Nucleic Acids

A1.2.1 - DNA as the Genetic Material of All Living Organisms

DNA – The Universal Genetic Code

• Deoxyribonucleic acid (DNA) is the molecule that stores the long-term genetic information in all living organisms.

  • This information provides the instructions for building proteins, which determine how cells function.

Because DNA can be accurately replicated, it allows genetic information to be:

• passed from cell to cell during growth

• passed from parent to offspring during reproduction

  
  

DNA as Evidence of Common Ancestry

• All living organisms use DNA as their genetic material.

  • This universality suggests that all life shares a common evolutionary origin.

  • This supports the idea that:

    • life evolved from a common ancestor

    • the genetic code is fundamentally shared across all species

Gene Expression: Same DNA, Different Cells

In multicellular organisms:

• all cells contain the same DNA

• but only specific genes are expressed in each cell

This leads to:

• cell specialization

• different structures and functions despite identical genetic material

What About Viruses?

Some viruses use:

• DNA as genetic material

• others use RNA

Examples:

• DNA viruses → bacteriophages

• RNA viruses → HIV, coronaviruses

Why This Does NOT Contradict the DNA Rule

• A virus is simply:

  • a nucleic acid core (DNA or RNA)

  • surrounded by a protein coat

  • with no cellular machinery of its own

• Because of this, viruses:

  • cannot reproduce independently

    • they must infect a host cell

  • do not carry out metabolism

  • do not respond to stimuli independently

So Why Doesn’t RNA in Viruses Falsify the DNA Claim?

• Since viruses are not living, they are not exceptions to the rule.

  • therefore, the presence of RNA in some viruses does not contradict the idea that DNA is the genetic material of all living organisms.

• In fact, the dependence of viruses on host cells highlights that only living cells with DNA have the full capacity for independent life.

🔍 IB EXAM FOCUS

• ⚠️ Always state: “viruses are not considered living”

• Be ready to explain:

  • DNA as evidence for common ancestry

  • Why universality of DNA supports evolution

• You MUST be able to link: DNA → genes → proteins → cell function

  • This is the foundation of gene expression and cell differentiation

• Common exam traps:

  • Saying “all organisms use DNA” ❎

    → Must say all living organisms ✅

A1.2.2 - Components of a nucleotide

What is a Nucleotide?

A nucleotide is the basic building block (monomer) of nucleic acids (DNA and RNA).

• Each nucleotide is made of three components:

  • Phosphate group - circle

  • Pentose sugar (5-carbon sugar) - pentagon

  • Nitrogenous base - rectangle

Key Structural Details

• The pentose sugar has 5 carbon atoms:

  • Base attaches to Carbon #1 (C1)

  • Phosphate attaches to Carbon #5 (C5)

• This positioning is what allows nucleotides to link together to form long chains (polymers)

🔍 IB EXAM FOCUS

• You may be asked to:

  • draw a nucleotide from memory

  • identify components in a diagram

  • label phosphate, sugar, and base correctly