyl-N6-Threonylcarbamoyladenosine in tRNA of Escherichia coli Modestly Improves the Efficiency of the tRNA", 1998
from which :

All tRNA species from the three domains, Archaea, Bacteria, andEucarya, contain modified nucleosides, which are derivatives of the four nucleosides, adenosine, guanosine, cytidine, and uridine. At present, more than 79 different modified nucleosides from the tRNA of various organisms have been characterized. Some of these are present in tRNA from only one domain, but a few are present in the same subset of and at the same position in the tRNAs from all three domains. One such conserved group of modified nucleosides is the threonylated adenosine (t⁶A) derivatives. These modified adenosines are present adjacent to and 3' of the anticodon (position 37) in the subset of tRNAs that reads codons starting with A. The universal presence of t⁶A derivatives suggests that these kinds of modifications may have been present in the tRNA of the progenitor, unless a convergent evolution has occurred. This conservation also suggests that the functions of these modified nucleosides may be principally the same in all organisms.

[End of 2022 Update]