<P> Deoxyribonucleic acid (DNA) is a nucleic acid containing the genetic instructions used in the development and functioning of all known living organisms . The DNA segments carrying this genetic information are called genes . Likewise, other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information . Along with RNA and proteins, DNA is one of the three major macromolecules that are essential for all known forms of life . DNA consists of two long polymers of simple units called nucleotides, with backbones made of sugars and phosphate groups joined by ester bonds . These two strands run in opposite directions to each other and are, therefore, anti-parallel . Attached to each sugar is one of four types of molecules called nucleobases (informally, bases). It is the sequence of these four nucleobases along the backbone that encodes information . This information is read using the genetic code, which specifies the sequence of the amino acids within proteins . The code is read by copying stretches of DNA into the related nucleic acid RNA in a process called transcription . Within cells DNA is organized into long structures called chromosomes . During cell division these chromosomes are duplicated in the process of DNA replication, providing each cell its own complete set of chromosomes . Eukaryotic organisms (animals, plants, fungi, and protists) store most of their DNA inside the cell nucleus and some of their DNA in organelles, such as mitochondria or chloroplasts . In contrast, prokaryotes (bacteria and archaea) store their DNA only in the cytoplasm . Within the chromosomes, chromatin proteins such as histones compact and organize DNA . These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed . </P> <P> Ribonucleic acid (RNA) functions in converting genetic information from genes into the amino acid sequences of proteins . The three universal types of RNA include transfer RNA (tRNA), messenger RNA (mRNA), and ribosomal RNA (rRNA). Messenger RNA acts to carry genetic sequence information between DNA and ribosomes, directing protein synthesis . Ribosomal RNA is a major component of the ribosome, and catalyzes peptide bond formation . Transfer RNA serves as the carrier molecule for amino acids to be used in protein synthesis, and is responsible for decoding the mRNA . In addition, many other classes of RNA are now known . </P> <P> Artificial nucleic acid analogues have been designed and synthesized by chemists, and include peptide nucleic acid, morpholino - and locked nucleic acid, glycol nucleic acid, and threose nucleic acid . Each of these is distinguished from naturally occurring DNA or RNA by changes to the backbone of the molecules . </P>

Which of the following is not a component within a single nucleotide of a nucleic acid