Biosensors
Biosensors are thin test strips that use capillary action to separate and detect specific DNA sequences, giving color lines as a positive signal.
The biosensor consists of four sections. From the bottom to the top these sections are: the immersion pad, which is made of cellulose and is the part of the biosensor that is placed in the running buffer; the conjugation pad, which is made of glass fiber, where the sample and the colored particles are added; the nitrocellulose membrane, where the sequences desired to be detected by using specific molecule interactions are bound; and the absorption pad, also made of cellulose, which absorbs all of the particles of the solution that were not previously bound .
To use a biosensor, first pipet the sample just above the glass fiber, then quickly pipet the beads onto the fiber, and stand the biosensor in the running buffer for 20 minutes. Since the entire strip is porous (contains many little holes), the solution travels up the strip due to capillary action. Colored lines are created at the points on the strip where specific chemicals have been spotted in a line, resulting in either one or two colored lines.
The top line is the control line – it should always be red unless there is something wrong with the beads. The top line is made with the long “TTTTTTTT” tails on the red beads stick to the AAAAAA that are baked into the biosensor.
The lower line is the experimental line – it will only be red if the bead attaches to the probe which attaches to the DNA which has been combined with a chemical called biotin. The second line is made of streptavidin, and biotin and streptavidin combine very strongly. Only if the bead attaches to the probe which is attached to the DNA sequence which has biotin on the end will we get the second line.
Questions
1. Why does the running buffer move up the biosensor?
2. What is the purpose of the poly A line on the biosensor?
3. What three things have to combine to form the second line? What does that line show you?