Prokaryotic vs Eukaryotic DNA (AQA A-Level Biology): Revision Notes
Prokaryotic vs Eukaryotic DNA
DNA structure in different cell types
The organisation and structure of DNA varies significantly between prokaryotic and eukaryotic cells. These differences reflect the distinct evolutionary pathways and cellular complexity of these two major cell types.
In prokaryotic cells (such as bacteria), the DNA exists as a single, circular molecule that floats freely in the cytoplasm. This DNA molecule is relatively short compared to eukaryotic DNA and typically contains fewer genes. Additionally, prokaryotic cells often contain smaller circular DNA molecules called plasmids, which carry additional genetic information.
The circular nature of prokaryotic DNA is a fundamental characteristic that distinguishes it from the linear DNA found in eukaryotic cells. This structural difference has significant implications for DNA replication and gene expression.
Eukaryotic cells show a more complex DNA organisation. Each cell contains multiple, separate DNA molecules that are linear in structure. These linear molecules are packaged with proteins called histones to form chromosomes. This packaging creates the characteristic rod-like structures visible under electron microscopy.
Key structural differences
DNA packaging and location
Prokaryotic DNA exists as a naked molecule without histone proteins. The circular chromosome sits directly in the cytoplasm, creating what appears to be a thick, coiled structure when viewed under high magnification.
Eukaryotic DNA associates closely with histone proteins, forming nucleosome structures that resemble beads on a string. This DNA-histone complex packages into the familiar chromosome structure found within the cell nucleus.
The presence or absence of histone proteins is a critical difference that affects how DNA is packaged, protected, and accessed for cellular processes. Histone proteins in eukaryotes allow for much more sophisticated gene regulation.
Gene organisation and structure
A gene can be defined as a specific DNA sequence that codes for either a polypeptide chain or a functional RNA molecule. Each gene occupies a fixed position on a chromosome, known as its locus.
Understanding the concept of a gene locus is crucial because it explains how genes are mapped and inherited. The locus remains constant even though the specific DNA sequence (allele) at that position may vary between individuals.
Eukaryotic genes contain both coding regions called exons and non-coding regions called introns. The presence of introns means that eukaryotic genes often appear interrupted, with coding sequences separated by non-coding DNA.
Prokaryotic genes lack introns entirely. Their genes consist only of continuous coding sequences, making them more compact and efficient in terms of space usage.
Non-coding DNA content
Eukaryotic cells contain substantial amounts of non-coding DNA. This includes introns within genes and repetitive DNA sequences found between genes. Surprisingly, less than 2% of human DNA actually codes for proteins.
This statistic often surprises students - the vast majority of human DNA does not directly code for proteins. Much of this "non-coding" DNA was once called "junk DNA," but scientists now recognise that many of these sequences have important regulatory functions.
Prokaryotic cells show much less non-coding DNA. Their genomes tend to be more streamlined, with most DNA sequences serving a coding function.
Comparative summary
| Feature | Prokaryotic cells | Eukaryotic cells |
|---|---|---|
| Molecule length | Short (fewer genes) | Long (many genes) |
| Molecule shape | Circular (closed loop) | Linear (part of chromosome) |
| Number of different molecules per cell | One main molecule | Multiple molecules |
| Protein association | Not associated with proteins | Associated with histones |
| Non-coding DNA | Absent | Present in genes (introns) and between genes |
Key Points to Remember:
- Prokaryotic DNA is circular and sits freely in the cytoplasm without histone proteins
- Eukaryotic DNA is linear, packaged with histones, and located in the nucleus
- Eukaryotic genes contain introns (non-coding regions) while prokaryotic genes do not
- Eukaryotic cells have much more non-coding DNA than prokaryotic cells
- Each gene occupies a specific locus (position) on its chromosome, regardless of cell type