What are some similarities and differences between prokaryotic and eukaryotic gene expression?

There are many differences between prokaryotic and eukaryotic cells. Some of these differences are structural whereas others are procedural. Two of the processes that are substantially different between prokaryotes and eukaryotes are gene expression and the regulation of it. Both types of cells transcribe DNA into mRNA, which is then translated into polypeptides, but the specifics of these processes differ.

Location

Prokaryotes lack nuclei and other organelles, which are specialized, membrane-bound compartments, whereas eukaryotes do have them. In fact, the word "eukaryote" means "true nucleus." In eukaryotes the cell's genome is located in the nucleus. Transcription thus occurs in the nucleus, and the mRNA transcript is subsequently exported through nuclear pores (pores in the nuclear envelope) to the cytoplasm for translation. By contrast, prokaryotic transcription and translation are not spatially or temporally segregated.

Initiation of Transcription

Promoter elements are short sequences of DNA that bind to a cell's transcriptional initiation factors. Prokaryotes have three promoter elements: one that is upstream of the gene being transcribed, one that is 10 nucleotides downstream of it and one that is 35 nucleotides downstream. Eukaryotes have a much larger set of promoter elements, the primary one being the TATA box. Eukaryotic transcription initiation factors assemble an initiation complex, which dissociates at the end of initiation. Prokaryotic transcription initiation factors do not assemble an initiation complex.

Ribosomes

Ribosomes are translation sites composed of RNA and protein that bind to a cell's mRNA and tRNA. Prokaryotes have 70S ribosomes whereas eukaryotes have 80S ribosomes. The "S" refers to the sedimentation coefficient, a measure of a particle's size, mass and shape. An 80S ribosome is composed of a 40S subunit and a 60S subunit while a 70S ribosome consists of a 30S subunit and a 50S subunit.

Polycistronic mRNA

In addition to having different transcription and translation machinery, prokaryotes and eukaryotes differ in their gene regulation. Eukaryotic regulation is much more complex and often relies on various feedback mechanisms, developmental processes and environmental factors. By contrast, prokaryotes regulate entire metabolic pathways rather than regulating each enzyme separately. Bacterial enzymes for a given pathway are adjacent to each other on a cell's DNA and are transcribed into one mRNA. This mRNA is called polycistronic mRNA. When a cell needs more or less of a pathway's enzymes, it simply transcribes more or less of that pathway's mRNA.

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 organisms	Eukaryotic organisms
Lack nucleus	Contain nucleus
RNA transcription and protein translation occur almost simultaneously	RNA 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 level	Gene expression is regulated at many levels (epigenetic, transcriptional, post-transcriptional, translational, and posttranslational)

References

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/

What are the similarities between prokaryotic and eukaryotic gene expression?

What are differences in gene expression between prokaryotes and eukaryotes?