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Nucleic Acids Overview

Aug 31, 2025

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Overview

This lecture introduces nucleic acids (DNA and RNA), their structures, component monomers (nucleotides), and the key differences between DNA and RNA relevant for protein synthesis.

Types of Nucleic Acids

  • Two main types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
  • DNA is usually double-stranded; RNA is usually single-stranded.
  • Both are large polymers made of repeating monomers called nucleotides.

Structure of Nucleotides

  • Each nucleotide has a phosphate group, a pentose (5-carbon) sugar, and a nitrogenous base.
  • Pentose sugar is either deoxyribose (in DNA) or ribose (in RNA).
  • The base can be a purine (large, two-ring structure: adenine/A, guanine/G) or a pyrimidine (small, single-ring: cytosine/C, thymine/T, uracil/U).

Purines and Pyrimidines

  • Purines: adenine (A) and guanine (G).
  • Pyrimidines: cytosine (C), thymine (T, DNA only), and uracil (U, RNA only).
  • Remember: "CUT pyrimidines"—cytosine, uracil, thymine are pyrimidines.

Differences Between DNA and RNA Nucleotides

  • DNA nucleotides have deoxyribose sugar (no oxygen on carbon 2); RNA nucleotides have ribose sugar (OH group on carbon 2).
  • DNA bases: adenine, guanine, cytosine, thymine.
  • RNA bases: adenine, guanine, cytosine, uracil.
  • The presence of T (thymine) indicates DNA; the presence of U (uracil) indicates RNA.

Identifying DNA vs. RNA Nucleotides

  • Check the sugar: deoxyribose (H on carbon 2) or ribose (OH on carbon 2).
  • Check the nitrogenous base: thymine (DNA only), uracil (RNA only), cytosine (can be in both).

Key Terms & Definitions

  • Nucleic acid — large molecule made of nucleotide monomers; includes DNA and RNA.
  • Nucleotide — monomer unit consisting of a phosphate, pentose sugar, and nitrogenous base.
  • Purine — large, two-ring nitrogenous base: adenine or guanine.
  • Pyrimidine — small, single-ring nitrogenous base: cytosine, thymine, or uracil.
  • Deoxyribose — 5-carbon sugar in DNA nucleotides, lacking oxygen at carbon 2.
  • Ribose — 5-carbon sugar in RNA nucleotides, with OH at carbon 2.

Action Items / Next Steps

  • Review the differences between DNA and RNA structures and their nucleotide components.
  • Memorize which bases are purines and which are pyrimidines.
  • Prepare for questions that ask you to identify DNA or RNA nucleotides based on their sugar or base.

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