Conditioned medium is medium that has
Answer: C
The factors secreted into it can support other cultures, which is a simple way to supply signals that are difficult to identify individually. It shares the drawback of serum in being undefined. Its usefulness lies in supplying factors nobody has yet identified.
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Consider the following statements about culture conditions.
1. Most mammalian cells are cultured at 37 degrees Celsius.
2. A carbon dioxide incubator maintains the pH of bicarbonate-buffered medium.
3. Cells grow equally well at room temperature.
Answer: D
The first two are correct. The third is wrong for mammalian cells, whose growth slows markedly below body temperature, though insect and fish lines are cultured cooler. The temperature follows the biology of the source organism.
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State whether the following is true or false.
All cultured cells are grown at 37 degrees Celsius.
Answer: A
Mammalian cells are, but insect cells are typically grown near 27 degrees and fish cells cooler still. The temperature chosen reflects the organism the cells came from rather than any universal rule. A line's optimum is inherited from the animal it was taken from.
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The single most important habit in avoiding contamination is
Answer: B
Contamination almost always enters through a lapse in handling rather than through the materials themselves. No additive compensates for careless work at the hood. Technique is learned by practice and is the real safeguard.
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State whether the following is true or false.
A contaminated culture can usually be rescued and should be treated rather than discarded.
Answer: C
Correct answer: False
A contaminated culture is discarded, and the incubator and hood are decontaminated. Antibiotics may suppress an infection without eliminating it, leaving a low-level contamination that alters the cells’ behaviour invisibly and seeds every other culture handled afterwards. Since a fresh vial can be thawed from the frozen stock, keeping a contaminated flask risks a great deal to save very little.
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The total number of live cells in a culture is counted using the frypan blue exclusion assay and is found to be 2.7 × 106 cells/ml. The culture is diluted 1 : 27 and then 100 μl seeded per well into a 96 well plate. What is the final cell density per well ?
MH SET - 2018, SET Exam - 2018,
Answer: D
The final cell density is \(1 \times 10^{4}\) cells per well.
Given 2.7 × 106 live cells/ml, a 1 : 27 dilution, and 100 μl (0.1 ml) seeded per well.
\[ \begin{aligned} \frac{2.7 \times 10^{6}}{27} &= 1 \times 10^{5}\ \text{per ml} \\ 10^{5} \times 0.1 &= 1 \times 10^{4}\ \text{per well} \end{aligned} \]
\(1 \times 10^{5}\) is the concentration per millilitre, the answer if the 100 μl volume is forgotten. \(2.7 \times 10^{5}\) and \(2.7 \times 10^{4}\) skip the dilution. (The paper prints 'frypan blue'; the dye is trypan blue, which enters only dead cells.)
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You are looking at a specimen with a compound microscope. The ocular lens has a magnification of 8X and the objective lens has a magnification of 2X. What is the total magnification ?
MH SET - 2020, SET Exam - 2020,
Answer: D
The total magnification is 16X.
In a compound microscope the objective forms a magnified image, and the ocular (eyepiece) magnifies that image again, so the two magnifications multiply:
\[ 8 \times 2 = 16 \]
10X adds the two powers instead of multiplying them, and 8X and 2X are the powers of the single lenses on their own.
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Cryopreservation is a process of :
MH SET - 2021, SET Exam - 2021,
Answer: A
Cryopreservation is ex situ in vitro conservation.
In cryopreservation, seeds, embryos, pollen, shoot tips or cell cultures are stored at very low temperature, usually in liquid nitrogen at −196°C, where metabolism effectively stops. It takes material away from its natural habitat, so it is ex situ, and it keeps tissues and cells in culture vessels and vials rather than as whole growing plants, so it is in vitro.
Ex situ ex vitro conservation means botanical gardens, field gene banks and zoos. In situ conservation keeps organisms in their habitats, such as national parks and biosphere reserves, and is never in vitro.
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