A routine microbiology lab at Harbor City Community College turned into an impromptu lesson on genome organization after students discovered that two bacterial cultures—one harmless, one carrying an antibiotic-resistance plasmid—had been swapped, prompting faculty to halt experiments and walk the class through how DNA is packaged in different cells and why it matters.

The mix-up surfaced late Friday when sophomore nursing major Lila Chen compared her results with a classmate’s and found their “control” bacteria behaving like the strain meant to withstand ampicillin.

“We thought we’d messed up the pipetting,” Chen said, standing outside the teaching lab as instructors bagged and labeled plates for disposal. “Then the numbers didn’t make sense. The ‘sensitive’ one kept growing.”

Within an hour, the department paused all sections of the course and began a trace of labels, storage racks and sign-out sheets. No one was injured and no students were exposed to dangerous pathogens, the college said. The bacteria were common teaching strains, handled under standard safety rules.

Still, the incident became a springboard for a broader review session—part damage control, part exam prep—about how organisms store genetic information and how that storage connects to reproduction and biological complexity.

“Prokaryotes have DNA. It’s just not in a nucleus.”

During an emergency review in a nearby lecture hall, instructor Daniel Ortiz held up a marker and wrote a word that several students said they had only memorized without understanding: nucleoid.

“The nucleoid is the DNA-containing region in many bacteria and archaea,” Ortiz told the class, emphasizing that it is not a membrane-bound compartment. “It’s in the cytosol—meaning the watery interior of the cell where many reactions happen.”

Ortiz asked students to describe what they pictured when they heard “genome.” Several said “a nucleus.” He shook his head.

“That’s a common exam trap: ‘prokaryotes have no DNA’ is false,” he said, drawing a circle to represent a bacterial cell and sketching a tangled loop inside it. “They often have a circular chromosome in a nucleoid region.”

Students flipped through lab manuals and compared diagrams to the culture behavior they had seen.

“If it’s not in a nucleus, how does it stay organized?” asked engineering student Mateo Rios.

Ortiz responded by pointing to the consequences of the day’s mistake.

“Organization is why your results change,” he said. “DNA packaging affects replication timing, gene access and, in bacteria, how extra DNA like plasmids can spread.”

Plasmids: the “bonus rings” that can rewrite a lab outcome

As instructors reviewed the plate swap, they focused on the small circle of DNA that likely caused the confusion: a plasmid.

“A plasmid is a small, usually circular piece of DNA separate from the main chromosome,” Ortiz said, adding that plasmids can carry genes that help a cell survive sudden pressures. “In this lab, that pressure was an antibiotic.”

The department later confirmed that the resistant strain carried a plasmid encoding a protein that breaks down ampicillin, allowing colonies to grow where the control should have failed.

“That’s why plasmids matter: adaptability,” said lab coordinator Priya Nand, who oversees inventory and safety logs. “A plasmid can turn a routine environment into a filter—only the cells with that extra DNA make it.”

Nand described plasmids as “portable,” noting that bacteria can sometimes pass them between cells.

“It’s not reproduction like having a baby cell,” she said, “but it changes what traits spread in a population.”

Circular vs. linear chromosomes: what students saw on the gel

To reinforce the point, faculty repeated a demonstration normally scheduled for later in the semester: a DNA extraction followed by an agarose gel run.

On the classroom projector, bands appeared at different positions. Students were asked to infer which samples contained a large chromosome and which contained smaller DNA circles.

“Bacterial chromosomes are often circular,” Ortiz said, gesturing to the gel image. “Eukaryotic chromosomes—like in plants, animals and fungi—are typically linear, packaged with proteins inside a nucleus.”

He paused to correct another misconception that had surfaced in whispered side conversations.

“Second exam trap: ‘all eukaryotes are multicellular’ is false,” he said. “Yeast are eukaryotes. Many protists are eukaryotes. One cell, nucleus and all.”

Reproduction links: binary fission vs. mitosis and meiosis

The lab’s bacterial cultures were multiplying rapidly, a pace that students said felt “out of control” once they realized plates had been swapped.

“That fast doubling is binary fission,” Ortiz said, referring to the common form of prokaryotic reproduction in which one cell copies its chromosome and splits into two. He contrasted it with mitosis, the eukaryotic process that separates duplicated linear chromosomes into two nuclei before a cell divides.

“It’s like the cell is running a more choreographed show,” said biology tutor Elena Park, who leads the campus study sessions. Park described meiosis—used to produce eggs and sperm in many organisms—as “the version with deliberate shuffling,” the source of genetic variety in sexual reproduction.

Students were asked to connect these processes to cell structure.

“If the DNA is in a nucleus, you need a way to divide that nucleus cleanly,” Park told the group, pointing to a model of chromosomes. “If DNA is in a nucleoid, the timing and mechanics are different.”

Mini map: where DNA is found (as students annotated it)

During the review, Ortiz asked students to build a “location map” in their notes. Several compared it to labeling a floor plan.

DNA LOCATION MINI MAP (labeled)

1) Prokaryotic cell (bacteria/archaea)

• Nucleoid region (in cytosol): main chromosome (often circular)
• Plasmids (in cytosol): extra small DNA circles; may carry traits like drug resistance
• No nucleus (no membrane around the nucleoid)

2) Eukaryotic cell (animal/plant/fungus/protist)

• Nucleus: main chromosomes (typically linear)
• Organelle genomes (extension point):
  • Mitochondria: own DNA (organelle = a specialized structure inside a cell)
  • Chloroplasts (plants/algae): own DNA
• Cytosol: where organelles sit and many reactions occur

Ortiz defined organelle in passing as “a specialized structure inside a eukaryotic cell,” then tapped the mitochondria slide.

“Notice how this complicates the simple question of ‘where is DNA?’” he said. “It’s not only one place in eukaryotes.”

Compare/contrast chart posted after the incident

By Monday morning, the department had posted a one-page handout outside the lab, summarizing the concepts students said were most likely to be tested.

The exam traps, in students’ own words

As the session ended, Park asked the room to volunteer “statements that sound true but aren’t.” Students called out examples while Ortiz wrote them on the board and underlined the corrections.

• “Prokaryotes have no DNA.” Chen said she had heard that in high school. Ortiz replied: “They do; it’s typically in a nucleoid.”
• “All eukaryotes are multicellular.” A student in the back offered it as a guess. Park countered by citing yeast and single-celled protists.
• “Plasmids are the same as chromosomes.” Nand noted that plasmids are separate, often smaller, and can be gained or lost, changing traits quickly.
• “Only the nucleus has DNA.” Ortiz pointed again to mitochondria and chloroplasts as organelles with their own genomes.

What changed in the lab after the swap

The college said it has added a second-person verification step for labeling cultures and will separate resistant and nonresistant strains into different storage bins.

Ortiz said the disruption cost the class a day of data but may have prevented a longer stretch of confusion.

“In a weird way, the cells did what they always do,” he said as students filed out, comparing notes. “They followed their DNA—where it sits, how it’s organized, and what extra pieces they’re carrying. Our job is to learn to read that story correctly.”