- Compare prokaryotic and eukaryotic gene regulation
To understand how gene expression is regulated, we must first understand how a gene codes for a functional protein in a cell. The process occurs in both prokaryotic and eukaryotic cells, just in slightly different manners.
Prokaryotic organisms are single-celled organisms that lack a cell nucleus, and their DNA therefore floats freely in the cell cytoplasm. To synthesize a protein, the processes of transcription and translation occur almost simultaneously. When the resulting protein is no longer needed, transcription stops. As a result, the primary method to control what type of protein and how much of each protein is expressed in a prokaryotic cell is the regulation of DNA transcription. All of the subsequent steps occur automatically. When more protein is required, more transcription occurs. Therefore, in prokaryotic cells, the control of gene expression is mostly at the transcriptional level.
Eukaryotic cells, in contrast, have intracellular organelles that add to their complexity. In eukaryotic cells, the DNA is contained inside the cell’s nucleus and there it is transcribed into RNA. The newly synthesized RNA is then transported out of the nucleus into the cytoplasm, where ribosomes translate the RNA into protein. The processes of transcription and translation are physically separated by the nuclear membrane; transcription occurs only within the nucleus, and translation occurs only outside the nucleus in the cytoplasm. The regulation of gene expression can occur at all stages of the process (Figure 1). Regulation may occur when the DNA is uncoiled and loosened from nucleosomes to bind transcription factors (epigenetic level), when the RNA is transcribed (transcriptional level), when the RNA is processed and exported to the cytoplasm after it is transcribed (post-transcriptional level), when the RNA is translated into protein (translational level), or after the protein has been made (post-translational level).
The differences in the regulation of gene expression between prokaryotes and eukaryotes are summarized in Table 1. The regulation of gene expression in these types of organisms is discussed in detail in subsequent sections.
|Table 1. Differences in the Regulation of Gene Expression of Prokaryotic and Eukaryotic Organisms
|DNA is found in the cytoplasm
|DNA is confined to the nuclear compartment
|RNA transcription and protein formation occur almost simultaneously
|RNA transcription occurs prior to protein formation, and it takes place in the nucleus. Translation of RNA to protein occurs in the cytoplasm.
|Gene expression is regulated primarily at the transcriptional level
|Gene expression is regulated at many levels (epigenetic, transcriptional, nuclear shuttling, post-transcriptional, translational, and post-translational)
Control of gene expression in eukaryotic cells occurs at which level(s)?
- only the transcriptional level
- epigenetic and transcriptional levels
- epigenetic, transcriptional, and translational levels
- epigenetic, transcriptional, post-transcriptional, translational, and post-translational levels
Post-translational control refers to the:
- regulation of gene expression after transcription
- regulation of gene expression after translation
- control of epigenetic activation
- period between transcription and translation