Cell Culture Techniques Questions and Answers

Q: A carbon dioxide incubator is used because the medium is buffered by

Correct answer: a bicarbonate system needing carbon dioxideMost media are buffered by bicarbonate, and that buffer only holds its pH in equilibrium with a defined partial pressure of carbon dioxide - typically five per cent. In ordinary air the dissolved carbon dioxide escapes and the medium turns alkaline, which the phenol red indicator shows as a shift towards purple. Media buffered with HEPES instead can be handled outside an incubator for the same reason.

11.

Assertion (A): The origin of the HeLa cell line is discussed in research ethics courses.
Reason (R): The cells were taken and distributed without the patient's knowledge or consent.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

Answer: D

Both are true and the reason explains why the case is taught: it illustrates in one example the questions of consent, of benefit and of what a patient is owed when their tissue proves valuable. The Lacks family reached an agreement over access to the genome in 2013.

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12.

Culture medium for animal cells typically contains

A.amino acids, vitamins, salts and glucose
B.only distilled water together with glucose
C.agar and a very small amount of salt
D.nothing except a buffered saline solution

Answer: A

Serum is usually added as well, supplying growth factors and attachment proteins. Its composition is poorly defined, which is why serum-free formulations are preferred for reproducible work. Defined media remove one uncontrolled variable from every experiment.

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13.

Foetal bovine serum is added to culture medium chiefly to supply

A.antibiotics to prevent contamination
B.growth factors and attachment proteins
C.the oxygen the cells will respire
D.the plastic surface for cells to grow on

Answer: B

It is an undefined mixture that varies between batches, which is why laboratories test each batch and why defined serum-free media are increasingly used. Batch variation can change a result without anything else altering.

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14.

Assertion (A): Serum-free media are preferred for experiments that must be reproducible.
Reason (R): The composition of animal serum varies from batch to batch.

A.A is true but R is false
B.A is false but R is true
C.Both A and R are true, and R is the correct explanation of A
D.Both A and R are true, but R is not the correct explanation of A

Answer: C

Both are true and the reason gives the difficulty: an undefined and variable ingredient makes it hard to attribute a result to the treatment rather than to the medium, so a defined formulation removes one source of doubt.

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15.

A carbon dioxide incubator is used because the medium is buffered by

A.a phosphate system needing no added gas at all
B.the plastic material of the culture vessel
C.the cultured cells themselves as they respire
D.a bicarbonate system needing carbon dioxide

Answer: D

Correct answer: a bicarbonate system needing carbon dioxide

Most media are buffered by bicarbonate, and that buffer only holds its pH in equilibrium with a defined partial pressure of carbon dioxide - typically five per cent. In ordinary air the dissolved carbon dioxide escapes and the medium turns alkaline, which the phenol red indicator shows as a shift towards purple. Media buffered with HEPES instead can be handled outside an incubator for the same reason.

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16.

Cells are cultured in a laminar flow hood chiefly to

A.keep the work area free of airborne contamination
B.supply the cultured cells with carbon dioxide gas
C.keep the medium at exactly the correct temperature
D.provide the light that the cells require in order to grow

Answer: A

A filtered stream of air moving across the work surface keeps microorganisms away from the open vessels. Temperature and gas are controlled by the incubator instead. The hood protects the culture, and in some designs the worker as well.

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17.

Contamination of a culture by mycoplasma is particularly troublesome because it

A.can be seen easily under a microscope
B.is invisible and often goes undetected
C.kills every cell within a few hours
D.turns the medium bright red at once

Answer: B

These very small bacteria cause no turbidity and are not visible in an ordinary microscope, yet they alter cell metabolism and behaviour. Regular specific testing is the only reliable way to find them. Many published results have been traced to undetected mycoplasma.

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18.

State whether the following is true or false.
Mycoplasma contamination is easily spotted by cloudiness in the medium.

A.False only at small scales
B.True
C.False
D.True only at large scales

Answer: C

The organisms are far too small to make the medium turbid, so a heavily contaminated culture can look entirely normal. This is exactly what makes them a serious problem for reproducible work. Absence of visible signs is not evidence of a clean culture.

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19.

Assertion (A): Laboratories test their cultures for mycoplasma routinely rather than waiting for signs of trouble.
Reason (R): Mycoplasma causes no visible change in the appearance of a culture.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the necessity. Mycoplasma are too small to see by light microscopy, do not cloud the medium and do not kill the cells, so an infected culture looks entirely normal - while its metabolism, gene expression and drug responses are all altered. Nothing about routine observation would reveal it, so periodic testing by PCR or staining is the only defence.

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20.

A phenol red indicator in culture medium changes colour to show

A.a change in the pH of the medium
B.the number of cells currently present
C.the concentration of dissolved oxygen
D.the age of the culture vessel itself

Answer: A

Yellow indicates acidity, usually from cells respiring heavily or from bacterial contamination, and purple indicates alkalinity from loss of carbon dioxide. It gives an immediate visual check without opening the flask. A quick glance at the colour tells an experienced worker a great deal.

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