Biophysics Questions and Answers – Centrifugation

This set of Biophysics Multiple Choice Questions & Answers (MCQs) focuses on “Centrifugation”.

1. Which of the following is not a criteria of separation through centrifugation?
a) Physical properties of sample
b) The viscosity of the medium
c) Speed of the centrifuge rotor
d) Size of the sample to be separated
View Answer

Answer: a
Explanation: Through centrifugation, we rotate the sample at a high speed, so that molecule gets separated on the basis of their size and also on the basis of their density. Sometimes we can separate two molecules on the basis of the viscosity of the medium too. So here the option “Physical properties of sample” is the wrong information that is Physical property does not matter anything in the separation of two molecules through centrifugation.

2. Which technique is preferably used for the separation of labeled and unlabeled DNA molecules?
a) CsCl gradient centrifugation
b) Sucrose gradient centrifugation
c) Discontinuous density gradient centrifugation
d) Gel gradient centrifugation
View Answer

Answer: a
Explanation: Differentiation between labeled and unlevelled DNA is very tough because of their very low molecular weight difference. This differentiation is only possible through a CsCl gradient because of the very high density of the medium. The rest of the two is not highly viscous to make differentiate between labeled and unlabeled DNA

3. We mostly use analytical ultracentrifuge to separate large proteins.
a) True
b) False
View Answer

Answer: b
Explanation: Ultra-centrifuge is a very sensitive machine that is rotated at a very high speed. So it is only used if the molecular weight of the molecule to be separated is very less or we need very efficient data. So it is false because the large protein can be easily centrifuged by common low centrifugation.

4. What is the speed of a common ultra-centrifugation machine in rpm (rotation per minute)?
a) 20000 g
b) 40000 g
c) 60000 g
d) 80000 g
View Answer

Answer: d
Explanation: 80k is the speed of ultra-centrifuge. This speed is achieved by the low air pressure inside the instrument. The rest of the options are for high speeds centrifugation.

5. What is the correct centrifugal force required for ribosome separation from a mixture of cell components?
a) 150000 g
b) 80000 g
c) 60000 g
d) 40000 g
View Answer

Answer: a
Explanation: Ribosome is a very low molecular weight organelle. This even cannot be using the common ultra-centrifugation technique. So researchers prefer to use very high ultra-centrifugation to separate ribosome by rotating at a speed of 150 k.

6. What is the minimum required centrifugal force to separate DNA from a mixture of cell components?
a) 10000 g
b) 20000 g
c) 40000 g
d) 60000 g
View Answer

Answer: a
Explanation: DNA is the most commonly available part compared to other cell organelles present in the cell. So just to separate DNA one can centrifuge the cell suspension at low centrifugation that is 10000 rpm.

7. Suppose you are given a sample containing phage virus. At getting a pallet of the virus at what speed will you rotate the ultra-centrifuge machine? [Here S is Svedberg unit]
a) 25 S
b) 100 S
c) 200 S
d) 1000 S
View Answer

Answer: b
Explanation: Bacteriophage is a very tiny thing having a very less molecular weight. So it is rotated at a very high Svedberg speed. 0 S – 200 s is used for different cellular organelle separation.

8. Which of the following procedure requires a vacuum for better functionality?
a) Common centrifugation
b) High-speed centrifugation
c) Preparative Ultra-centrifugation
d) Analytical Ultra-centrifugation
View Answer

Answer: d
Explanation: Among the above options Analytical Ultra-centrifugation requires a very high speed of rotation. This can be achieved by creating a vacuum. Moreover, the vacuum reduces the frictional heat generation by friction with air while rotation, thereby it indirectly protects the instrument.

9. What is the meaning of Svedberg unit?
a) 10-11 sec
b) 10-12 sec
c) 10-13 sec
d) 10-14 sec
View Answer

Answer: c
Explanation: This unit is named after the scientist Svedberg. This unit is basically a way of representation of the weight of centrifuging molecules in terms of the velocity of sedimentation or the time taken to be sediment. More S means more time taken to be sediment that means the molecule has a very high molecular weight.

