Structure of DNA and RNA
CIE A-Level BiologyΒ· Unit 6: Nucleic Acids and Protein SynthesisΒ· 15 min read
1. Nucleotide Structureβ β ββββ± 4 min
Nucleotide
The monomer subunit of nucleic acids (DNA and RNA), consisting of three covalently bonded components: a pentose sugar, a nitrogenous base, and one or more phosphate groups.
Example:
A deoxyribose nucleotide is the monomer that makes up DNA
All nucleotides share a common core structure. The pentose sugar is a 5-carbon sugar: DNA uses deoxyribose, while RNA uses ribose. Deoxyribose lacks an oxygen atom on carbon 2 compared to ribose.
Nitrogenous bases are divided into two groups:
Purines: Double-ring structures: adenine (A) and guanine (G)
Pyrimidines: Single-ring structures: cytosine (C), thymine (T) (DNA only) and uracil (U) (RNA only)
A student draws an RNA nucleotide with thymine attached to ribose. Identify two mistakes and explain why they are incorrect.
- 1
First mistake: Thymine is not found in RNA
- 2
RNA uses uracil instead of thymine. Thymine is only present in DNA nucleotides.
- 3
Second mistake (common alternative): The phosphate group is not attached to the correct carbon
- 4
Phosphate groups always attach to carbon 5 of the pentose sugar, and the base attaches to carbon 1.
- 5
A corrected RNA nucleotide would be: ribose sugar + uracil base + phosphate group.
2. Polynucleotide Formationβ β ββββ± 4 min
Individual nucleotides are joined by condensation reactions to form a polynucleotide chain, which has a sugar-phosphate backbone with nitrogenous bases projecting sideways out from the backbone.
Phosphodiester bond
The covalent bond formed between the 5' phosphate group of one nucleotide and the 3' hydroxyl (-OH) group of the next nucleotide in a chain, formed during a condensation reaction that releases one water molecule.
All polynucleotide chains have directionality: one end has a free 5' phosphate group (the 5' end) and the other has a free 3' hydroxyl group (the 3' end).
Explain how three adjacent nucleotides join to form a short polynucleotide chain.
- 1
- Each nucleotide has a phosphate on carbon 5 of its pentose, and a hydroxyl group on carbon 3.
- 2
- A condensation reaction occurs between the 5' phosphate of the second nucleotide and the 3' hydroxyl of the first nucleotide.
- 3
- A water molecule is removed, forming a phosphodiester bond between the two nucleotides.
- 4
- A second condensation reaction occurs between the 5' phosphate of the third nucleotide and the 3' hydroxyl of the second nucleotide, forming a second phosphodiester bond.
- 5
The resulting chain has a free 5' end and a free 3' end, with an uninterrupted sugar-phosphate backbone.
3. DNA Double Helix Structureβ β β βββ± 4 min
DNA is made of two polynucleotide strands twisted around each other to form a stable double helix. The two strands run in opposite 5' to 3' directions, so they are described as antiparallel.
Bases on opposite strands pair specifically via hydrogen bonds: adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). This is called complementary base pairing.
A double-stranded DNA molecule has 18% adenine bases. Calculate the percentage of guanine bases.
- 1
Final answer: Guanine makes up 32% of the DNA molecule.
4. RNA Structure and Typesβ β ββββ± 3 min
RNA is usually a single-stranded polynucleotide. Key differences from DNA: it has ribose instead of deoxyribose, and uracil instead of thymine. There are three main types of RNA involved in protein synthesis, each with a unique structure suited to its function:
RNA Type | Structure | Main Function |
|---|---|---|
mRNA (messenger) | Linear single chain | Carries genetic code from DNA to ribosomes |
tRNA (transfer) | Folded clover shape, held by hydrogen bonds between bases | Carries specific amino acids to ribosomes during translation |
rRNA (ribosomal) | Folded globular structure | Forms the structural core of ribosomes |
List three structural differences between DNA and messenger RNA (mRNA).
- 1
Three valid differences (any three are accepted in exams):
- 2
- DNA contains deoxyribose sugar, mRNA contains ribose sugar
- 3
- DNA contains thymine, mRNA contains uracil instead of thymine
- 4
- DNA is double-stranded double helix, mRNA is single-stranded linear
- 5
Other valid differences: DNA is much longer than mRNA, DNA has two antiparallel strands while mRNA is a single strand
5. Common Pitfalls
Wrong move:
Attaching the phosphate group to the nitrogenous base instead of the pentose sugar in a nucleotide drawing
Why:
All three components of a nucleotide bond to the pentose sugar, not to each other
Correct move:
Phosphate bonds to C5 of pentose, base bonds to C1 of pentose
Wrong move:
Claiming phosphodiester bonds hold complementary base pairs together
Why:
Phosphodiester bonds link adjacent nucleotides in the same strand, not bases on opposite strands
Correct move:
Phosphodiester bonds form the sugar-phosphate backbone; hydrogen bonds link complementary base pairs
Wrong move:
Stating RNA is always double-stranded or DNA is always single-stranded
Why:
Cellular DNA is double-stranded, cellular RNA is typically single-stranded; viral exceptions are not expected for this syllabus
Correct move:
For CIE 9700, state DNA is double-stranded and RNA is usually single-stranded
Wrong move:
Calculating base percentages incorrectly for double-stranded DNA
Why:
Forgetting that complementary bases have equal percentages
Correct move:
Double the given base percentage to get the total for its pair, subtract from 100, divide by 2 to get the percentage of the other base pair
Wrong move:
Calling deoxyribose a 6-carbon sugar
Why:
The sugar in nucleotides is always a 5-carbon pentose sugar
Correct move:
Both deoxyribose and ribose are 5-carbon (pentose) sugars
6. Quick Reference Cheatsheet
Feature | DNA | RNA |
|---|---|---|
Pentose sugar | Deoxyribose | Ribose |
Nitrogenous bases | A, T, C, G | A, U, C, G |
Number of strands | 2 (antiparallel double helix) | Usually 1 |
Relative size | Very long | Much shorter than DNA |
Bond between nucleotides | Phosphodiester | Phosphodiester |
Base pairing bonds | A-T (2 H bonds), G-C (3 H bonds) | Only in folded regions (e.g. tRNA) |
7. Frequently Asked
What is the difference between a nucleoside and a nucleotide?
A nucleoside consists only of a pentose sugar and a nitrogenous base. A nucleotide has an additional phosphate group covalently bonded to the pentose sugar.
Why do purines only pair with pyrimidines?
Pairing a double-ring purine with a single-ring pyrimidine maintains a constant uniform width of the DNA double helix. Two purines would make the helix too wide, and two pyrimidines would make it too narrow.
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.
- 2022 Β· 22
Compare DNA and RNA structure
- 2023 Β· 11
Draw a DNA nucleotide structure
- 2024 Β· 21
Explain complementary base pairing
Going deeper
What's Next
The rules of nucleotide structure and complementary base pairing you learned here are the foundation for every topic in the nucleic acids unit. Complementary base pairing enables accurate semi-conservative DNA replication, the process that copies genetic information before cell division. Structural differences between DNA and RNA explain their distinct roles in protein synthesis: DNA stores genetic information long-term, while RNA carries short-lived copies of genes for translation. Mastering this subtopic makes understanding replication, transcription and translation much more straightforward, as all these processes rely on the core principles of base pairing and polynucleotide structure you have covered here.