MCAT® ExamBiological and Biochemical Foundations of Living SystemsEasy
A scientist is studying a newly discovered prokaryotic organism. Analysis of its genetic material reveals a circular double-stranded DNA molecule located in the cytoplasm, along with several smaller, extrachromosomal DNA elements. Which of the following best describes these extrachromosomal DNA elements?
- AMitochondrial DNA
- BCentromeres
- CRibosomal RNA
- DPlasmids
Show answer & explanationAnswer & explanation
Correct answer: D. Plasmids
In prokaryotes, circular double-stranded DNA is typical for the main chromosome. Smaller, extrachromosomal circular DNA molecules that can replicate independently are known as plasmids. These often carry genes for antibiotic resistance or other advantageous traits.
Why the other options are wrong
- A. Mitochondrial DNA is found in eukaryotes, not prokaryotes, and is typically circular but within mitochondria.
- B. Centromeres are regions on eukaryotic chromosomes essential for proper segregation during cell division, not found in prokaryotes as distinct extrachromosomal elements.
- C. Ribosomal RNA (rRNA) is a component of ribosomes, involved in protein synthesis, not an extrachromosomal genetic element.
Bacterial Plasmids
Small, circular, double-stranded DNA molecules distinct from the bacterial chromosome, capable of independent replication.
- Found in prokaryotes.
- Extrachromosomal.
- Carry non-essential genes (e.g., antibiotic resistance).
- Replicate independently.
Memory trick: Plasmids: Extra perks, independent circles!