# Nucleic acids

> IB Biology SL · Theme A: Unity and Diversity
> Source: https://www.owlsprep.com/study/ib-biology-sl-u1-nucleic-acids/

This sub-topic covers the core structure and properties of DNA and RNA, the molecular building blocks of genetic information. You will learn key structural features that underpin nucleic acid function across all living organisms.

**Prerequisites:** [Introduction to biological macromolecules](https://www.owlsprep.com/study/ib-biology-sl-u1-biological-molecules/)

## Learning objectives

- Describe the three-component structure of a nucleotide
- Distinguish between purines and pyrimidines
- Explain directionality in polynucleotide strands
- Compare the structure and function of DNA and RNA
- Outline the double helix structure of DNA

## Nucleotide Structure and Components

**Nucleotide** — The monomer (building block) of all nucleic acids, consisting of three covalently bonded components: a 5-carbon pentose sugar, a nitrogen-containing base, and a phosphate group.

*Example:* A single deoxyribonucleotide is the basic monomer unit of DNA.

Nitrogenous bases are divided into two structural groups: purines have a double-ring structure (adenine and guanine), while pyrimidines have a single-ring structure (cytosine, thymine, and uracil). The pentose sugar differs between DNA and RNA: DNA uses deoxyribose, while RNA uses ribose.

> **mnemonic**
>
> Pure As Gold (Purines = Adenine, Guanine); CUT the Py (Cytosine, Uracil, Thymine = Pyrimidines)

**Worked example:** A question asks to classify adenine, cytosine, guanine, and uracil into purines and pyrimidines. What is the correct classification?

1. Recall the size difference between the two groups: purines have a double-ring structure, pyrimidines have a single-ring.
2. Recall the composition of each group: purines are only adenine and guanine, pyrimidines are cytosine, thymine, and uracil.
3. Final classification: Purines = adenine + guanine; Pyrimidines = cytosine + uracil

> **Exam tip:** In multiple choice, you will often be asked to classify bases — always confirm the ring size, not just the name.

## Polynucleotide Structure and Directionality

**Phosphodiester Bond** — A strong covalent bond that links adjacent nucleotides in a polynucleotide strand. It forms between the phosphate group of one nucleotide and the 3' carbon of the pentose sugar of the next nucleotide.

All polynucleotide strands have directionality (chemical polarity): one end has a free phosphate group attached to the 5' carbon of the terminal sugar (the 5' end), and the opposite end has a free hydroxyl group attached to the 3' carbon of the terminal sugar (the 3' end). All nucleic acid processes (replication, transcription, translation) proceed in the 5' → 3' direction.

**Worked example:** Explain why the sequence 5' ATGC 3' can not be rewritten as 3' ATGC 5' without changing the identity of the strand.

1. Recall that directionality defines the order of nucleotides in the strand. The first nucleotide in the sequence is at the 5' end, and the last is at the 3' end.
2. For 5' ATGC 3', A is at the 5' end, connected to T, which connects to G, which connects to C at the 3' end.
3. If the sequence was written 3' ATGC 5', A would be at the 3' end and C at the 5' end, reversing the order of nucleotides. This is a different strand.
4. Conclusion: Directionality changes alter the strand sequence, so the original orientation must be preserved.

## Comparison of DNA and RNA Structure

In cellular organisms, DNA is a double-stranded molecule that forms a stable double helix. The two strands run antiparallel (opposite directions) and are held together by hydrogen bonds between complementary base pairs: A pairs with T, G pairs with C. RNA is typically single-stranded, though it can fold into complex shapes held together by internal base pairing.

**Worked example:** An unknown nucleic acid sample has base composition: 21% adenine, 29% guanine, 21% cytosine, 29% uracil. Is this sample DNA or RNA, and is it double or single-stranded?

1. First check for the presence of uracil: DNA contains thymine instead of uracil, RNA contains uracil. Uracil is present, so this is RNA.
2. Check complementary base percentages: for double-stranded nucleic acid, %A = %U (or %T) and %G = %C.
3. In this sample, %A = 21% = %U, and %G = 29% = %C. This matches double-stranded base pairing rules.
4. Conclusion: The sample is double-stranded RNA. Note that while most cellular RNA is single-stranded, some viral RNA is double-stranded.

| Feature | DNA | RNA |
| --- | --- | --- |
| Pentose Sugar | Deoxyribose | Ribose |
| Bases | A, G, C, T | A, G, C, U |
| Typical Structure | Double-stranded double helix | Single-stranded |
| Core Function | Long-term genetic storage | Short-term genetic transfer and catalysis |

## Common pitfalls

- **Wrong:** Confusing purine and pyrimidine ring size
  - Why it fails: Many students mix up the groups, memorizing only the names not the structure
  - Correct: Use the CUT the Py mnemonic to remember pyrimidines are single-ring; purines are double-ring
- **Wrong:** Stating hydrogen bonds hold the DNA backbone together
  - Why it fails: Confusing base pairing bonds with backbone bonds
  - Correct: The sugar-phosphate backbone is held together by covalent phosphodiester bonds; hydrogen bonds only link complementary base pairs between strands
- **Wrong:** Drawing phosphodiester bonds between nitrogenous bases
  - Why it fails: Misunderstanding how nucleotides link into a polymer
  - Correct: Phosphodiester bonds always link the 3' sugar carbon of one nucleotide to the 5' phosphate of the next
- **Wrong:** Claiming all nucleic acids are double-stranded
  - Why it fails: Overgeneralizing DNA structure to all nucleic acids
  - Correct: For IB SL, assume cellular DNA is double-stranded and cellular RNA is single-stranded unless told otherwise

## Cheatsheet

| Concept | Key Fact |
| --- | --- |
| Nucleotide | Pentose + phosphate + nitrogenous base |
| Purines | Double ring: Adenine, Guanine |
| Pyrimidines | Single ring: Cytosine, Thymine, Uracil |
| Directionality | Synthesis/reading always 5' → 3' |
| DNA | Deoxyribose, Thymine, double-stranded, stores genetic info |
| RNA | Ribose, Uracil, single-stranded, transfers genetic info |
| Backbone bonds | Covalent phosphodiester bonds |
| Base pairing bonds | Hydrogen bonds between complementary bases |

## What's next

Understanding nucleic acid structure is the foundation for all molecular genetics topics in IB Biology. The directionality and complementary base pairing rules you learned here directly underpin core processes like DNA replication, transcription, and translation, which are frequently tested in exams. This topic also connects to the Theme A core idea of unity and diversity: all living organisms use the same nucleic acid structure for genetic storage, demonstrating shared ancestry, while variation in nucleic acid sequence generates the genetic diversity that drives evolution. Use the links below to continue building your understanding of molecular biology.

- [Cell Structure](https://www.owlsprep.com/study/ib-biology-sl-u1-cell-structure/)
- [Cell membranes and transport](https://www.owlsprep.com/study/ib-biology-sl-u1-cell-membranes-and-transport/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-biology-sl-u1-nucleic-acids/