<P> Some noncoding DNA sequences play structural roles in chromosomes . Telomeres and centromeres typically contain few genes but are important for the function and stability of chromosomes . An abundant form of noncoding DNA in humans are pseudogenes, which are copies of genes that have been disabled by mutation . These sequences are usually just molecular fossils, although they can occasionally serve as raw genetic material for the creation of new genes through the process of gene duplication and divergence . </P> <P> A gene is a sequence of DNA that contains genetic information and can influence the phenotype of an organism . Within a gene, the sequence of bases along a DNA strand defines a messenger RNA sequence, which then defines one or more protein sequences . The relationship between the nucleotide sequences of genes and the amino - acid sequences of proteins is determined by the rules of translation, known collectively as the genetic code . The genetic code consists of three - letter' words' called codons formed from a sequence of three nucleotides (e.g. ACT, CAG, TTT). </P> <P> In transcription, the codons of a gene are copied into messenger RNA by RNA polymerase . This RNA copy is then decoded by a ribosome that reads the RNA sequence by base - pairing the messenger RNA to transfer RNA, which carries amino acids . Since there are 4 bases in 3 - letter combinations, there are 64 possible codons (4 combinations). These encode the twenty standard amino acids, giving most amino acids more than one possible codon . There are also three' stop' or' nonsense' codons signifying the end of the coding region; these are the TAA, TGA, and TAG codons . </P> <P> Cell division is essential for an organism to grow, but, when a cell divides, it must replicate the DNA in its genome so that the two daughter cells have the same genetic information as their parent . The double - stranded structure of DNA provides a simple mechanism for DNA replication . Here, the two strands are separated and then each strand's complementary DNA sequence is recreated by an enzyme called DNA polymerase . This enzyme makes the complementary strand by finding the correct base through complementary base pairing and bonding it onto the original strand . As DNA polymerases can only extend a DNA strand in a 5 ′ to 3 ′ direction, different mechanisms are used to copy the antiparallel strands of the double helix . In this way, the base on the old strand dictates which base appears on the new strand, and the cell ends up with a perfect copy of its DNA . </P>

The section of dna being used to make the strand of rna is known as