Big picture

• DNA is the genetic material of all living organisms. Some viruses use RNA as genetic material, but viruses are not considered living.

• Nucleic acids store hereditary information because their base sequence can vary enormously while the overall molecule keeps a stable structure.

• The two key questions in this topic are: how information is stored and how DNA structure allows accurate replication.

Nucleotide structure

• A nucleotide has 3 parts: phosphate group, pentose sugar, and nitrogenous base.

• In IB diagrams, show these in the correct relative positions using circles, pentagons and rectangles.

• The pentose sugar is deoxyribose in DNA and ribose in RNA.

• The bases you must know are adenine (A), thymine (T), cytosine (C), guanine (G) and uracil (U).

• Exam focus: be able to recognize and draw a single nucleotide clearly.

A clear structural overview of DNA showing how nucleotides are arranged. It is useful for identifying the backbone and where the bases project inward. Source

DNA and RNA as polymers

• DNA and RNA are polymers made by condensation reactions joining many nucleotide monomers.

• Adjacent nucleotides are linked by sugar–phosphate bonding, forming a continuous sugar–phosphate backbone.

• This backbone contains strong covalent bonds, which make each strand structurally stable.

• RNA is usually considered as a single-stranded polymer in this topic.

• Exam focus: describe the backbone as the repeating sugar–phosphate chain rather than a random mixture of parts.

DNA double helix and complementary base pairing

• DNA consists of two antiparallel strands of nucleotides.

• The strands are linked by hydrogen bonds between complementary base pairs.

• A pairs with T and G pairs with C.

• In IB diagrams, draw the strands antiparallel; you are not required to draw the helical twist.

• You do not need to memorize the numbers of hydrogen bonds or the relative base lengths.

• Complementary base pairing is the key reason DNA can be copied accurately and genetic information can be expressed.

This diagram is ideal for revising A–T and G–C pairing and the arrangement of the backbone on the outside of DNA. It also helps with IB-style drawing conventions. Source

DNA vs RNA

• DNA is double-stranded; RNA is usually single-stranded.

• DNA contains deoxyribose; RNA contains ribose.

• DNA uses thymine; RNA uses uracil instead of thymine.

• Both molecules contain adenine, cytosine and guanine.

• You should be able to sketch the difference between ribose and deoxyribose.

• Be familiar with examples of nucleic acids, especially DNA and RNA.

This visual highlights the most testable differences between DNA and RNA: strand number, sugar type, and base type. It is especially useful for quick side-by-side revision. Source

Why DNA can store so much information

• The order of bases along a DNA strand is the code.

• Any base sequence is possible, and DNA molecules can be any length, so the number of possible sequences is effectively limitless.

• This gives DNA an enormous information-storage capacity with great economy.

• Because each strand can act as a template for the other through complementary base pairing, information can be replicated accurately.

• Link idea: base sequence varies, but the overall double-helix structure stays consistent.

Genetic code and common ancestry

• The genetic code is conserved across all life forms.

• This conservation is evidence for universal common ancestry.

• You do not need to memorize specific examples of organisms sharing the code.

• Exam link: a shared genetic code strongly supports the idea that all life descended from a common ancestor.

This diagram shows how DNA wraps around histone proteins to form nucleosomes. It is useful for HL because it makes chromatin packing much easier to visualize. Source

This figure shows the core logic of the Hershey–Chase experiment: DNA entered the bacterial cells, protein did not. That is why the experiment supports the conclusion that DNA is the genetic material. Source