Nucleic acids: DNA and RNA structure

Learning objectives

Describe a nucleotide as a pentose sugar, a phosphate group and a nitrogenous base, and compare DNA and RNA in terms of sugar, bases and the number of strands

Explain how phosphodiester bonds join nucleotides into a sugar–phosphate backbone, and describe the two strands of DNA as antiparallel and wound into a double helix

Explain complementary base pairing between adenine and thymine with two hydrogen bonds and between guanine and cytosine with three, and relate it to the stability of the molecule

Explain how an almost unlimited number of base sequences allows DNA to store the genetic information of every organism, and why the same four bases in all organisms is evidence of a common ancestor

(HL) Describe the evidence provided by Chargaff's data on base ratios, by X-ray diffraction images and by the Hershey–Chase experiment, and explain how Watson and Crick used it to build the double-helix model

(HL) Explain the directionality of a polynucleotide strand from the 5′ to the 3′ end, distinguish purine from pyrimidine bases, and describe how DNA is packaged with histone proteins into nucleosomes in eukaryotes