Compare and Contrast Prokaryotic and Eukaryotic Gene Expression

Prokaryotic and eukaryotic gene expression are the two cellular processes responsible for the expression of genes in the genome to produce a functional gene product. In general, both processes proceed through two steps: transcription and translation. This article aims to compare and contrast prokaryotic and eukaryotic gene expression.

In prokaryotes, the closely related genes are clustered to form operons, and thus, producing a polycistronic mRNA molecule. Meanwhile, the functional genes undergo expression individually, and thus, producing monocistronic mRNAs. Furthermore, transcription and translation simultaneously occur in the cytoplasm of prokaryotes. But, in eukaryotes, they occur separately; the former inside the nucleus while the latter in the cytoplasm. Moreover, eukaryotes undergo both post-transcriptional and translational modifications. 

Key Areas Covered 

1. What is Prokaryotic Gene Expression
     – Definition, Process, Significance
2. What is Eukaryotic Gene Expression
     – Definition, Process, Significance
3. Compare and Contrast Prokaryotic and Eukaryotic Gene Expression
     – Similarities and Differences

Key Terms 

Eukaryotic Gene Expression, mRNAs, Prokaryotic Gene Expression, Translation, Transcription

What is Prokaryotic Gene Expression 

Prokaryotic gene expression is the process of synthesizing a gene product according to the information in prokaryotic genes. The significance of the prokaryotic gene expression is that the 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. Furthermore, both transcription and translation of prokaryotes occur in the cytoplasm simultaneously. Therefore, the primary transcript is readily used for the translation whose transcription is still going on.

Figure 1: Prokaryotic Gene Structure

Furthermore, a single type of RNA polymerase is responsible for the transcription in prokaryotes. It requires the sigma factor and a specific DNA sequence called the Pribnow box for the initiation of transcription. 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 synthesizing gene products depending on the information in the eukaryotic genes. Significantly, eukaryotic DNA occurs inside the nucleus; hence, transcription also occurs inside the nucleus. Usually, three RNA polymerases are responsible for the transcription of different types of RNAs: RNA polymerase 1, which transcribes rRNA, RNA polymerase 2, which transcribes mRNA, and RNA polymerase 3, which transcribes tRNA. In addition, each eukaryotic gene is under the control of an individual promoter. Hence, transcription results in a monocistronic mRNA. 

Figure 2: Prokaryotic and Eukaryotic Gene Expression

Moreover, the primary transcript of mRNA undergoes post-transcriptional modifications including the addition of a 5’ cap and a 3’ poly-A tail. On the other hand, the introns, which 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. Eukaryotic proteins undergo post-translational modifications such as phosphorylation, acetylation, methylation, glycosylation, etc. The regulation of gene expression in eukaryotes mostly occurs at the transcriptional level. However, it can also occur at the levels of epigenetics, post-transcription, translation, and post-translation.  

Compare and Contrast Prokaryotic and Eukaryotic Gene Expression  

Similarities Between Prokaryotic and Eukaryotic Gene Expression  

• Prokaryotic and eukaryotic gene expression are two processes in living organisms responsible for the synthesis of a functional gene product inside the cell.  
• Their gene products include functional proteins, resulting in the expression of coding genes and noncoding RNAs, resulting in the expression of noncoding DNA.  
• The two main processes of gene expression are transcription and translation.    
• Protein coding genes undergo both transcription and translation while noncoding DNA only undergoes transcription.    
• Meanwhile, RNA polymerase is the enzyme responsible for transcription. The transcription of protein-coding genes results in mRNAs while the transcription of noncoding DNA results in noncoding RNAs, including tRNAs, rRNAs, miRNAs, etc.  
• However,  only mRNAs undergo translation, which occurs in the cytoplasm. Also,  ribosomes formed from rRNAs facilitates translation, while the tRNAs carry appropriate amino acids. 

Difference Between Prokaryotic and Eukaryotic Gene Expression 

Definition 

Prokaryotic gene expression refers to the process which synthesizes a functional gene product depending on the information on the prokaryotic genes. On the other hand, eukaryotic gene expression refers to the process which synthesizes a functional gene product depending on the information on the eukaryotic genes. 

Spatial Segregation 

Prokaryotic gene expression completely occurs in the cytoplasm while in eukaryotes, transcription occurs inside the nucleus and translation occurs in the cytoplasm.  

Temporal Segregation 

Transcription and translation occur simultaneously in prokaryotic gene expression, while transcription and translation are temporally segregated in eukaryotic gene expression. 

Occurrence of DNA  

Moreover, prokaryotic DNA occurs in the cytoplasm while eukaryotic DNA occurs inside the nucleus. 

Epigenetic Factors 

In addition, prokaryotic DNA is not in a 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, the second is ten nucleotides downstream to it, and the third is 35 nucleotides downstream to it. But, eukaryotes contain a much larger set of promoter elements including the TATA box.  

Transcription Initiation Factors 

Transcription initiation factors of prokaryotes do not assemble with the initiation complex. But, 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 

Moreover, prokaryotic gene expression is equipped with a single type of RNA polymerase while three RNA polymerases are employed in the eukaryotic gene expression. 

Size of Genes 

While prokaryotic genes are small, eukaryotic genes are larger. 

Extra DNA 

Prokaryotes rarely have extra DNA while eukaryotes have large regions of repetitive DNA. 

Coding to Non-Coding DNA 

95% of the prokaryotic genome contains protein-coding genes, while 98% of the eukaryotic genome contains noncoding DNA. 

Genes 

Several functionally-related genes occur in clusters called operons in prokaryotes while eukaryotic genes occur individually. 

Type of mRNA 

Prokaryotic gene expression results in polycistronic mRNA, while eukaryotic gene expression results in monocistronic mRNA.  

Post-Transcriptional Modifications 

Post-transcriptional modifications do not occur in prokaryotic gene expression while post-transcriptional modifications occur in eukaryotic gene expression. 

Ribosomes 

Furthermore, prokaryotes have 70S ribosomes while eukaryotes have 80S ribosomes. 

Post-Translational Modifications 

Post-translational modifications do not occur in prokaryotic gene expression. But, post-translational modifications occur in eukaryotic gene expression. 

Regulation of Gene Expression 

The regulation of the prokaryotic gene expression occurs at the transcriptional level. But, the regulation of the eukaryotic gene expression can occur at the epigenetic level, transcriptional level, post-transcriptional level, translational level, or post-translational level. 

Conclusion 

Prokaryotic and eukaryotic gene expression are the processes of producing a functional gene product according to the information in the genome. Generally, they proceed through two processes: transcription and translation. Typically, translation occurs in the cytoplasm. Functionally-related prokaryotic genes cluster together to form operons, which transcribe together to form a polycistronic mRNA. Also, transcription and translation occur in the cytoplasm of prokaryotes simultaneously. In contrast, transcription of eukaryotes occurs in the nucleus while mRNAs pass into the cytoplasm for translation. Therefore, transcription and translation are not coupled. In addition to that, eukaryotic genes undergo transcription individually, forming monocistronic mRNAs. 

References:

1. Ussery, David. “An Introduction to the Control of Gene Expression in Eukaryotes.” Biology 210 – GENETICS, Spring, 1998, Available Here.
2. “16.2B: Prokaryotic versus Eukaryotic Gene Expression.” Biology LibreTexts, Libretexts, 19 Nov. 2019, 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. “Figure_16_01_01” By OpenStax CNX (CC BY 4.0) via OpenStax College.