Backbones of the right-handed double helix of DNA are composed of deoxyribose linked by phosphate groups. A given amino acid can have more than one codon. There are 61 codons for 20 amino acids in the general code. Transcription is suppressed when DNA is bound by protein and rendered inaccessible by a polymerase. Structures of most nuclear histones are well conserved within and between species.
Inactive genes tend to be highly methylated.
Protein synthesized along the ribosome is transferred into and through the endoplasmic reticulum and Golgi complex. Protein hormones are packaged into secretory granules within the Golgi apparatus. Residual hormone is stored in granules and thereby made readily available for secretion upon a subsequent bout of cellular stimulation. Peptide bonds linking amino acids are susceptible to enzymatic attack. Terminal residues of sialic acid on glycosylated proteins are removed by plasma glycosidases.