Eukaryotic vs Prokaryotic Transcription

  Prokaryotes and eukaryotes differ in their transcriptional processes in several ways. Here are some key differences:

1. Transcriptional machinery:

                                                   Prokaryotes have a single RNA polymerase enzyme that transcribes all genes, while eukaryotes have three different RNA polymerases (RNA polymerase I, II, and III), each with specific functions. Prokaryotes and eukaryotes use different transcriptional machinery to transcribe DNA into RNA.

   In prokaryotes, there is a single type of RNA polymerase that is responsible for transcribing all              types of genes, including mRNA, rRNA, and tRNA. The prokaryotic RNA polymerase consists of a core enzyme composed of several subunits, plus a sigma factor that helps the RNA polymerase recognize the promoter sequence on DNA.

In contrast, eukaryotes have three different types of RNA polymerase: RNA polymerase I, RNA polymerase II, and RNA polymerase III. Each of these RNA polymerases is responsible for transcribing specific types of genes. For example, RNA polymerase II is responsible for transcribing protein-coding genes (mRNA), while RNA polymerase I and III are responsible for transcribing other types of RNA, such as rRNA, tRNA, and small nuclear RNA (snRNA).

In addition, eukaryotic RNA polymerases are more complex than prokaryotic RNA polymerase. They consist of multiple subunits, each with specific functions. For example, RNA polymerase II consists of over 12 subunits, including the Rpb1 subunit that forms the catalytic core of the enzyme and other subunits that play important roles in transcriptional regulation and RNA processing.

Overall, the transcriptional machinery in eukaryotes is more complex and diverse than that in prokaryotes, reflecting the higher complexity of eukaryotic gene expression and the need for multiple RNA polymerases to transcribe the various types of genes in the genome.

 

2. Transcriptional initiation: In prokaryotes, transcriptional initiation is facilitated by the sigma factor, which binds to the promoter sequence and helps RNA polymerase to initiate transcription. In eukaryotes, transcriptional initiation involves the assembly of a pre-initiation complex (PIC) containing various transcription factors and RNA polymerase at the promoter.
3. Transcriptional regulation: Prokaryotic transcriptional regulation often involves the binding of regulatory proteins (transcription factors) to DNA near the promoter, while in eukaryotes, transcriptional regulation is more complex and often involves enhancers, silencers, and other cis- and trans-acting elements.
4. RNA processing: In prokaryotes, the transcribed RNA is often used directly as mRNA without any processing, while in eukaryotes, the pre-mRNA transcript undergoes extensive processing including capping, splicing, and polyadenylation to generate the mature mRNA.

RNA processing refers to the modifications that occur on pre-mRNA transcripts to generate mature mRNA in eukaryotes and some archaea. Prokaryotes do not undergo the same degree of RNA processing as eukaryotes, and their transcripts can often be used directly as mRNA.

In eukaryotes, RNA processing includes several steps:

1.       Capping: The pre-mRNA is modified at the 5' end by the addition of a 7-methylguanosine cap. This cap protects the mRNA from degradation and helps to recruit the ribosome during translation.

2.       Splicing: The pre-mRNA is also spliced to remove non-coding introns and join together the exons to generate a continuous coding sequence. This process is carried out by the spliceosome, a complex of proteins and small nuclear RNA (snRNA).

3.       Polyadenylation: The pre-mRNA is modified at the 3' end by the addition of a poly(A) tail consisting of multiple adenosine nucleotides. This tail also helps to protect the mRNA from degradation and is involved in the export of the mRNA from the nucleus to the cytoplasm.

4.       RNA editing: Some mRNAs in eukaryotes undergo RNA editing, which involves the post-transcriptional modification of nucleotides within the mRNA. This can include the deamination of cytosine to uracil, or the insertion or deletion of nucleotides.

In contrast, prokaryotes do not undergo splicing or capping, and their transcripts typically do not have a poly(A) tail. However, some prokaryotic transcripts may undergo other types of processing, such as endonucleolytic cleavage or RNA modification.

Overall, the RNA processing in eukaryotes is more complex than that in prokaryotes, reflecting the higher degree of post-transcriptional regulation and the need for additional mechanisms to protect and regulate the mRNA.

5. Transcriptional termination: In prokaryotes, transcriptional termination occurs when RNA polymerase encounters a terminator sequence in the DNA, while in eukaryotes, transcriptional termination is more complex and involves the cleavage of the pre-mRNA and release of the RNA polymerase.

Overall, the transcriptional processes in prokaryotes are simpler and more streamlined than those in eukaryotes, reflecting the simpler overall organization and lower complexity of prokaryotic genomes.