Review of Prokaryotic vs. Eukaryotic Gene Expression: Videos & Practice Problems

Prokaryotic gene expression involves sigma factors for transcription initiation, while eukaryotes require transcription factors. Prokaryotes utilize 70S ribosomes, and transcription occurs in the nucleoid, allowing simultaneous transcription and translation. Eukaryotes, with 80S ribosomes, perform transcription in the nucleus and translation in the cytoplasm or rough endoplasmic reticulum, lacking simultaneous processes. Eukaryotic mRNA undergoes processing, including intron removal and exon splicing, while prokaryotic mRNA does not. Prokaryotes can have mono- or polycistronic mRNA, whereas eukaryotes typically have only monocistronic mRNA.

Review of Prokaryotic vs. Eukaryotic Gene Expression

Gene expression is a fundamental biological process that varies significantly between prokaryotic and eukaryotic organisms. Understanding these differences is crucial for grasping how genes are regulated and expressed in different cellular contexts.

In prokaryotes, transcription initiation requires sigma factors, which assist RNA polymerase in binding to DNA. In contrast, eukaryotes utilize transcription factors for this purpose. Both mechanisms are essential for the initiation of transcription, allowing RNA polymerase to begin synthesizing RNA from the DNA template.

The ribosome size also differs between these two groups. Prokaryotic ribosomes are classified as 70S, composed of a 30S small subunit and a 50S large subunit. Eukaryotic ribosomes are larger, at 80S, consisting of a 40S small subunit and a 60S large subunit. This size difference reflects the complexity of the protein synthesis machinery in eukaryotes.

Location plays a critical role in gene expression. Prokaryotes lack a membrane-bound nucleus; instead, they have a nucleoid region where transcription occurs. Both transcription and translation happen in the cytoplasm or nucleoid, allowing for simultaneous processes. Conversely, eukaryotic transcription occurs within the nucleus, while translation takes place in the cytoplasm or on the rough endoplasmic reticulum (RER). This separation means that eukaryotes do not have simultaneous transcription and translation.

Another key distinction is the processing of mRNA. Prokaryotic organisms do not process their messenger RNAs, while eukaryotic mRNAs undergo significant modifications. This processing includes the addition of a modified guanine cap at the 5' end and a poly-A tail at the 3' end, as well as the removal of introns and splicing together of exons. Prokaryotes typically do not have introns, whereas eukaryotes often contain both introns and exons in their genes.

Regarding mRNA types, prokaryotes can produce both monocistronic and polycistronic mRNAs. Monocistronic mRNA contains a single gene, while polycistronic mRNA can encode two or more genes. Eukaryotes predominantly produce monocistronic mRNAs, which simplifies the regulation of gene expression.

In summary, the differences in gene expression between prokaryotes and eukaryotes highlight the complexity and regulation of biological processes. Understanding these distinctions is essential for studying molecular biology and genetics.

PRACTICE: Which of the following statements concerning transcription in bacteria is FALSE:

• A. There are a variety of sigma factors that affect transcription.
• B. Sigma factors are parts of RNA polymerase that recognize promoter regions.
• C. It occurs in the nucleus.
• D. Termination occurs when a stem-loop is formed or due to the presence of Rho protein.
• E. The prokaryotic mRNA does not include introns & does not need to be processed.

Answer: (C)

PRACTICE: All the following are involved in prokaryotic transcription but:

• A. RNA polymerase.
• B. Transcription factors.
• C. A Rho protein.
• D. A promoter.
• E. A sigma factor.

Answer: (B)

PRACTICE: Which of these answers are major differences between eukaryotes and prokaryotes in gene expression?

• A. Eukaryotes have monocistronic mRNA while prokaryotes can have polycistronic mRNA.
• B. Eukaryotic transcription occurs in the nucleus, while prokaryotic transcription occurs in the cytoplasm.
• C. Eukaryotes use transcription factor proteins in transcription, while prokaryotes use sigma factors.
• D. Simultaneous transcription and translation occur in prokaryotes but not eukaryotes.
• E. All of the above are major differences between eukaryotic and prokaryotic transcription.

Answer: (E)

PRACTICE: Why can prokaryotes perform transcription and translation simultaneously while eukaryotes cannot?

• A. Because in prokaryotes, transcription and translation both occur in the cytoplasm.
• B. Because in eukaryotes, transcription and translation do not occur in the same location within the cell.
• C. Because prokaryotes have polycistronic mRNA while eukaryotes have monocistronic mRNA.
• D. Because prokaryotes use sigma factors while eukaryotes use transcription factors.
• E. A and B.
• F. B and C.
• G. C and D.

Answer: (E)

Here’s what students ask on this topic:

What are the key differences between prokaryotic and eukaryotic gene expression?

Prokaryotic gene expression involves sigma factors for transcription initiation, while eukaryotes require transcription factors. Prokaryotes use 70S ribosomes, and transcription occurs in the nucleoid, allowing simultaneous transcription and translation. Eukaryotes, with 80S ribosomes, perform transcription in the nucleus and translation in the cytoplasm or rough endoplasmic reticulum, lacking simultaneous processes. Eukaryotic mRNA undergoes processing, including intron removal and exon splicing, while prokaryotic mRNA does not. Prokaryotes can have mono- or polycistronic mRNA, whereas eukaryotes typically have only monocistronic mRNA.

Why do prokaryotes and eukaryotes have different ribosome sizes?

Prokaryotes have 70S ribosomes, composed of a 30S small subunit and a 50S large subunit, while eukaryotes have 80S ribosomes, consisting of a 40S small subunit and a 60S large subunit. These differences are due to variations in ribosomal RNA (rRNA) and protein composition, reflecting the evolutionary divergence between prokaryotes and eukaryotes. The larger size of eukaryotic ribosomes is associated with their more complex cellular machinery and the need for more intricate regulation of protein synthesis.

How does mRNA processing differ between prokaryotes and eukaryotes?

In eukaryotes, mRNA undergoes extensive processing before translation. This includes the addition of a 5' cap, a poly-A tail at the 3' end, and the removal of introns with the splicing together of exons. These modifications are crucial for mRNA stability, export from the nucleus, and efficient translation. In contrast, prokaryotic mRNA does not undergo such processing. Prokaryotic mRNA is often ready for translation immediately after transcription, reflecting the simpler and more direct gene expression mechanisms in prokaryotes.

What is the significance of simultaneous transcription and translation in prokaryotes?

Simultaneous transcription and translation in prokaryotes allow for rapid gene expression, which is crucial for their quick adaptation to environmental changes. Since prokaryotes lack a membrane-bound nucleus, ribosomes can begin translating mRNA while it is still being transcribed. This coupling of processes enhances the efficiency and speed of protein synthesis, enabling prokaryotes to respond swiftly to stimuli and maintain cellular functions effectively.

Why do eukaryotes not have simultaneous transcription and translation?

Eukaryotes do not have simultaneous transcription and translation because these processes occur in different cellular compartments. Transcription takes place in the nucleus, where the DNA is housed, while translation occurs in the cytoplasm or on the rough endoplasmic reticulum. This separation allows for more complex regulation and processing of mRNA, including splicing, capping, and polyadenylation, which are essential for the proper expression and function of eukaryotic genes.