Question

If the following RNA polymerases were missing from a eukaryotic cell, what types of genes would not be transcribed? A. RNA polymerase I C. RNA polymerase III B. RNA polymerase II

Short answer

If RNA polymerase I were missing, ribosomal RNA genes (excluding 5S rRNA) would not be transcribed. If RNA polymerase II were missing, all protein-coding genes, creating mRNA, certain snRNA, and microRNA would not be transcribed. If RNA polymerase III were missing, genes coding for tRNAs and 5S rRNA would not be transcribed.

Step-by-step solution

Identify genes transcribed by RNA polymerase I

RNA polymerase I is responsible for synthesis of ribosomal RNA (rRNA), except for 5S rRNA. Therefore, absent RNA polymerase I would result in failure to transcribe genes coding for rRNA, except for 5S rRNA.

Identify genes transcribed by RNA polymerase II

RNA polymerase II transcribes all protein-coding genes, thus produces messenger RNA (mRNA). It also transcribes certain small nuclear RNA (snRNA) and microRNA. So, a deficiency of RNA polymerase II would stop transcription of protein-coding, certain snRNA, and microRNA genes.

Identify genes transcribed by RNA polymerase III

RNA polymerase III is responsible for transcribing genes coding for transfer RNAs (tRNAs) and 5S rRNA. So, in absence of RNA polymerase III, no transcription of tRNA and 5S rRNA genes would occur.

Key concepts

Ribosomal RNA (rRNA)

Ribosomal RNA, or rRNA, is a critical component of the ribosome, the cell's protein factory. In eukaryotic cells, RNA polymerase I is chiefly responsible for transcribing rRNA genes, which then assemble into ribosomes within the nucleolus. The ribosomes are then transported to the cytoplasm where they play a pivotal role in translating the genetic code from messenger RNA (mRNA) into proteins.

Without RNA polymerase I, the production of rRNA would halt, leading to a severe impediment in the cell's ability to create proteins. rRNA sequences vary in size and form the structural and enzymatic core of ribosomes. While most rRNA is produced by RNA polymerase I, the smaller 5S rRNA component is transcribed by RNA polymerase III, indicating a unique division of labor within the cell.

Messenger RNA (mRNA)

Messenger RNA, or mRNA, serves as the direct template for protein synthesis within a cell. It is transcribed from DNA by RNA polymerase II. This enzyme is tasked with transcribing protein-coding genes into mRNA, which carries the genetic blueprint from the DNA in the nucleus to the ribosomes in the cytoplasm.

RNA polymerase II is also involved in the transcription of small nuclear RNA (snRNA) and microRNA, which are involved in the splicing and regulation of gene expression. The lack of RNA polymerase II would be detrimental to the cell because it would block the transcription of all protein-coding genes, leading to a failure in the production of new proteins, which are essential for virtually all cellular functions. mRNA's role is so central to gene expression that without it, cells cannot produce the proteins necessary for survival.

Transfer RNA (tRNA)

Transfer RNA, or tRNA, is the molecular link between the triplet codons on mRNA and the amino acid sequence of proteins. These small RNA molecules are transcribed by RNA polymerase III, which is charged with synthesizing tRNA and the 5S rRNA component. Each tRNA molecule has an anticodon region that pairs with complementary codons on mRNA during translation and carries a specific amino acid.

In the absence of RNA polymerase III, cells would lose their ability to produce tRNA. This would be catastrophic for the cell, as the process of translating mRNA into a polypeptide chain requires tRNA to deliver the appropriate amino acids to the growing protein chain. As a result, protein synthesis would come to a standstill, disrupting numerous cellular processes and ultimately leading to cellular dysfunction.