How is genetic information stored differently in prokaryotic and eukaryotic cells, and what does a genome actually contain?
In prokaryotic cells DNA molecules are short, circular and not associated with proteins. In the nucleus of eukaryotic cells DNA molecules are very long, linear and associated with proteins called histones. A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA. The genome is the complete set of genes in a cell and the proteome is the full range of proteins a cell can produce.
An exam-focused answer to the AQA A-Level Biology 3.4.1 dot point on DNA, genes and chromosomes. Compares prokaryotic and eukaryotic DNA, defines gene, locus, allele, genome and proteome, and explains exons, introns and the triplet code.
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What this dot point is asking
AQA wants you to contrast the way DNA is organised in prokaryotic and eukaryotic cells, define the key vocabulary (gene, locus, allele, genome, proteome), and explain how a gene's base sequence relates to a polypeptide through the triplet code. You also need to handle the eukaryotic complication of non-coding introns within genes.
Prokaryotic versus eukaryotic DNA
| Feature | Prokaryotic DNA | Eukaryotic nuclear DNA |
|---|---|---|
| Length | Short | Very long |
| Shape | Circular | Linear |
| Histone proteins | Absent (naked DNA) | Present (wound around histones) |
| Location | Free in cytoplasm | Enclosed in nucleus |
| Plasmids | Often present (small circular DNA) | Absent |
Mitochondria and chloroplasts also contain DNA, and that DNA is short, circular and not associated with histones, making it structurally like prokaryotic DNA.
In eukaryotes, a long linear DNA molecule wound around histone proteins forms a chromosome. The DNA-histone complex condenses tightly so that metres of DNA fit inside a microscopic nucleus.
Key vocabulary
A diploid cell carries two alleles of each gene, one on each chromosome of a homologous pair, at the same locus. Different alleles differ in their base sequence and so may code for slightly different polypeptides.
The triplet code
A sequence of three DNA bases (a triplet, or codon when in mRNA) codes for one amino acid. Three features of the genetic code are examinable:
- The code is degenerate. Most amino acids are coded for by more than one triplet, because there are possible triplets but only about 20 amino acids.
- The code is non-overlapping. Each base is read once, as part of only one triplet.
- The code is universal. The same triplet codes for the same amino acid in (almost) all organisms, which is what makes genetic engineering across species possible.
Some triplets do not code for an amino acid. Stop codons signal the end of a polypeptide chain.
Exons and introns
In eukaryotes a gene is not one continuous coding stretch. The coding sequences, called exons, are interrupted by non-coding sequences called introns.
- Exons carry the base sequence that is eventually translated into amino acids.
- Introns are transcribed into pre-mRNA but are removed before translation by splicing.
Eukaryotic DNA also contains many other non-coding sequences, including multiple repeats of base sequences between genes that do not code for polypeptides at all. Prokaryotic DNA, by contrast, does not contain introns within its genes.
Common mistakes
Try this
Q1. Define the terms genome and proteome. [2 marks]
- Cue. Genome = the complete set of genes in a cell. Proteome = the full range of proteins a cell can produce.
Q2. The proteome of a cell is larger than the number of genes in its genome. Suggest why. [2 marks]
- Cue. Splicing can join exons in different combinations from the same gene, and post-translational modification can produce several proteins from one polypeptide.
Q3. Describe how a chromosome differs in structure from a circular prokaryotic DNA molecule. [3 marks]
- Cue. Chromosome is linear, very long, and DNA is wound around histone proteins; prokaryotic DNA is circular, short, and not associated with histones.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 AQA Paper 23 marksDescribe the differences between the DNA in a prokaryotic cell and the DNA in the nucleus of a eukaryotic cell.Show worked answer →
A 3-mark "describe the differences" answer needs three clear contrasts, each stated as a comparison.
- Length and shape. Prokaryotic DNA is shorter and circular; eukaryotic nuclear DNA is much longer and linear.
- Association with protein. Prokaryotic DNA is not associated with histones (it is "naked"); eukaryotic nuclear DNA is wound around histone proteins to form chromosomes.
- Location. Prokaryotic DNA lies free in the cytoplasm (with smaller circular plasmids); eukaryotic DNA is enclosed in a nucleus.
Markers reward genuine comparative phrasing (prokaryotic versus eukaryotic in the same sentence), not two separate descriptions.
2021 AQA Paper 22 marksMitochondria and chloroplasts contain DNA. Describe how the DNA in a mitochondrion is similar to the DNA in a prokaryotic cell.Show worked answer →
A 2-mark answer needs two specific similarities.
- The DNA is short and circular in both.
- The DNA is not associated with histone proteins in both.
This reflects the endosymbiotic origin of mitochondria, though AQA only asks for the structural similarities, not the theory.
Related dot points
- The genetic code is universal, non-overlapping and degenerate. Transcription produces mRNA from DNA, in eukaryotes pre-mRNA is spliced to remove introns, and translation at ribosomes uses tRNA and the genetic code to assemble a polypeptide from amino acids.
An exam-focused answer to the AQA A-Level Biology 3.4.2 dot point on protein synthesis. Walks through transcription, splicing of pre-mRNA, the roles of mRNA, tRNA and the ribosome, and translation, with the properties of the genetic code.
- Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include base substitution and base deletion. Because the genetic code is degenerate, not all mutations result in a change to the amino acid sequence. Mutagens increase the rate of mutation, and mutations are one source of genetic diversity within a gene pool.
An exam-focused answer to the AQA A-Level Biology 3.4.3 dot point on mutation. Explains base substitution and deletion, frameshift effects, why the degenerate code buffers some mutations, the role of mutagens, and how mutation contributes to genetic diversity.
- Meiosis produces haploid daughter cells from a diploid parent cell, halving the number of chromosomes so that fertilisation restores the diploid number. Genetic variation arises from independent segregation of homologous chromosomes and from crossing over between homologous chromosomes during meiosis, and the number of possible combinations can be calculated.
An exam-focused answer to the AQA A-Level Biology 3.4.4 dot point on meiosis. Explains how two divisions halve the chromosome number, how independent segregation and crossing over generate variation, and how to calculate the number of possible chromosome combinations.
- Genetic diversity within a population, expressed as the number of different alleles in a gene pool, is acted on by natural selection. Random mutation produces new alleles, and selection results in changes in allele frequency. Directional and stabilising selection produce different effects, and selection leads to anatomical, physiological and behavioural adaptations that increase the chance of survival and reproduction.
An exam-focused answer to the AQA A-Level Biology 3.4.5 dot point on natural selection. Explains how selection changes allele frequencies, contrasts stabilising, directional and disruptive selection, and covers anatomical, physiological and behavioural adaptations.
- A species is a group of similar organisms able to reproduce to give fertile offspring. Each species is given a binomial name. Courtship behaviour helps members of a species to recognise each other and is used in classification. Phylogenetic classification arranges species into a hierarchy of groups that share a common ancestor, and the taxa from domain to species reflect evolutionary relationships.
An exam-focused answer to the AQA A-Level Biology 3.4.6 dot point on species and taxonomy. Defines species and the binomial system, explains phylogenetic classification and the taxonomic hierarchy from domain to species, and covers the role of courtship behaviour.