Question

RECALL What are the products of the reactions of the three eukaryotic RNA polymerases?

Short answer

RNA Polymerase I produces rRNA; RNA Polymerase II produces mRNA and snRNA; RNA Polymerase III produces tRNA and 5S rRNA.

Step-by-step solution

Identify RNA Polymerase I Products

RNA Polymerase I is responsible for transcribing ribosomal RNA (rRNA). Specifically, it transcribes the genes for the 18S, 5.8S, and 28S rRNAs, which are essential components of the ribosome.

Identify RNA Polymerase II Products

RNA Polymerase II transcribes messenger RNA (mRNA) and several small nuclear RNAs (snRNAs). The mRNA serves as the template for protein synthesis during translation.

Identify RNA Polymerase III Products

RNA Polymerase III transcribes transfer RNA (tRNA) and 5S rRNA, along with some other small RNAs. tRNAs are crucial for the translation of mRNA into protein.

Key concepts

RNA Polymerase I

RNA Polymerase I is a key enzyme in the process of transcription, specifically dedicated to producing ribosomal RNA (rRNA). It transcribes genes that code for the 18S, 5.8S, and 28S rRNAs. These rRNAs are essential components of the ribosome, the cellular machinery responsible for synthesizing proteins.

The ribosome itself is composed of both rRNA and protein, and the rRNA forms the core structural and functional components.

• 18S rRNA is part of the small ribosomal subunit
• 5.8S and 28S rRNAs are part of the large ribosomal subunit

Together, these rRNAs ensure that ribosomes function efficiently, facilitating the translation of mRNA into proteins required for various cellular processes.

RNA Polymerase II

RNA Polymerase II is essential for transcribing messenger RNA (mRNA) and several small nuclear RNAs (snRNAs). mRNA serves as the template from which proteins are synthesized during translation. The process starts with the transcription of genes into mRNA molecules.

This polymerase plays a critical role in gene expression because mRNA carries the genetic instructions from DNA to the ribosome, where they are translated into protein.

• mRNA results from the transcription of protein-coding genes.
• snRNAs are involved in the splicing process, where introns are removed from pre-mRNA to produce mature mRNA.

By producing mRNA, RNA Polymerase II enables the cell to produce the diverse range of proteins necessary for life.

RNA Polymerase III

RNA Polymerase III transcribes transfer RNA (tRNA) and 5S ribosomal RNA (rRNA), along with other small RNAs. Transfer RNA is vital for translating the mRNA sequence into a protein.

Each tRNA molecule carries a specific amino acid that corresponds to a three-nucleotide codon in the mRNA.

• tRNAs are key players in protein synthesis by bringing corresponding amino acids to the ribosome.
• 5S rRNA, on the other hand, is part of the ribosome's large subunit and plays a role in its stability and function.

Other small RNAs synthesized by RNA Polymerase III provide various cellular functions, reinforcing the importance of this enzyme in cell biology.

Transcription Products

Transcription products are the RNA molecules synthesized by the three different RNA polymerases. Each type of RNA polymerase is responsible for producing specific types of RNA, which have unique roles in the cell.

• RNA Polymerase I produces rRNAs that are crucial for ribosome structure and function.
• RNA Polymerase II produces mRNAs and snRNAs, which are vital for protein synthesis and mRNA processing, respectively.
• RNA Polymerase III produces tRNAs and 5S rRNA, which are essential for translating mRNA into protein and maintaining ribosome structure.

Together, these transcription products form the backbone of gene expression and protein synthesis, orchestrating the cellular processes necessary for life.

rRNA

Ribosomal RNA (rRNA) is a fundamental component of ribosomes, the cellular structures where protein synthesis occurs. Produced by RNA Polymerase I and III, rRNAs form the core scaffolding and functional elements of ribosomes.

• 18S rRNA forms part of the small ribosomal subunit, involved in binding mRNA and the initial steps of translation.
• 28S and 5.8S rRNAs are part of the large ribosomal subunit and facilitate peptide bond formation between amino acids.
• 5S rRNA, also part of the large subunit, contributes to ribosomal stability.

Without rRNA, ribosomes could not function properly, which would halt protein synthesis and disrupt cellular function.

mRNA

Messenger RNA (mRNA) is a type of RNA synthesized by RNA Polymerase II, and it plays a critical role in the process of gene expression. mRNA molecules carry coding sequences for proteins from the DNA in the nucleus to the ribosome in the cytoplasm.

Once at the ribosome, the sequence of nucleotides in the mRNA is translated into a sequence of amino acids, forming a protein.

• Each mRNA molecule is a transcript of a specific gene that includes the template for making a particular protein.
• During translation, tRNAs bring the appropriate amino acids to the ribosome, which are added to the growing polypeptide chain based on the codons in the mRNA sequence.

Thus, mRNA serves as the crucial intermediary between the genetic information stored in DNA and the protein products that perform cellular functions.

tRNA

Transfer RNA (tRNA) is essential for translating the genetic code in mRNA into an amino acid sequence, which will become a functioning protein. Transcribed by RNA Polymerase III, tRNA molecules are adapters that match an amino acid to its corresponding mRNA codon through its anticodon region.

• Each tRNA molecule carries a specific amino acid and has a region that pairs with a specific mRNA codon.
• During translation, tRNAs bring the correct amino acids to the ribosome, which are then added to the growing polypeptide chain in the proper sequence.

tRNA ensures that the genetic code in mRNA is translated accurately and efficiently into proteins, which are crucial for many cellular processes.