A 58-year-old man comes to the office with a persistent dry cough. He has experienced an involuntary weight loss of 10 kg (22 lb) over the last 3 months. The patient drinks 2-3 beers daily and has a 40-pack-year smoking history. Physical examination shows dullness to percussion over the right lower lung base. A chest CT reveals a right-sided pleural effusion and a mass in the lower lobe of the right lung. Transbronchial biopsy of the mass demonstrates malignant cells with large nuclei that contain prominent, round, basophilic bodies as shown below. Which of the following enzymes is most likely to function only within this basophilic region of the nucleus?

Question

A 58-year-old man comes to the office with a persistent dry cough.  He has experienced an involuntary weight loss of 10 kg (22 lb) over the last 3 months.  The patient drinks 2-3 beers daily and has a 40-pack-year smoking history.  Physical examination shows dullness to percussion over the right lower lung base.  A chest CT reveals a right-sided pleural effusion and a mass in the lower lobe of the right lung.  Transbronchial biopsy of the mass demonstrates malignant cells with large nuclei that contain prominent, round, basophilic bodies as shown below.  Which of the following enzymes is most likely to function only within this basophilic region of the nucleus?

Quizplus · Accepted Answer

11ec160b_3772_461a_9249_7fae66531626_MD0001_00 The nucleolus is a round, dense, basophilic (dark-staining) body that can be identified within the nucleus on standard light microscopy. It is the primary site of ribosomal RNA (rRNA) transcription. Although there are several hundred copies of the 45S pre-rRNA gene divided among multiple chromosomes, these are arranged into clusters called nucleolar organizing regions that position themselves so that all copies are located within the nucleolus. RNA polymerase I functions exclusively to transcribe the 45S pre-rRNA gene into a single template that is subsequently processed into mature 18S, 5.8S, and 28S rRNAs. As such, the function of RNA polymerase I is restricted to the nucleolus. In addition to transcribing rRNA, the nucleolus is also involved in the maturation and assembly of ribosomal subunits. Ribosomal protein components are synthesized in the cytoplasm and transported into the nucleolus, where they combine with rRNA to form immature 40S and 60S subunits that are then shuttled out of the nucleus via nuclear pores. Regulation of ribosomal synthesis occurs in part by controlling the number of active rRNA genes. Generally, as cells become more differentiated, their growth slows and they require fewer numbers of ribosomes for protein production. In contrast, malignant cells with high metabolic activity usually have a large number of active rRNA genes and prominent nucleoli. (Choice A) Endonucleases break the phosphodiester bond within the nucleotide chain in DNA and RNA. They are important in DNA repair and RNA splicing and are present throughout the nucleoplasm. (Choice B) DNA polymerases are the primary enzymes responsible for DNA replication and repair. They are found throughout the nucleus. (Choice D) RNA polymerase II synthesizes messenger RNA (mRNA), small nuclear RNA (snRNA), and micro RNA (miRNA). It is the most highly regulated of the 3 RNA polymerases, with its function determined by multiple transcription factors and regulatory processes. (Choice E) RNA polymerase III transcribes DNA to form small RNA molecules such as transfer RNA (tRNA) and 5S rRNA. Unlike other rRNA molecules, the genes for the 5S rRNA are located outside the nucleolus. Educational objective: The nucleolus is the site of ribosomal subunit maturation and assembly. RNA polymerase I functions exclusively within the nucleolus to transcribe the 45S pre-rRNA gene, which codes for most of the ribosomal RNA components (18S, 5.8S, and 28S rRNAs). __________ References: What the nucleolus says to a tumour pathologist. (http://www.ncbi.nlm.nih.gov/pubmed/19178588)

A 58-Year-Old Man Comes to the Office with a Persistent