<P> Non-standard nucleosides are also found in both RNA and DNA and usually arise from modification of the standard nucleosides within the DNA molecule or the primary (initial) RNA transcript . Transfer RNA (tRNA) molecules contain a particularly large number of modified nucleosides . </P> <P> Double - stranded nucleic acids are made up of complementary sequences, in which extensive Watson - Crick base pairing results in a highly repeated and quite uniform double - helical three - dimensional structure . In contrast, single - stranded RNA and DNA molecules are not constrained to a regular double helix, and can adopt highly complex three - dimensional structures that are based on short stretches of intramolecular base - paired sequences including both Watson - Crick and noncanonical base pairs, and a wide range of complex tertiary interactions . </P> <P> Nucleic acid molecules are usually unbranched, and may occur as linear and circular molecules . For example, bacterial chromosomes, plasmids, mitochondrial DNA, and chloroplast DNA are usually circular double - stranded DNA molecules, while chromosomes of the eukaryotic nucleus are usually linear double - stranded DNA molecules . Most RNA molecules are linear, single - stranded molecules, but both circular and branched molecules can result from RNA splicing reactions . The total amount of pyrimidine is equal to the total amount of purines . The diameter of the helix is about 20A . </P> <P> One DNA or RNA molecule differs from another primarily in the sequence of nucleotides . Nucleotide sequences are of great importance in biology since they carry the ultimate instructions that encode all biological molecules, molecular assemblies, subcellular and cellular structures, organs, and organisms, and directly enable cognition, memory, and behavior (see Genetics). Enormous efforts have gone into the development of experimental methods to determine the nucleotide sequence of biological DNA and RNA molecules, and today hundreds of millions of nucleotides are sequenced daily at genome centers and smaller laboratories worldwide . In addition to maintaining the GenBank nucleic acid sequence database, the National Center for Biotechnology Information (NCBI, https://www.ncbi.nlm.nih.gov) provides analysis and retrieval resources for the data in GenBank and other biological data made available through the NCBI web site . </P>

Which of the following acids is most similar to the acidic part of rna and dna