10. Among these options which best describes the settling and sedimentation process?
a) Settling is the period when the suspended molecule is falling off and sedimentation is the completion of settling
b) Sedimentation is related to centrifugation that is artificial but settling in natural phenomenon
c) Sedimentation and settling are both means the same phenomenon
d) Sedimentation is a fast process whereas settling is a slow process
View Answer

Answer: a
Explanation: These two terms confuses a lot. The actual meaning is that Settling is the period before the molecule gets completely sediment. But the sedimentation means the molecule is no more settling.

11. Among the following options which best describe the Svedberg unit S? Where r is the radius of the molecule, w is angular momentum, m is the mass of the molecule to be centrifuged.
a) S = v r/w2
b) S = v/w2rm
c) S = v/w2r
d) S = v m/w2r
View Answer

Answer: c
Explanation: Firstly, the Svedberg unit does not depend on the mass of that molecule. This represents the time taken by the molecule to be sediment. Among the above options, only the option “S = v/w2r” doesn’t contain a mass factor and also signifies the proper representation of the unit, so it’s the correct answer.

12. Suppose you have a solution mixture which contains proteins, N15 labeled DNA and N14 labeled DNA, how will you separate this mixture using centrifugation method?
a) Low-speed centrifugation followed by high-speed centrifugation
b) Low-speed centrifugation followed by Cesium chloride gradient centrifugation
c) Low-speed centrifugation followed by ultra-centrifugation
d) Low-speed centrifugation followed by high-speed centrifugation in sucrose gradient
View Answer

Answer: b
Explanation: Initially low-speed centrifugation will be performed because through that the proteins will be separated. Secondly to separate radioactively labelled DNA, one needs to perform density gradient centrifugation with CsCl gradient because it is a very sensitive gradient to differentiate minute molecular weight differentiate.

13. Suppose during centrifugation, a molecule with 1000 Dalton mass is rotating at a π/30 average angular velocity with 20 cm radius of rotation. What will be the centrifugal force that the molecule will experience?
a) 200 Neuton
b) 500 Dyn
c) 219 Dyn
d) 400 Dyn
View Answer

Answer: c
Explanation: If M denotes the mass of that particle r, ω means the average angular velocity and r means the radius of rotation then the centrifugal force will be
F = Mω2r
That is F = 1000*(π/30)2*20 = 219 Dyn
As everything is taken is CGS unit so the unit of the result will be in the CGS unit of force that is Dyn.

14. When will you perform density gradient centrifugation in the following cases?
a) Two molecules with a high molecular weight difference
b) Two molecules with a very low molecular weight difference
c) Two molecules with high shape difference
d) Two molecules with very low shape difference
View Answer

Answer: b
Explanation: Density gradient centrifugation is used to separate molecules with very low molecular weight difference because the density gradient is very sensitive if the molecules have a very high molecular weight difference than the centrifuge tube must have been very long which is impossible. Secondly, it is costly, so for large molecular weight difference, we use normal centrifugation technique rather than a density gradient.

15. The following picture shows that between two hypothetical proteins, protein A denoted by red color and protein B is denoted by green color. When they normally centrifuged at 15k rpm in a sucrose gradient, they give the following patterns of the band. What do you infer from this image?
a) Protein A and B have a different shape
b) Protein A has a higher molecular weight than protein B
c) The density of Protein A is higher than Protein B
d) Protein B has a higher molecular weight than protein A
View Answer

Answer: b
Explanation: In a sucrose gradient, the more bottom you go into a tube higher the concentration of sucrose is. So A has a higher density than B. Most importantly, yes, A has a higher molecular weight than B too, as it could travel more distance than B in centrifugation.

Sanfoundry Global Education & Learning Series – Biophysics.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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