Question 1

What does it mean for E. coli cells to be competent?

Accepted Answer

Competent E. coli cells are capable of taking in DNA molecules from their environment. Chemical competence is induced by incubating cells in a calcium chloride solution, which makes the cell membrane permeable to DNA. This treatment allows negatively-charged DNA to overcome the repulsion from the negatively-charged bacterial cell surface.

Question 2

Why is a heat shock step necessary during bacterial transformation?

Accepted Answer

The heat shock at 42 degrees Celsius for 45 seconds drives DNA across the bacterial cell wall and membranes into competent cells. This temperature change, followed by rapid cooling on ice, favors DNA uptake by creating a temperature imbalance that enhances the transformation process and increases efficiency.

Question 3

How do antibiotic resistance genes help identify successfully transformed bacteria?

Accepted Answer

Plasmids like pUC19 carry antibiotic resistance genes that serve as selectable markers. When transformed bacteria are plated on selective media containing ampicillin, only cells that have taken up the plasmid survive and express the resistance gene product. Non-transformed cells cannot survive on antibiotic-containing media.

Question 4

What is the purpose of calculating transformation efficiency in this experiment?

Accepted Answer

Transformation efficiency measures how many bacterial cells successfully take up plasmid DNA per microgram of DNA used. It is calculated by dividing the colony-forming units (CFUs) counted on selective plates by the amount of DNA plated and the dilution factor, providing a quantitative assessment of the transformation procedure's success.

Question 5

Why must E. coli cells be in mid-exponential growth phase before transformation?

Accepted Answer

Bacterial cells in mid-exponential growth phase are physiologically robust enough to withstand the heat shock treatment during transformation. Cells in other growth stages are too sensitive to the temperature stress, resulting in loss of viability and significantly decreased transformation efficiency.

Question 6

How does restriction enzyme digestion confirm successful plasmid transformation?

Accepted Answer

After isolating plasmid DNA from transformed colonies, a restriction enzyme digest cuts the DNA at predictable locations. When digested pUC19 is run on an agarose gel, it produces a single band at 2,686 base pairs, confirming the expected plasmid size and verifying successful transformation of the desired DNA.

Question 7

What different conformational states can circular plasmid DNA adopt in a gel?

Accepted Answer

Uncut circular plasmid DNA can exist in multiple conformational states: supercoiled, open circular, or more linear forms. Each conformation migrates through the gel at different rates, causing the undigested plasmid lane to display two or three bands of varying brightness rather than a single band.

טרנספורמציה של תאי E. coli באמצעות הליך סידן כלוריד מותאם - חידון הערכה