Structure of DNA
• All organisms have a genetic code
• DNA (deoxyribonucleic acid) contains the
genetic code, and is the blueprint for the
expression of physiological traits
• Each individual organism has its own unique
blueprint
• The DNA structure is a double-stranded helical shape, twisted around a
common axis
• The width is 20 Å (angstroms)
• Normally, the DNA strand can be found wrapped around specialized proteins
called histones. When it not wrapped, the double-helical shape is more
evident
• The two backbones are comprised of repeating sugar and phosphate
molecules
• Each backbone is linked to the other via nitrogenous bases connected by
hydrogen bonds
• The right-handed helix makes a complete turn every 10 bases and 3.4 nm
• The two backbones are anti-parallel—one strand running 5’ (prime) to 3’, the
other from 3’ to 5’
• The “rungs” of the double-helical ladder are comprised of four nitrogenous
bases (adenine, thymine, cytosine, and guanine)
• Each base pair is held together by a hydrogen bond
• A binds with T, and C binds with G (Chargaff’s rule)
• The pentose sugar and phosphate group comprise the backbone
• Each sugar-phosphate group is bonded to the next by a covalent
phosphodiester bond
• Each phosphate connects the 5’ carbon of one nucleotide to the 3’ carbon of
another
• The nitrogenous bases is bound to the pentose sugar
• There are two classes of nitrogenous bases: pyrimidines and purines
• Cytosine and thymine are pyrimidines, a one-ring structure
• Adenine and guanine are purines, a two-ring structure