Gene expression in eukaryotes is the process by which information in DNA is used to make functional RNA molecules and proteins. It matters because different cell types contain nearly the same DNA but produce very different sets of proteins. A neuron, muscle cell, and skin cell look and act differently because they regulate which genes are active.
This regulation allows development, repair, immune responses, and adaptation to changing conditions.
Eukaryotic gene expression is controlled at many checkpoints, starting with how tightly DNA is packed into chromatin. Transcription factors and enhancers help RNA polymerase begin transcription, while RNA processing changes the primary transcript into mature mRNA. The mRNA must leave the nucleus, survive in the cytoplasm, and be translated by ribosomes.
Even after a protein is made, its activity can be changed by folding, chemical modification, transport, or degradation.
Key Facts
- DNA to RNA to protein is the central flow of gene expression: DNA is transcribed into RNA, and mRNA is translated into protein.
- Chromatin accessibility controls whether transcription machinery can reach a gene; tightly packed heterochromatin is usually less active than open euchromatin.
- Transcription factors bind regulatory DNA sequences and help control the rate of transcription initiation.
- Enhancers can act far from a gene by looping DNA so regulatory proteins contact the promoter.
- Eukaryotic pre-mRNA is processed by 5' capping, splicing, and poly-A tail addition before export from the nucleus.
- Protein production can be estimated as protein made = number of mRNA molecules × translation rate × mRNA lifetime.
Vocabulary
- Chromatin
- Chromatin is the complex of DNA and proteins that packages eukaryotic DNA inside the nucleus.
- Transcription factor
- A transcription factor is a protein that binds DNA and helps increase or decrease transcription of specific genes.
- Enhancer
- An enhancer is a regulatory DNA sequence that can increase transcription of a gene, often from a distance.
- RNA splicing
- RNA splicing is the process that removes introns from pre-mRNA and joins exons together.
- Post-translational modification
- A post-translational modification is a chemical change to a protein after translation that can alter its activity, location, or stability.
Common Mistakes to Avoid
- Thinking every gene is active in every cell is wrong because most cells regulate gene expression and use only a selected portion of their genome.
- Ignoring chromatin structure is wrong because a gene cannot be efficiently transcribed if transcription machinery cannot access the DNA.
- Confusing enhancers with promoters is wrong because promoters are near transcription start sites, while enhancers can be far away and work through DNA looping.
- Assuming mRNA amount always equals protein amount is wrong because translation rate, mRNA degradation, protein folding, and protein degradation also affect final protein levels.
Practice Questions
- 1 A gene produces 40 mature mRNA molecules. Each mRNA is translated 15 times before it is degraded. How many protein molecules are produced from this gene?
- 2 A pre-mRNA contains 5 exons and 4 introns. If all introns are removed and all exons are joined, how many exon junctions are present in the mature mRNA?
- 3 A mutation removes an enhancer for a liver-specific gene but leaves the coding sequence unchanged. Explain how this could reduce protein production without changing the amino acid sequence of the protein.