Question

a) What is the total net charge of double-stranded DNA that is 500 basepairs (bp) in length? b) In agarose gel electrophoresis, two parallel and oppositely charged electrodes are situated at opposite ends of the gel. The surrounding solution is a “buffer”, an aqueous salt solution maintained at constant pH. The gel is made in the same buffer, with a small concentration of agarose. Because the concentration of agarose is small , everything between the electrodes can be considered to have the same dielectric constant. In the schematic below, sketch the electric field lines within the apparatus, taking care to note their direction. c) In what direction would the DNA feel a force and move spontaneously because of this electric field? d) Generally, the user “runs a gel” by applying a constant (DC) voltage between the two electrodes. A common operating voltage is 90V. Under this potential difference, the current flowing between the two electrodes is measured to be 60 mA. What is the resistance of the gel + buffer solution between the electrodes? Show your work. e) If the distance between the electrodes were doubled and the voltage were held constant, what would you expect to happen to the current? Justify your answer. f) What is the electric power needed to run the experiment in part (d)? g) If the “gel runs” (voltage is applied) for 1 hour, how much energy is used?

Answer 1

Hi,

Hope you are doing well.

(a)

Net charge of a double-stranded DNA is Negative charge. Phosphate groups in the backbone of DNA have negatively charged oxygen molecules which gives the backbone of DNA an overall negative charge and thus DNA a total negative charge.

(b)

We know that electric field direction will be from positive electrode (Anode) to negative electrode (Cathode) as shown below.

Cathode Direction of electric field +) Anode

(c)

Since DNA is negatively charged, it will have a tendency to move towards the Anode (positive electrode). So the DNA Fragments move towards Anode through agarose gel/gel medium.

(d)

According to Ohm's Law, We know that, Resistance,

$R=rac{V}{I}Omega$

Where,

'' is the Resistance of gel. (ohms)

'' is the Potential difference between electrodes. (Volts)

'' is the Current flowing through gel/medium.(Ampere)

Given that,

Potential difference between electrodes, $V=90mathbf{V}$

Current flowing through gel/medium,
60mA= 60 x 10-3A

(e)

We know that,

pl

Where,

'' is the Resistance of gel. (ohms)

'' is the Resistivity of gel. (ohm-meter) [Constant]

'' is the Area of gel. (ohms)

From the above equation, It is clear that, When distance is doubled, Resistance of medium will be doubled.

New resistance, $R_{1}$

According to Ohm's Law, We know that, Current,

$I=rac{V}{R}mathbf{A}$

Where,

'' is the Current flowing through gel/medium.(Ampere)

'' is the Resistance of gel. (ohms)

'' is the Potential difference between electrodes. (Volts)

Given that,

Potential difference between electrodes, $V=90mathbf{V}$

New Resistance of gel, $R_{1}=3000Omega$

90V 30 × 10-3A = 30mA R1 300012

ie; Current flowing between electrode is halved, when the distance between electrodes is doubled.

(f)

We know that,

Power consumed,

$P=VI$

Where,

'' is the Power consumed.(Watts)

'' is the Current flowing through gel/medium.(Ampere)

'' is the Potential difference between electrodes. (Volts)

Given that,

Potential difference between electrodes, $V=90mathbf{V}$

Current flowing through gel/medium,
60mA= 60 x 10-3A

$mathbf{ie;}P=5.4mathbf{W}$

(g)

We know that,

Energy used,

$E=VIt$

Where,

'' is the Energy used. (Joules)

'' is the Current flowing through gel/medium.(Ampere)

'' is the Potential difference between electrodes. (Volts)

'' is the time consumed .(seconds)

Given that,

Potential difference between electrodes, $V=90mathbf{V}$

Current flowing through gel/medium,
60mA= 60 x 10-3A

Time consumed,

$mathbf{ie;}E=19440mathbf{J}$

Hope this helped for your studies. Keep learning. Have a good day.

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