<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> <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>

Where are nucleic acids found in the cell