<P> Due to the chemical composition of the pentose residues of the bases, DNA strands have directionality . One end of a DNA polymer contains an exposed hydroxyl group on the deoxyribose; this is known as the 3' end of the molecule . The other end contains an exposed phosphate group; this is the 5' end . The two strands of a double - helix run in opposite directions . Nucleic acid synthesis, including DNA replication and transcription occurs in the 5' → 3' direction, because new nucleotides are added via a dehydration reaction that uses the exposed 3' hydroxyl as a nucleophile . </P> <P> The expression of genes encoded in DNA begins by transcribing the gene into RNA, a second type of nucleic acid that is very similar to DNA, but whose monomers contain the sugar ribose rather than deoxyribose . RNA also contains the base uracil in place of thymine . RNA molecules are less stable than DNA and are typically single - stranded . Genes that encode proteins are composed of a series of three - nucleotide sequences called codons, which serve as the "words" in the genetic "language". The genetic code specifies the correspondence during protein translation between codons and amino acids . The genetic code is nearly the same for all known organisms . </P> <P> The total complement of genes in an organism or cell is known as its genome, which may be stored on one or more chromosomes . A chromosome consists of a single, very long DNA helix on which thousands of genes are encoded . The region of the chromosome at which a particular gene is located is called its locus . Each locus contains one allele of a gene; however, members of a population may have different alleles at the locus, each with a slightly different gene sequence . </P> <P> The majority of eukaryotic genes are stored on a set of large, linear chromosomes . The chromosomes are packed within the nucleus in complex with storage proteins called histones to form a unit called a nucleosome . DNA packaged and condensed in this way is called chromatin . The manner in which DNA is stored on the histones, as well as chemical modifications of the histone itself, regulate whether a particular region of DNA is accessible for gene expression . In addition to genes, eukaryotic chromosomes contain sequences involved in ensuring that the DNA is copied without degradation of end regions and sorted into daughter cells during cell division: replication origins, telomeres and the centromere . Replication origins are the sequence regions where DNA replication is initiated to make two copies of the chromosome . Telomeres are long stretches of repetitive sequence that cap the ends of the linear chromosomes and prevent degradation of coding and regulatory regions during DNA replication . The length of the telomeres decreases each time the genome is replicated and has been implicated in the aging process . The centromere is required for binding spindle fibres to separate sister chromatids into daughter cells during cell division . </P>

The location of a gene on a chromosome is called a(n)