Nucleic acids
IB Biology SLΒ· Theme A.1.1 to A.1.4Β· 5 min read
1. Nucleotide Structure and Componentsβ β βββSL onlyβ± 15 min
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.
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 β always confirm the ring size, not just the name.
2. Polynucleotide Structure and Directionalityβ β β ββSL onlyβ± 20 min
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' β 3' direction.
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.
3. Comparison of DNA and RNA Structureβ β βββSL onlyβ± 15 min
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.
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 |
4. Common Pitfalls
Wrong move:
Confusing purine and pyrimidine ring size
Why:
Many students mix up the groups, memorizing only the names not the structure
Correct move:
Use the CUT the Py mnemonic to remember pyrimidines are single-ring; purines are double-ring
Wrong move:
Stating hydrogen bonds hold the DNA backbone together
Why:
Confusing base pairing bonds with backbone bonds
Correct move:
The sugar-phosphate backbone is held together by covalent phosphodiester bonds; hydrogen bonds only link complementary base pairs between strands
Wrong move:
Drawing phosphodiester bonds between nitrogenous bases
Why:
Misunderstanding how nucleotides link into a polymer
Correct move:
Phosphodiester bonds always link the 3' sugar carbon of one nucleotide to the 5' phosphate of the next
Wrong move:
Claiming all nucleic acids are double-stranded
Why:
Overgeneralizing DNA structure to all nucleic acids
Correct move:
For IB SL, assume cellular DNA is double-stranded and cellular RNA is single-stranded unless told otherwise
5. Quick Reference 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' β 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 |
6. Frequently Asked
Do I need to draw the full structure of nitrogenous bases for exams?
No. IB only requires you to distinguish between double-ring purines and single-ring pyrimidines, you do not need to memorize or draw full base structures.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· Paper 1
DNA vs RNA multiple choice
- 2024 Β· Paper 2
Draw and label a nucleotide
- 2023 Β· Paper 1
Directionality of nucleic acid question
Going deeper
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.