Introns vs. Exons: Within a single eukaryotic gene, coding sequences called exons are interrupted by non-coding sequences called introns. While exons contain the information for protein synthesis, introns must be removed during RNA processing.
Splicing: The process of removing introns and joining exons together to form a continuous coding sequence of mRNA is called splicing. This occurs in the nucleus before the mRNA is exported for translation.
Multiple Repeats: Large portions of the eukaryotic genome consist of non-coding DNA located between genes. These often take the form of multiple repeats, which are sequences of bases that recur many times throughout the genome.
Extranuclear DNA: Eukaryotic cells contain DNA outside the nucleus within mitochondria (mtDNA) and chloroplasts (cpDNA). This DNA is essential for the function of these organelles, coding for specific enzymes and proteins involved in energy transformation.
Prokaryotic Characteristics: Unlike nuclear DNA, organelle DNA is typically short, circular, and not associated with histone proteins. This structural similarity to bacterial DNA supports the endosymbiotic theory of organelle evolution.
Localization: Mitochondrial DNA is found within the mitochondrial matrix, while chloroplast DNA is located in the stroma. Both organelles can replicate their DNA independently of the nuclear cell cycle.
| Feature | Nuclear Eukaryotic DNA | Organelle (mtDNA/cpDNA) | Prokaryotic DNA |
|---|---|---|---|
| Shape | Linear | Circular | Circular |
| Length | Very Long | Short | Short |
| Histones | Associated (Chromatin) | Not associated (Naked) | Not associated (Naked) |
| Introns | Present in most genes | Generally absent | Absent |
Structural Complexity: Nuclear DNA requires histones for extreme compaction, whereas organelle and prokaryotic DNA are small enough to remain 'naked' or associated with different types of proteins.
Gene Density: Eukaryotic nuclear DNA has a lower gene density due to the presence of vast non-coding regions (introns and intergenic repeats), whereas prokaryotic and organelle DNA are much more compact with fewer non-coding sequences.
Terminology Precision: Always distinguish between chromatin (the DNA-protein complex), chromatids (the identical arms of a replicated chromosome), and chromosomes (the overall condensed structure). Using these interchangeably can lead to lost marks.
Organelle DNA Trap: When asked about 'eukaryotic DNA', remember to specify if the question refers to nuclear DNA or DNA found within the cell as a whole. Mitochondria and chloroplasts contain DNA that behaves like prokaryotic DNA.
Splicing Context: Remember that splicing is a unique requirement for eukaryotic nuclear genes because prokaryotes and eukaryotic organelles lack introns. If a question asks why a bacterial cell cannot express a human gene correctly from a genomic DNA fragment, the answer usually involves the inability to remove introns.
