DNA and RNA: Structure, Bonding, and Function Revision Notes | A Level AQA Biology

1. Why DNA and RNA Matter

Every instruction your body has ever followed — from building an enzyme to triggering an immune response — originates in a molecule so thin it cannot be seen with a light microscope. That molecule is DNA, and understanding its structure is the key to understanding how life works at a molecular level.

Here is the central idea: DNA holds the genetic information, and RNA carries that information from the nucleus to the ribosomes, where proteins are made. These two molecules are the information-carrying backbone of all living cells, and their structures are exquisitely matched to their functions.

2. Building Blocks: The Nucleotide

Before you can understand DNA or RNA, you need to understand the monomer they are both built from: the nucleotide. Picture a nucleotide as having three components joined together — a pentose sugar (a five-carbon sugar), a phosphate group, and a nitrogenous base.

• In DNA nucleotides, the pentose sugar is deoxyribose (missing one oxygen atom compared to ribose).
• In RNA nucleotides, the pentose sugar is ribose.
• The nitrogenous bases in DNA are adenine (A), thymine (T), guanine (G), and cytosine (C).
• RNA uses the same bases except thymine is replaced by uracil (U).

The difference between deoxyribose and ribose is a single hydroxyl group. That one chemical difference is enough to give DNA and RNA distinct structural properties and distinct roles in the cell.

3. Forming the Polynucleotide Chain: Phosphodiester Bonds

Two nucleotides do not stick together at random. A specific type of covalent bond — the phosphodiester bond — links them, and it forms through a condensation reaction: a water molecule is released as the phosphate group of one nucleotide (attached to its 5' carbon) forms a bond with the 3'-OH group of the pentose sugar of the preceding nucleotide, creating a 3'-5' phosphodiester linkage.

Repeat this reaction hundreds of millions of times and you build a polynucleotide chain. The result is a sugar-phosphate backbone running along the outside of the chain, with the nitrogenous bases projecting inward — a structure that becomes critical when two chains come together to form DNA.

The phosphodiester bond is the structural glue of the entire polynucleotide chain. Every DNA and RNA molecule you will ever study is held together along its backbone by these bonds — which is why understanding their formation is a non-negotiable foundation for everything that follows.

4. DNA Structure: The Double Helix

A single polynucleotide chain is impressive, but DNA takes things further. Two polynucleotide chains run antiparallel to each other — one runs 5' to 3', the other runs 3' to 5' — and they are held together by hydrogen bonds between their nitrogenous bases. This produces the famous double helix.

The base pairing is not random. Adenine always pairs with thymine via two hydrogen bonds, and guanine always pairs with cytosine via three hydrogen bonds. This is complementary base pairing, and it is the reason DNA can be copied accurately and the reason the two strands fit together so precisely.

The two strands then twist around a central axis to form the helical shape. The sugar-phosphate backbones run along the outside of the helix, and the base pairs stack on the inside like rungs on a twisted ladder. This arrangement protects the genetic information held in the base sequence from chemical damage.

Notice the elegance here: the covalent phosphodiester bonds give each strand its structural integrity, while the hydrogen bonds between strands are individually weak but collectively strong enough to maintain the double helix — yet weak enough to be broken apart during DNA replication and transcription.

5. RNA Structure: A Shorter, Single-Stranded Molecule

RNA shares the same basic nucleotide structure as DNA, but its overall architecture is strikingly different. An RNA molecule is a relatively short, single-stranded polynucleotide chain. There is no second complementary strand, no double helix, and no thymine — uracil takes thymine's place instead.

This single-stranded nature is not a weakness — it is a functional advantage. RNA molecules can fold back on themselves to form localised double-stranded regions, giving them a variety of three-dimensional shapes suited to different roles. Messenger RNA (mRNA) carries the genetic code from DNA to the ribosome; transfer RNA (tRNA) brings amino acids to the ribosome during translation.

The relative shortness of RNA molecules is also functionally significant. Each mRNA molecule carries the code for one or a small number of proteins, and once that protein has been made, the mRNA can be degraded. DNA, by contrast, must persist for the lifetime of the cell and beyond — which is why its double-stranded, helical structure provides far greater chemical stability.

