The ballistics of gene bombardment
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The ballistics of gene bombardment
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Gene gun bombardment is a method of inserting foreign genetic material into a host cell. In this procedure, a viable host cell is repeatedly shot with spherical gold nanoparticles that have been coated with plasmid DNA. While the collision damages many host cells, a few successfully survive and incorporate the DNA into their genome.
In a typical gene gun, a plate containing a fixed weight of coated gold nanoparticles faces a plate containing many samples of the host cell type. A screen is positioned between the two objects. During bombardment, the plate and gold particles are rapidly accelerated using an explosive charge, which transfers a fixed quantity of energy, E, to the plate and particles over a very short time interval. The plate then collides with the screen, which stops the plate immediately but which allows the gold nanoparticles to continue towards the sample at the impact velocity. Upon reaching the sample, the gold nanoparticles are ballistically scattered as they collide with various tissues and structures, making the final location of the nanoparticles inherently random. Figure 1 shows the basic setup of a gene gun.
Figure 1. A: An explosive charge drives the plate bearing the nanoparticles towards the screen. B: The screen stops the plate, but the nanoparticles pass through. C: The nanoparticles enter the sample and scatter.
[post_title] => The ballistics of gene bombardment
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[question] => Over the entire time interval from when the explosion ends to when the gold particles strike the tissue, which of the following quantities is the smallest?
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[description] => Reason for the Correct Answer:
After the explosion ends, the nanoparticles maintain the same speed until they strike the sample.
Their average speed is thus equal to their maximum instantaneous speed.
The plate has the same initial, maximum instantaneous speed as the nanoparticles.
But the average speed of the plate, including the time after it has stopped at the screen, must be much lower than that of the nanoparticles.
The average speed of the plate is lower than the other three quantities, which are all equal.
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[answers] => Array
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[each_answer] => A. Average speed of the plate
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[1] => Array
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[each_answer] => B. Average speed of the gold nanoparticles
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[each_answer] => C. Maximum instantaneous speed of the plate
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[each_answer] => D. Maximum instantaneous speed of the gold nanoparticles
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => After the explosion (and thus energy transfer) has completely finished, which of these properties of the plate remains both a constant and non-zero value until the plate reaches the screen?
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[answer] => 2
[description] => Reason for the Correct Answer:
The definition of acceleration is a = Δv/Δt.
The slope of a velocity vs. time graph gives the acceleration.
Choose two points on the cheetah’s graph and solve for slope by using (v2-v1)/(t2-t1)
Plug in v₂=20m/s, t₂=4sec and v₁=0m/s, t₁=0sec into the formula for slope to get (20m/s)/(4sec) = 5m/s²
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[0] => Array
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[each_answer] => A. Force
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[each_answer] => B. Velocity
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[each_answer] => C. Work
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[each_answer] => D. Acceleration
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[question] => **Which of the following quantities determines the final velocity with which gold nanoparticles will strike the target?
I. The mass of the plate
II. The total mass of the nanoparticles
III. The mass of a single nanoparticle**
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[answer] => 1
[description] => Reason for the Correct Answer:
The explosion used to propel the plate and particles imparts a fixed amount of kinetic energy.
The velocity of the plate and particles is determined by their kinetic energy.
After colliding with the screen, the plate stops but the gold nanoparticles retain their impact velocity.
Before the screen strike, the nanoparticles and plate together act as a single mass in the equation.
The final velocity of the gold nanoparticles is determined by their total mass and the mass of the plate.
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[each_answer] => A.I and II
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[each_answer] => B. II and III
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[each_answer] => C. I and III
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[each_answer] => D. III only
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[question] => For certain gun geometries, the explosion causes the spherical nanoparticles to spin after the plate collides with the screen and releases them. Which of the following effects would this have on their motion?
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[answer] => 2
[description] => Reason for the Correct Answer:
The energy of the particles can only decrease or remain the same after the explosion ends.
Rotation causes some of the translational kinetic energy to become rotational kinetic energy.
The spinning particles will have decreased impact velocity
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[answers] => Array
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[0] => Array
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[each_answer] => A. An increased impact velocity
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[each_answer] => B. A decreased impact velocity
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[each_answer] => C. A hyperbolic trajectory
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[each_answer] => D. A spiral trajectory
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[question] => Which of the following protocols would improve the consistency with which the DNA plasmid is taken up into the host cells?
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[answer] => 2
[description] => Reason for the Correct Answer:
The consistency is affected by the scattering of the particles off of multiple obstacles as they pass through the tissue.
Lower-energy particles will penetrate less deeply and thus scatter off of fewer obstacles before stopping
A lower explosion energy will result in the particles having lower kinetic energy
Thinner tissue samples and less explosion energy will increase the consistency of the experiment.
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[0] => Array
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[each_answer] => A. Use thicker tissue samples and more explosion energy
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[each_answer] => B. Use thinner tissue samples and less explosion energy
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[each_answer] => C. Increase the number of gold nanoparticles used
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[each_answer] => D. Decrease the number of gold nanoparticles used
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