Researchers analyzing eukaryotic genome structure and function perform an experiment to extract DNA from exocrine pancreatic cells. During the purification process, they isolate small circular DNA molecules that resemble a bacterial chromosome. Further analysis shows that these molecules code for proteins, transfer RNA, and ribosomal RNA. From which of the following cellular structures did these DNA molecules most likely originate?

Question

Researchers analyzing eukaryotic genome structure and function perform an experiment to extract DNA from exocrine pancreatic cells.  During the purification process, they isolate small circular DNA molecules that resemble a bacterial chromosome.  Further analysis shows that these molecules code for proteins, transfer RNA, and ribosomal RNA.  From which of the following cellular structures did these DNA molecules most likely originate?

Quizplus · Accepted Answer

11ec160b_36b3_6389_9249_1163d3b75581_MD0001_00 Nuclear chromosomes contain most of the DNA found in human cells. However, mitochondria also contain their own DNA called mitochondrial DNA (mtDNA). This DNA exists as a small circular chromosome with a slightly different genetic code than that of nuclear DNA, consistent with the endosymbiotic theory that mitochondria originated as prokaryotic cells that were later engulfed by ancient eukaryotes. Over time, most of the genes coding for mitochondrial proteins have migrated to nuclear DNA. However, mtDNA still codes for about 14 proteins (some involved in oxidative metabolic pathways) and the ribosomal and transfer RNA needed for mitochondrial protein synthesis. Each mitochondrion contains 1-10 copies of maternally derived mtDNA. As a result, diseases arising from mutations in mtDNA are transmitted from the mother to all of her offspring. Mitochondria can be identified on electron microscopy by their characteristic double membrane and wavy cristae. (Choice A) The rough endoplasmic reticulum has a stippled appearance secondary to the presence of numerous ribosomes bound to its membranes. These ribosomes are involved in the synthesis of integral membrane proteins and proteins destined for export or packaging into granules or organelles. (Choice B) The dark region identified within the nucleus is the nucleolus, the site of synthesis and assembly of eukaryotic ribosomal components. There is no lipid membrane separating the nucleolus from the rest of the nucleus. (Choice C) The lighter "electron-lucent" regions within the nucleus signify euchromatin (unpackaged DNA being actively transcribed). (Choice E) This electron-dense membrane-bound spherical structure represents an exocrine granule containing enzymes and other proteins packaged for secretion. Educational objective: Mitochondrial DNA (mtDNA) is the most common non-nuclear DNA found in eukaryotic cells. It resembles prokaryotic DNA and is maternally derived. Mutations involving mtDNA (or nuclear DNA that codes for mitochondrial proteins) can cause a variety of mitochondrial disorders, including Leigh syndrome and MELAS.

Researchers Analyzing Eukaryotic Genome Structure and Function Perform an Experiment