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A 1.2 Nucleic Acids

Guiding Questions:
- How does the structure of nucleic acids allow hereditary information to be stored?
- How does the structure of DNA facilitate accurate replication?

A 1.2 Nucleic Acids

SL and HL Content 

Learning Objectives


A1.2.1—DNA as the Genetic Material

  • Discuss why DNA is the genetic material in all living organisms.

  • Explain why some viruses use RNA instead of DNA.

  • Debate why viruses are not considered living organisms.


  • Deoxyribonucleic Acid (DNA) is a molecule composed of two long chains (strands) coiled around each other to form a double helix.


  • It carries the genetic instructions used in the growth, development, functioning, and reproduction of all known living organisms and many viruses.


Structure of DNA:

  • DNA is made up of nucleotides, each consisting of:

    • a phosphate group

    • a sugar molecule (deoxyribose)

    • a nitrogenous base (adenine [A], thymine [T], cytosine [C], or guanine [G])

  • The sequence of these bases encodes genetic information.


Function of DNA:

  • DNA holds the instructions for the synthesis of proteins, which are essential for cell structure and function.

  • Genes, segments of DNA, are transcribed into RNA and then translated into proteins.


Universal Genetic Code:

  • The genetic code is nearly universal, with the same codons coding for the same amino acids in almost all organisms, demonstrating the common ancestry of life.

RNA in Viruses:

  • Viruses are infectious agents that replicate only inside the living cells of an organism.

  • Some viruses use RNA (Ribonucleic Acid) instead of DNA as their genetic material.

  • RNA viruses can have single-stranded or double-stranded RNA genomes.

    • Examples of RNA Viruses:

    • Influenza virus, HIV, and coronaviruses (e.g., SARS-CoV-2) are examples of RNA viruses.

A1.2.2—Components of a Nucleotide

  • Identify and describe the three components of a nucleotide: phosphates, pentose sugars, and nitrogenous bases.

  • Discuss the use of circles, pentagons, and rectangles in diagrams to represent these components.


A1.2.3—Sugar–Phosphate Bonding

  • Explain how sugar-phosphate bonds form the backbone of DNA and RNA.

  • Discuss the significance of the continuous chain of covalently bonded atoms in nucleotides.


A1.2.4—Nitrogenous Bases

  • List the names of the nitrogenous bases in nucleic acids.

  • Discuss the role of these bases in the genetic code.


A1.2.5—RNA Structure

  • Describe how RNA is formed by the condensation of nucleotide monomers.

  • Draw and recognize diagrams of single nucleotides and RNA polymers.


A1.2.6—DNA Structure

  • Explain the structure of DNA as a double helix with antiparallel strands.

  • Discuss hydrogen bonding between complementary base pairs: adenine (A) with thymine (T) and guanine (G) with cytosine (C).


A1.2.7—Differences between DNA and RNA

  • Compare and contrast DNA and RNA in terms of strand number, types of nitrogenous bases, and type of pentose sugar.

  • Sketch the differences between ribose and deoxyribose sugars.


A1.2.8—Complementary Base Pairing

  • Discuss the role of complementary base pairing in DNA replication and expression.

  • Explain how hydrogen bonding is crucial for complementarity.


A1.2.9—DNA’s Capacity for Storing Information

  • Explain the diversity of possible DNA base sequences and their implications.

  • Emphasize the enormous capacity of DNA for storing genetic information.


A1.2.10—Universal Genetic Code

  • Discuss the conservation of the genetic code across all life forms as evidence of common ancestry.


Additional Higher Level


A1.2.11—Directionality of RNA and DNA

  • Explain the significance of 5’ to 3’ linkages in the sugar-phosphate backbone.

  • Discuss the importance of directionality for replication, transcription, and translation.


A1.2.12—Purine-to-Pyrimidine Bonding

  • Discuss how purine-to-pyrimidine bonding contributes to DNA helix stability.

  • Explain why adenine–thymine (A–T) and cytosine–guanine (C–G) pairs result in a consistent three-dimensional structure.


A1.2.13—Structure of a Nucleosome

  • Describe the structure of a nucleosome: DNA wrapped around histone proteins.

  • Use molecular visualization software to study the association between proteins and DNA within a nucleosome.


A1.2.14—Hershey–Chase Experiment

  • Explain the significance of the Hershey–Chase experiment in identifying DNA as the genetic material.

  • Discuss how technological advancements enable new experimental possibilities.


A1.2.15—Chargaff’s Data

  • Analyze Chargaff’s data on the relative amounts of pyrimidine and purine bases across different life forms.

  • Discuss how the data falsified the tetranucleotide hypothesis, addressing the “problem of induction” through “certainty of falsification”.





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