What is the Difference Between Prokaryotic and Eukaryotic Gene Expression

The main difference between prokaryotic and eukaryotic gene expression is that the entire prokaryotic gene expression occurs in the cytoplasm whereas a part of the eukaryotic gene expression occurs inside the nucleus while rest occurs in the cytoplasm. Furthermore, the regulation of prokaryotic gene expression mainly occurs at the transcriptional level while the regulation of eukaryotic gene expression occurs at various steps of gene expression, facilitated by the compartmentalization of the gene expression.

Prokaryotic and eukaryotic gene expression are two processes involved in the transcription of genes, producing mRNAs and translating mRNA into functional proteins.

Key Areas Covered

1. What is Prokaryotic Gene Expression
     – Definition, Process, Regulation
2. What is Eukaryotic Gene Expression
     – Definition, Process, Regulation
3. What are the Similarities Between Prokaryotic and Eukaryotic Gene Expression
     – Outline of Common Features
4. What is the Difference Between Prokaryotic and Eukaryotic Gene Expression
     – Comparison of Key Differences

Key Terms

Eukaryotic Gene Expression, Prokaryotic Gene Expression, Regulation of Gene Expression, Transcription, Translation

What is Prokaryotic Gene Expression

Prokaryotic gene expression is the process of the production of a gene product based on the information in prokaryotic genes. The two main steps of prokaryotic gene expression are transcription and translation. Also, the main significance of the prokaryotic gene expression is that their transcription occurs in the cytoplasm. This is due to the lack of a nucleus in prokaryotic cells. In prokaryotes, a single type of RNA polymerase is responsible for transcription. It requires the sigma factor and a specific DNA sequence called the Pribnow box for the initiation of transcription.

Figure 1: Prokaryotic Gene Structure

On the other hand, genes in prokaryotes occur in operons, which are the clusters of functionally-related genes. Some examples of operons include Lac operon and Trp operon. Therefore, genes in an operon are transcribed together, forming a polycistronic mRNA molecule. In addition, prokaryotic transcription always couples with translation since both occur in the cytoplasm. Therefore, the primary transcript is readily used for the translation whose transcription is still going on. 70S Ribosomes are responsible for the translation of the prokaryotic polycistronic mRNA. Most importantly, the regulation of prokaryotic gene expression occurs at the transcriptional level, either by increasing or decreasing the level of transcription.

What is Eukaryotic Gene Expression

Eukaryotic gene expression is the process of the production of gene products based on the information in the eukaryotic genes. It also occurs through transcription and translation. Here, since eukaryotic DNA occurs inside the nucleus, the transcription also occurs inside the nucleus. Three RNA polymerases are responsible for the transcription of different types of RNAs: RNA polymerase 1, which synthesizes rRNA, RNA polymerase 2, which synthesizes mRNA, and RNA polymerase 3, which synthesizes tRNA. Moreover, each eukaryotic gene is under the control of an individual promoter. Hence, transcription produces a monocistronic mRNA.

Figure 2: Prokaryotic and Eukaryotic Gene expression

On the other hand, the primary transcript of mRNA undergoes post-transcriptional modifications including the addition of a 5’ cap and a 3’ poly A tail. In addition, the introns that interrupt the protein coding region of the eukaryotic mRNA are spliced out in a process called RNA splicing. The ultimate mRNA molecule is the mature mRNA which leaves the nucleus to the cytoplasm and it is ready for the translation. 80S Ribosomes are responsible for the translation of the eukaryotic mRNA.

Similarities Between Prokaryotic and Eukaryotic Gene Expression

• Prokaryotic and eukaryotic gene expression are the processes responsible for the production of a functional protein based on the information encoded by genes.
• Also, both processes occur through transcription and translation.
• Furthermore, translation in both prokaryotes and eukaryotes occurs in the cytoplasm.
• Besides, both prokaryotic and eukaryotic gene expression have post-translational modifications including phosphorylation, acetylation, etc.

Difference Between Prokaryotic and Eukaryotic Gene Expression

Definition

Prokaryotic gene expression refers to the process by which information from the prokaryotic genes is used in the synthesis of a functional gene product while eukaryotic gene expression refers to the process by which information from eukaryotic genes is used in the synthesis of a functional gene product.

Spatial Segregation

Prokaryotic gene expression occurs in the cytoplasm. However, the transcription of eukaryotic gene expression occurs inside the nucleus and translation occurs in the cytoplasm. Hence, this is one main difference between prokaryotic and eukaryotic gene expression.

Temporal Segregation

Another difference between prokaryotic and eukaryotic gene expression is that the transcription and translation occur simultaneously during prokaryotic gene expression while the transcription and translation are temporally segregated in eukaryotic gene expression.

Epigenetic Factors

Moreover, prokaryotic DNA is not in permanently condensed form while eukaryotic DNA forms a stable, condensed complex with histones. 

Promoter Elements

Prokaryotes contain three promoter elements: one is upstream to the gene, second is 10 nucleotides downstream to it, and the third is 35 nucleotides downstream to it while eukaryotes contain a much larger set of promoter elements including the TATA box. Thus, this is another difference between prokaryotic and eukaryotic gene expression.

Transcription Initiation Factors

Furthermore, the transcription initiation factors of prokaryotes do not assemble with the initiation complex while the transcription initiation factors of eukaryotes assemble with the initiation complex.

Open Reading Frame

Introns do not interrupt the open reading frame of the prokaryotic genes while introns interrupt the open reading frame of the eukaryotic genes.

RNA Polymerase

One other difference between prokaryotic and eukaryotic gene expression is that the prokaryotic gene expression is equipped with a single type of RNA polymerase while three RNA polymerases are employed in the eukaryotic gene expression.

Genes

Moreover, several functionally-related genes occur in clusters called operons in prokaryotes while eukaryotic genes occur individually.

Type of mRNA

Also, prokaryotic gene expression results in polycistronic mRNA while eukaryotic gene expression results in monocistronic mRNA. This is another difference between prokaryotic and eukaryotic gene expression.

Post-Transcriptional Modifications

Another difference between prokaryotic and eukaryotic gene expression is that prokaryotic gene expression does not include post-transcriptional modifications while eukaryotic gene expression includes post-transcriptional modifications.

Ribosomes

One more difference between prokaryotic and eukaryotic gene expression is that the prokaryotes contain 70S ribosomes while eukaryotes contain 80S ribosomes.

Regulation of Gene Expression

Besides, the regulation of the prokaryotic gene expression occurs at the transcriptional level while the regulation of the eukaryotic gene expression can occur at epigenetic level, transcriptional level, post-transcriptional level, translational level, and post-translational level.

Conclusion

Prokaryotic gene expression completely occurs in the cytoplasm and is controlled at transcriptional level. Its transcription couples with the translation. In eukaryotic gene expression, transcription occurs inside the nucleus while translation occurs in the cytoplasm. In between the transcription and translation, the primary mRNA transcript undergoes post-transcriptional modifications in eukaryotes. The main difference between prokaryotic and eukaryotic gene expression is the location and the coupling of transcription and translation.

Reference:

1. “Biology for Majors I.” Lumen, Lumen Waymaker, Available Here

Image Courtesy:

1. “Gene structure prokaryote 2 annotated” By Thomas Shafee – Shafee T, Lowe R (2017). “Eukaryotic and prokaryotic gene structure”. WikiJournal of Medicine 4 (1). DOI:10.15347/wjm/2017.002. ISSN 20024436. (CC BY 4.0) via Commons Wikimedia  
2. “Regulation of Gene Expression” – OpenStax CNX (CC BY 4.0) via OpenStax College