RNA Processing 3c'

Summary

This lecture delves into the complexities of transcription and RNA processing, focusing on initiation, elongation, and termination in both prokaryotes and eukaryotes. It highlights the differences, such as the presence of poly-A tails and 5' caps in eukaryotic mRNA, which are post-transcriptionally added. The lecture also explores the fascinating process of RNA splicing, where introns are removed, and exons are joined to form mature mRNA. It touches on alternative splicing, a mechanism that allows for the production of different proteins from the same gene, and mentions self-splicing RNA, showcasing the intricate nature of gene expression.

Takeaways

• 📚 The lecture discusses the third part of a series on transcription and RNA processing, focusing on the initiation, elongation, and termination of transcription in eukaryotes.
• 🔬 Phosphorylation of the carboxy-terminal domain in RNA polymerase II initiates the process of transcription, which is regulated by enhancer sequences.
• 🌟 Elongation in eukaryotes is similar to that in prokaryotes, but termination in eukaryotes is less well defined, with less-known terminator sequences.
• 📖 The addition of a poly(A) tail to the end of the RNA is a key feature of eukaryotic transcription, occurring after a poly(A) recognition site, although the exact termination process is not well understood.
• 🧬 Eukaryotic mRNA differs from prokaryotic mRNA in that it has a five prime cap and a poly(A) tail, which are added post-transcriptionally, and it is monocistronic, coding for only one polypeptide chain.
• 🌀 Introns are non-coding sections of the transcript that are removed during processing to produce mature mRNA, which only contains exons that code for amino acids.
• ✂️ RNA splicing is the process of removing introns and joining exons together, which occurs in the nucleus and is facilitated by small nuclear ribonucleoproteins (snRNPs).
• 🔄 The 'GU-AG' rule describes the consensus sequences at the intron-exon boundaries that are recognized during splicing, with 'GU' at the 5' end and 'AG' at the 3' end of introns.
• 🔗 The branch point adenine, located 15 to 45 nucleotides upstream from the 3' end of the intron, plays a crucial role in the splicing mechanism by forming a lariat structure.
• 🔄 Alternative splicing allows for the production of different mRNAs and proteins from the same gene, which can occur in different cell types and is an important aspect of gene regulation.

Q & A

What is the role of the carboxy-terminal domain (CTD) in RNA polymerase during transcription initiation?

The carboxy-terminal domain (CTD) of RNA polymerase plays a crucial role in the initiation of transcription. Phosphorylation of the CTD is part of the process that enhances transcription by recruiting factors necessary for the initiation complex assembly.

How does the addition of a poly(A) tail influence mRNA stability and translation in eukaryotes?

The poly(A) tail added to eukaryotic mRNA increases its stability in the cytoplasm by preventing rapid degradation. It may also facilitate translation, although the exact mechanism is debated, with some studies suggesting it enhances translation and others showing no effect.

What is the difference between polycistronic and monocistronic mRNA in terms of the number of polypeptides they code for?

Polycistronic mRNA codes for multiple polypeptides, whereas monocistronic mRNA codes for only one polypeptide. This distinction is based on the number of genes or loci responsible for generating proteins, with polycistronic mRNAs being characteristic of prokaryotes and monocistronic mRNAs of eukaryotes.

What are introns and exons, and how do they relate to the mature mRNA in eukaryotes?

Introns are non-coding sequences within a gene that are transcribed but later removed from the pre-mRNA during processing. Exons, on the other hand, are the coding sequences that become part of the mature mRNA. The mature mRNA in eukaryotes consists only of exons after introns have been removed through a process called splicing.

What is the function of the five prime (5') cap in eukaryotic mRNA?

The five prime (5') cap is a modified guanosine added to the 5' end of eukaryotic mRNA. It serves to protect the mRNA from degradation and is also required for the initiation of translation.

How does RNA splicing ensure the correct removal of introns and joining of exons?

RNA splicing is facilitated by the spliceosome, a complex of small nuclear RNAs (snRNAs) and proteins. It recognizes consensus sequences at the exon-intron boundaries (GU at the 5' end and AG at the 3' end of introns) and a branch point adenine within the intron. This ensures the correct removal of introns and the ligation of exons.

What is the significance of the 'R loop' experiment in understanding RNA splicing?

The 'R loop' experiment was significant as it provided early evidence for the existence of introns. By hybridizing DNA with its mature mRNA, loops in the DNA that did not pair with the mRNA were observed, indicating the presence of intronic sequences in the DNA that were not present in the mature mRNA.

What is alternative splicing, and how does it contribute to protein diversity?

Alternative splicing is a process where different exons are included or excluded from the mature mRNA, leading to the production of different mRNA variants from the same gene. This can result in the synthesis of different proteins with distinct functions, contributing to the complexity and diversity of the proteome.

How does the process of transcription termination differ between prokaryotes and eukaryotes?

In prokaryotes, transcription termination is well defined and involves specific sequences recognized by the cell. In contrast, eukaryotic transcription termination is less well defined, with less known about the specific sequences or factors involved. Eukaryotes do have poly(A) recognition sites where the poly(A) tail is added post-transcriptionally, but the exact mechanism of how transcription is terminated is not as clear as in prokaryotes.

What is the role of the poly(A) polymerase in eukaryotic mRNA processing?

Poly(A) polymerase is an enzyme that adds a poly(A) tail to the 3' end of eukaryotic mRNA after transcription. This tail consists of multiple adenine nucleotides and is added without a DNA template, contributing to mRNA stability and potentially aiding in translation.