Prokaryotes don't contain introns. So splicing of introns and joining of exons are not needed. But in eukaryotics, splicing of introns and joining of exons is needed.
To understand how gene expression is regulated, we must first understand how a gene becomes a functional protein in a cell. The process occurs in both prokaryotic and eukaryotic cells, just in slightly different fashions.
Because prokaryotic organisms lack a cell nucleus, the processes of transcription and translation occur almost simultaneously. When the protein is no longer needed, transcription stops. As a result, the primary method to control what type and how much protein is expressed in a prokaryotic cell is through the regulation of DNA transcription into RNA. All the subsequent steps happen automatically. When more protein is required, more transcription occurs. Therefore, in prokaryotic cells, the control of gene expression is almost entirely at the transcriptional level.
Eukaryotic cells, in contrast, have intracellular organelles and are much more complex. Recall that in eukaryotic cells, the DNA is contained inside the cell’s nucleus and it is transcribed into mRNA there. The newly synthesized mRNA is then transported out of the nucleus into the cytoplasm, where ribosomes translate the mRNA into protein. The processes of transcription and translation are physically separated by the nuclear membrane; transcription occurs only within the nucleus, and translation only occurs outside the nucleus in the cytoplasm. The regulation of gene expression can occur at all stages of the process (Figure 2):
- Epigenetic level: regulates how tightly the DNA is wound around histone proteins to package it into chromosomes
- Transcriptional level: regulates how much transcription takes place
- Post-transcriptional level: regulates aspects of RNA processing (such as splicing) and transport out of the nucleus
- Translational level: regulates how much of the RNA is translated into protein
- Post-translational level: regulates how long the protein lasts after it has been made and whether the protein is processed into an active form
Figure 2: Eukaryotic gene expression is regulated during transcription and RNA processing, which take place in the nucleus, as well as during protein translation, which takes place in the cytoplasm. Further regulation may occur through post-translational modifications of proteins.The differences in the regulation of gene expression between prokaryotes and eukaryotes are summarized in Table 1.
Table 1: Differences in the Regulation of Gene Expression of Prokaryotic and Eukaryotic Organisms
Prokaryotic organismsEukaryotic organismsLack nucleusContain nucleusRNA transcription and protein translation occur almost simultaneouslyRNA transcription occurs prior to protein translation, and it takes place in the nucleus. RNA translation to protein occurs in the cytoplasm.RNA post-processing includes addition of a 5′ cap, poly-A tail, and excision of introns and splicing of exons.
Gene expression is regulated primarily at the transcriptional levelGene expression is regulated at many levels (epigenetic, transcriptional, post-transcriptional, translational, and posttranslational)Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.
OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016 http://cnx.org/contents/[email protected]
Sproul, D., Gilbert, N., & Bickmore, W. The role of chromatin structure in regulating the expression of clustered genes. Nature Reviews Genetics 6, 775–781 (2005) doi:10.1038/nrg1688 (link to article) This page has been archived and is no longer updated
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