6. Ribosomes: Where RNA and Protein Meet

Ribosomes are the molecular machines that translate the genetic code into protein. Their composition is worth knowing precisely for the exam: ribosomes are formed from RNA and proteins. Specifically, ribosomal RNA (rRNA) combines with ribosomal proteins to form two subunits — a large subunit and a small subunit — that come together during translation.

This means RNA plays a structural role inside the ribosome as well as an informational role as mRNA. The ribosome reads the mRNA sequence in triplets (codons), and tRNA molecules bring the corresponding amino acids. The result is a growing polypeptide chain — the protein product encoded by the original DNA sequence.

7. DNA vs RNA: A Functional Comparison

Now that you have seen both molecules in detail, it is worth stepping back to compare them directly. The structural differences between DNA and RNA are not accidents — each feature is tied to a specific function.

8. Why Scientists Doubted DNA Carried the Genetic Code

Here is a fascinating piece of scientific history that the AQA specification asks you to appreciate. For much of the early twentieth century, many scientists believed that proteins, not DNA, carried the genetic code. Their reasoning was logical given what they knew at the time.

DNA appeared structurally simple: it is built from only four different nucleotide monomers (differing only in their base). Proteins, by contrast, are built from twenty different amino acids, giving them enormous structural and chemical diversity. Scientists reasoned that a molecule capable of encoding the complexity of life must itself be complex — and DNA, with its repetitive sugar-phosphate backbone and just four bases, seemed far too simple for the job.

The irony is that this apparent simplicity is precisely what makes DNA so powerful. The genetic code is written in sequences of bases — combinations of just four 'letters' that, in triplets, can encode twenty amino acids and all the regulatory signals a cell needs. The simplicity of the monomer does not limit the complexity of the message; it enables it.

This historical episode is a reminder that scientific understanding develops through evidence, not assumption. The experiments of Avery, MacLeod, and McCarty (1944) and later Hershey and Chase (1952) provided compelling evidence that DNA, not protein, was the genetic material — overturning the prevailing view.

9. Exam Technique and Common Mistakes

Questions on DNA and RNA structure appear in almost every AQA Biology paper. The marks are there to be taken — but only if you use precise language. Here is what separates a full-mark answer from a near-miss.

Worked example: Answering a 'Describe the Structure of DNA' Question

Describe the structure of a DNA molecule. [4 marks]

1. DNA is a double helix formed from two polynucleotide chains. — [B1] This earns the first mark. Name the shape and the number of strands explicitly.
2. Each polynucleotide chain has a sugar-phosphate backbone, with nucleotides linked by phosphodiester bonds formed through condensation reactions. — [B1] Naming the phosphodiester bond and the condensation reaction secures this mark. Do not just say 'covalent bonds'.
3. The two strands are held together by hydrogen bonds between complementary base pairs: adenine pairs with thymine (two hydrogen bonds) and guanine pairs with cytosine (three hydrogen bonds). — [B1] Specifying the number of hydrogen bonds for each pair is a common mark-winner that many students omit.
4. The two strands run antiparallel to each other — one runs 5' to 3' and the other runs 3' to 5'. — [B1] The antiparallel orientation is a structural feature that AQA expects at A Level. Including it distinguishes a full-mark answer from a three-mark answer.

Answer: DNA is a double helix of two antiparallel polynucleotide chains. The chains are linked by phosphodiester bonds (formed by condensation reactions) along a sugar-phosphate backbone. The two strands are held together by hydrogen bonds between complementary base pairs: A-T (two hydrogen bonds) and G-C (three hydrogen bonds).

Every spec point in this lesson connects back to one central theme: structure determines function. The double-stranded, helical structure of DNA makes it stable and replicable. The single-stranded, shorter structure of RNA makes it mobile and versatile. The phosphodiester bonds give each chain its integrity, and the hydrogen bonds between strands allow the helix to be opened and read. Master these relationships and you will be able to answer any question this topic can generate.