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1.6 Nucleic Acids

Syllabus
2025
Topic
1.6
Level
—

Learning objectives

1.6A—Describe the structure and function of DNA and RNADescribe the structure and function of DNA and RNA.• In nucleic acids (DNA and RNA), biological information is encoded in sequences of nucleotide monomers. Each nucleotide has the following structural components: a five-carbon sugar (deoxyribose or ribose), a phosphate, and a nitrogenous base (adenine, thymine, guanine, cytosine, or uracil).• Nucleic acids have a linear sequence of nucleotides that have ends, defined by the 3’ (three prime) hydroxyl and 5’ (five prime) phosphates of the sugar in the nucleotide. During nucleic acid synthesis, nucleotides are added to the 3’ end of the growing strand, resulting in the formation of covalent bonds between nucleotides.- Exclusion: The molecular structure of specific nucleotides is beyond the scope of the AP Exam.• DNA is structured as an antiparallel double helix, with two strands of nucleotides running in opposite 5’ to 3’ orientation. In DNA, adenine nucleotides pair with thymine nucleotides via hydrogen bonds (A-T), and cytosine nucleotides pair with guanine nucleotides via hydrogen bonds (C-G). In RNA, adenine pairs with uracil (A-U).• Structural differences between DNA and RNA include:- i. DNA contains the sugar deoxyribose, and RNA contains the sugar ribose.- ii. DNA contains the nitrogenous base thymine, and RNA contains the nitrogenous base uracil.- iii. DNA is typically double stranded, while RNA is typically single stranded.

Nucleotide Sequence, Direction, and Pairing Build Nucleic Acids

DNA and RNA encode biological information in the linear order of nucleotide monomers. Each nucleotide contains a five-carbon sugar, a phosphate, and one nitrogenous base. The base sequence carries information; covalent bonds connect nucleotides into a strand.

Nucleotide component DNA or RNA options
Five-carbon sugar Deoxyribose in DNA; ribose in RNA
Phosphate Part of each nucleotide and the strand connection
Nitrogenous base Adenine, guanine, cytosine, plus thymine in DNA or uracil in RNA

A nucleic-acid strand has a 5′ end defined by a phosphate and a 3′ end defined by a hydroxyl group on the sugar. During synthesis, each new nucleotide is added to the 3′ end, forming a covalent bond. The growing strand therefore extends in the 5′→3′ direction.

Structure Complementary pairing
DNA antiparallel double helix A–T and C–G through hydrogen bonds; the two strands run in opposite 5′→3′ orientations
RNA A pairs with U when complementary pairing occurs
Feature DNA RNA
Sugar Deoxyribose Ribose
Distinctive base Thymine Uracil
Typical strand form Double stranded Single stranded

Antiparallel means the two DNA strands have opposite directional orientations; it does not mean their bases are unrelated. The molecular structure of specific nucleotides is explicitly outside AP Exam scope, so track components, direction, bonds, and pairing rather than detailed structural formulas.