Nucleic Acids (College Board AP® Biology)

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Molecular Structure of Nucleic Acids

Nucleotide Bases & Nucleic Acids

  • DNA and RNA are nucleic acid polymers that are made up of many repeating units called nucleotides

  • Each nucleotide is formed from:

    • A pentose sugar (a sugar with 5 carbon atoms)

    • A nitrogen-containing organic base (with either 1 or 2 rings of atoms)

      • There are four bases in DNA: adenine (A), cytosine (C), guanine (G) and thymine (T)

      • In RNA, a fifth base, uracil (U), appears in place of thymine

    • A phosphate group (this is acidic and negatively charged)

  • The base and phosphate group are both covalently bonded to the sugar

Diagram of the General Structure of a DNA Nucleotide

general-structure-of-a-nucleotide

The basic structure of a nucleotide

Components of Nucleic Acid Structure

  • Nucleotide bases join together in chains to form DNA or RNA strands

  • The phosphate group of one nucleotide forms a covalent bond to the pentose sugar of the next one

    • This forms a "sugar phosphate backbone" with a base linked to each sugar

  • The polymer of nucleotides is known as a strand

  • Nucleotide bases can be joined in any combination/sequence

  • The sequence of nucleotide bases acts as a store of genetic information for an organism

RNA & DNA

  • RNA & DNA different in structure and function

    • DNA is double-stranded, RNA is usually single-stranded

    • DNA contains the bases A, T, C, and G, whereas RNA contains the bases A, U, C and G

    • DNA has a deoxyribose sugar whereas RNA has a ribose sugar

    • The similarities and difference between DNA and RNA are dealt with in Topic 1.6 Nucleic Acids

Examiner Tip

A common error is to describe DNA or RNA as polymers of bases; more correctly, they are polymers of nucleotides

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Phil

Author: Phil

Expertise: Biology Content Creator

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.