Nucleic acids are the molecules that hold and use the genetic code. DNA stores the full instruction set as a stable, double-stranded archive. RNA carries the working copies out into the cell, where they direct protein synthesis.
Nucleotides are the three-part monomers of every nucleic acid.
Nucleic acids are polymers built from nucleotide monomers. Every nucleotide carries three components covalently joined together, and two of those components decide what the nucleotide will go on to do. The sugar fixes whether the nucleotide belongs to DNA or RNA; the base decides which information position the nucleotide occupies in the polymer.
Every nucleotide has three covalently bonded parts: a pentose sugar, a phosphate group, and a nitrogenous base. The pentose is deoxyribose in a DNA nucleotide and ribose in an RNA nucleotide. The phosphate group is identical in every nucleotide; the base is one of four.
DNA uses adenine, thymine, cytosine, guanine. RNA uses adenine, uracil, cytosine, guanine. Three bases — adenine, cytosine, guanine — are shared between DNA and RNA. Thymine appears only in DNA; uracil appears only in RNA and replaces thymine at every position.
Write
nucleotidesorbases. Don't writenucleotide basesas a hybrid term. AQA rejects the hybrid in both directions: a nucleotide is the full three-part monomer; a base is just the nitrogenous component.
In extended writing, use the full base names —
adenine,thymine,cytosine,guanine,uracil. Single-letter shorthand (A, T, C, G, U) does not score on extended-response questions.
Two nucleotides joined produce a dinucleotide. Three or more produce a polynucleotide. DNA molecules run into millions of base units per chromosome because the molecule has to hold the full genetic instruction set. RNA molecules are much shorter, proportional to the single immediate task each one serves.
Nucleotides join by condensation to form a polynucleotide strand.
Polynucleotides form through condensation reactions between adjacent nucleotides. Each reaction joins one nucleotide to the next by a single covalent bond, releases one water molecule, and adds one position to the growing strand. The reverse reaction, hydrolysis, breaks a polynucleotide back into its monomers by adding water across the bond.
The phosphate group of one nucleotide reacts with the pentose sugar of the next. A water molecule is released, and a covalent phosphodiester bond forms between the two nucleotides. Hydrolysis of a phosphodiester bond — the reverse — splits the polymer at that point.
The product of repeated condensation is a polynucleotide strand with a regular architecture. The phosphate groups and pentose sugars alternate to form a continuous sugar-phosphate backbone along the exterior of the strand; the nitrogenous bases project laterally from the backbone. The bond is the phosphodiester bond; the reaction is condensation. These two terms account for the chemistry of polymer formation in this topic, and they are the same for DNA and RNA.
Open any extended DNA structure description with
polymer of nucleotides. This is the most-omitted credit point on 5- and 6-mark structure questions; AQA expects the polymer framing before the structural detail.
DNA is a double helix of two antiparallel strands held by complementary base pairing.
A complete DNA molecule is two polynucleotide strands wound together around a shared axis, forming a double helix. The two strands run in opposite chemical directions along their length — they are antiparallel. They are held together along their full length by hydrogen bonds between complementary bases on opposite strands.
Write
double helix. AQA does not credithelixalone. The double-stranded geometry is the credited structural fact, not the spiral itself.
Write
antiparallel, notopposite directions. The credited term carries the chemical-direction meaning; the looser phrasing does not.
Adenine pairs with thymine through two hydrogen bonds. Guanine pairs with cytosine through three hydrogen bonds. No other stable pairings occur under normal cellular conditions. The pairing rule is specific and obligatory, and the hydrogen-bond count difference between the two pairs is itself a mark-earning fact.
The architecture of the helix places the backbones on the outer surface of the molecule and the paired bases stacked in the interior. The backbone is charged and hydrophilic; it interacts with the aqueous surroundings. The bases sit in a hydrophobic environment, shielded from attack. This positional protection is one reason DNA works as a long-term genetic archive.
G-C content sets the thermal stability of a given DNA region. A region rich in G-C base pairs has more total hydrogen-bond energy holding the two strands together than a region rich in A-T pairs, because each G-C pair contributes three hydrogen bonds and each A-T pair contributes two. A high-G-C region therefore requires a higher temperature to separate; this is the credited mechanism behind the G-C thermal stability fact.
Individual hydrogen bonds are weak; collectively, the thousands running the length of the molecule hold the two strands together robustly under normal cellular conditions. The same property is what lets DNA helicase achieve strand separation progressively during replication, breaking hydrogen bonds position by position rather than all at once.
Prokaryotic DNA differs from eukaryotic DNA in form, not in chemistry.
Both prokaryotic and eukaryotic DNA share the nucleotide chemistry, the base pairing rules, and the double-stranded architecture. The differences between them are structural and organisational, not chemical.
Prokaryotic DNA is
double-stranded. The credited differences arecircular,no histones,no introns. Describing prokaryotic DNA assingle-strandedis explicitly rejected.
Shape: prokaryotic — circular; eukaryotic — linear. Histones: prokaryotic — not associated with histones; eukaryotic — wrapped around histones. Introns: prokaryotic — no introns; eukaryotic — contain introns. Prokaryotic cells also carry plasmids: small, additional, circular DNA molecules separate from the main chromosome.
