Competent Host

Competence is the ability to take DNA and transformation is the process of vector entry. This pair is the core of the topic.

Natural competence is the natural state or ability of certain bacterial cells to take up free external DNA from their surroundings. This DNA uptake may subsequently lead to genetic transformation. Protein digestion is a different cellular process and does not define competence, so option B is incorrect. Exam tip: link competence with the ability to take up DNA during transformation.

Artificial competence is made by lab methods such as chemical treatment or electroporation. It is common in cloning.

Calcium chloride is a common chemical treatment for E coli competence. It can make DNA uptake easier.

Cold calcium chloride treatment makes bacterial cells competent for plasmid uptake. Heat shock can later increase entry.

Heat shock can help cause temporary permeability change. This can allow plasmid DNA to enter cells.

Electroporation uses an electric pulse to make cells temporarily permeable. Chemical methods commonly use salts and heat shock.

Actively growing cells can respond better during competence preparation. Therefore culture stage is important.

In an overgrown culture cells may be stressed or in stationary phase. This can reduce transformation efficiency.

The direct answer is A: competent cells can be fragile, so viability may decrease. Competent cells have been prepared to take up external DNA, and that preparation can make their membranes or cell envelopes sensitive. During transformation, gentle pipetting and mixing help keep the cells alive and preserve their ability to accept DNA. Harsh mixing, repeated strong vortexing, unsuitable heat, or rough handling can damage cells; fewer cells survive and fewer may remain competent. A is correct. B is absurd because cells are living biological structures, not stones. C is wrong because competent cells are specifically prepared to take up DNA; “never take DNA” contradicts the definition. D is wrong because competent cells are not always heat resistant; excessive heat can reduce viability and competence. Handling must follow the particular protocol, since some protocols use a controlled heat shock, but uncontrolled or excessive heat is harmful. Memory cue: competent cells are “ready but delicate.”

Ice helps preserve stability and viability of competent cells. The heat shock step is kept controlled.

Excess DNA or impurities can stress cells. An optimal amount is better for transformation efficiency.

In transformation, competent cells must take up the plasmid DNA. Excess salts, proteins, phenol, or other contaminants in a DNA preparation can stress cells and reduce DNA uptake. Therefore, clean plasmid DNA can improve transformation efficiency and cell survival. Option B is incorrect because contaminants do not always increase efficiency; they often reduce it. Exam tip: link DNA purity in transformation with uptake and cell viability.

In simple cloning the goal is to make copies of insert DNA. An expression vector has a different purpose.

Host strains can differ in mutations and properties. Therefore strain choice is important in cloning and protein expression.

A recombination-deficient host can reduce unwanted rearrangement. This can keep the cloned insert stable.

Some host mutations can reduce plasmid DNA degradation. This can improve plasmid preparation quality.

Protease-deficient strains can reduce recombinant protein degradation. This is useful in expression experiments.

Strains like DH5 alpha are common for plasmid cloning and maintenance. Understand them separately from expression strains.

BL21 type strains are commonly used for protein expression. Distinguish them from cloning-only hosts.

The direct answer is D: plasmid remains maintained in the host. A plasmid is a small, usually circular DNA molecule that is separate from the main chromosome and can carry a useful gene. In a competent host, stability means that after the plasmid enters the cell, it is copied or retained and passed to daughter cells instead of being rapidly lost. The reasoning is: plasmid enters the host; the host keeps it; the plasmid is replicated or distributed during cell division; therefore the plasmid remains present. Selection pressure, such as an antibiotic, gives plasmid-containing cells a growth advantage and helps remove cells that have lost the plasmid, although stability also depends on the vector and host. Option A is wrong because a plasmid is DNA, not water. Option B is the opposite of stability: constant disappearance means instability. Option C is wrong because plasmids do not digest proteins; they may carry genes that encode proteins. Option D is correct because maintenance in the host is exactly the meaning of plasmid stability. Memory cue: stable plasmid means “stays inside and is retained.”

Without selection pressure cells may lose the plasmid. Marker-based selection helps plasmid maintenance.

A freezer stock preserves a useful strain for long term. This allows the same host to be used in future experiments.

Glycerol stock helps protect bacteria from freezing damage. It is a common method for strain preservation.

Each host has a suitable growth temperature. Wrong temperature can affect growth or plasmid stability.