Pitfall — Both halves of every difference
Comparison questions need both halves stated directly opposite one another.
"Prokaryotic DNA is circular whereas eukaryotic DNA is linear" earns the mark. "Prokaryotic DNA is circular" alone earns nothing — the second half is implied, not stated, and AQA does not credit the implication.
The list rule applies here too: a wrong difference cancels a correct one. Pair two or three differences carefully rather than scatter five attempts.
RNA is single-stranded and exists in three functional types.
RNA is a polynucleotide built from the same condensation chemistry as DNA. It differs from DNA in four respects: pentose sugar (ribose, not deoxyribose), one base (uracil, not thymine), strand number (single-stranded), and length (much shorter). These four differences together account for RNA's role as a transient working molecule rather than a long-term archive.
Produced from a DNA template during transcription. Carries the coding sequence of a gene from the nucleus to ribosomes in the cytoplasm, where it directs protein synthesis. Linear; does not adopt a defined folded shape. Contains uracil, not thymine.
Around 80 nucleotides long. Folds via intrastrand hydrogen bonding into a compact clover-leaf shape. Carries an anticodon at one end and an amino acid binding site at the other; delivers a specific amino acid to the ribosome during translation. Contains uracil, not thymine.
The most abundant RNA in the cell. A structural and functional component of the ribosome — the molecular machine on which translation takes place. Combined with ribosomal proteins to form the assembled ribosome.
Both mRNA and tRNA contain
uracil, not thymine. Neither contains thymine; thymine is a DNA-only base.
Write
amino acid binding siteon tRNA, notan amino acidorcarries an amino acid. The credited phrasing names the structural site itself.
DNA and RNA compared.
| Attribute | DNA | RNA |
|---|---|---|
| Sugar | deoxyribose | ribose |
| Bases | adenine, thymine, cytosine, guanine | adenine, uracil, cytosine, guanine |
| Strands | double-stranded | single-stranded |
| Length | very long (millions of base pairs) | short (tens to thousands of nucleotides) |
| Function | stores the genetic code | carries the code, delivers amino acids, forms the ribosome |
DNA replication is semi-conservative, driven by helicase and DNA polymerase.
DNA replication produces two identical copies of a DNA molecule before cell division. The process is described as semi-conservative because each daughter molecule contains one strand from the original parent and one newly synthesised complementary strand. Neither daughter is wholly new; neither is wholly old.
Each daughter DNA molecule contains one original template strand and one newly synthesised complementary strand.
- DNA helicase breaks the hydrogen bonds between complementary bases on the two strands. The double helix unwinds and the strands separate, exposing the base sequence of each strand for use as a template.
- Free activated nucleotides in the nucleoplasm align opposite the exposed template strands by complementary base pairing — adenine with thymine, guanine with cytosine — and are held in position by hydrogen bonds.
- DNA polymerase catalyses condensation between adjacent aligned nucleotides, forming phosphodiester bonds along the new sugar-phosphate backbone. Two complete double-stranded molecules result, each with one original strand and one new strand.
Helicase
breakshydrogen bonds. Don't writehydrolyses. Hydrolysis is the addition of water across a covalent bond; hydrogen bonds are not covalent and do not undergo hydrolysis.
DNA polymerase
joins adjacent nucleotidesby catalysingphosphodiester bondformation. Don't writeforms hydrogen bondsorjoins complementary bases— the polymerase builds the covalent backbone, not the hydrogen-bond pairing.
Replication is initiated by regulatory proteins called cyclins. Cyclin A accumulates and binds to the enzymes that initiate replication; without cyclin binding, replication does not begin. Cyclin is a protein, not an enzyme. The semi-conservative outcome itself was confirmed by the Meselson-Stahl density-gradient experiment, in which a single intermediate-density band after one round of replication ruled out conservative replication, and a persistent bimodal pattern in later rounds ruled out dispersive replication.
Multi-treatment cyclin questions reward biochemical mechanism per treatment, not data restatement. For an antibody block: the antibody binds cyclin, so cyclin cannot bind its target enzyme, so replication is not initiated. For a rescue control: re-introducing cyclin restores function, confirming cyclin's role. Each treatment carries its own mark and its own chain.
Pitfall — Cyclin is a protein, not an enzyme
Cyclin is a regulatory protein, and cyclin questions reward mechanism, not data.
AQA explicitly rejects descriptions of cyclin as an enzyme. Cyclin does not catalyse a reaction; it activates the enzymes that do.
On multi-treatment datasets, each treatment carries its own mark and its own biochemical chain. "Antibody binds cyclin, so cyclin cannot activate the replication enzyme, so replication does not begin" earns the mark. "Fewer cells replicated" without a biochemical mechanism earns nothing — restating the data is not credited.
Key terms
- DNA polymerase
- helicase
- nucleotides
- hydrogen bonds
- RNA
- uracil
- thymine
- phosphodiester bond
- double helix
- condensation
- deoxyribose
- ribose