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A diagram of the biological half adder in culture is shown above, depicting the possible outcomes for each combination of inputs. Tetracycline was used as a second input since it inactivates the repressor TetR, thereby derepressing expression at the PtetR locus.ĭepending on the result of the calculation, the bacteria will produce either red fluorescent protein (RFP), green fluorescent protein (GFP), or neither. The red light will be detected by Cph8, the previously used Cph1/EnvZ fusion protein. We chose to use light as an input because it does not require altering the media, it can be turned on and off rapidly, and it has extremely high resolution and accuracy. For the second input, the absence of tetracycline represents a 0 and the presence represents a 1. For the first input, the presence of red light represents a 0 and the absence represents a 1. This is the type of approach used to construct our biological adder.Īs a half-adder, our device requires two binary inputs the possible 1 and 0 values of these inputs are represented by the presence and absence of two stimuli. If the inputs A and B are on and together can turn on expression of gene G, the expressed protein X represents the output of an AND gate. By default, the gene is not expressed (to be precise, there is a low basal level of expression that we can disregard). Whether or not the gene is turned 'on' or 'off' depends on the logic gate implemented.įor two inputs, the "presence" or "absence" of input A and the "presence" or "absence" of protein B (both representing 0 or 1 as inputs) would result in an output of either the "presence" or "absence" of protein X (where "presence" and "absence" should be read as high and low intensity/concentration, respectively).įor example, consider a regulated gene G that codes for protein X. The output of a logical operation is the protein product of a certain gene the inputs are external stimuli that cause allosteric changes in the conformation of proteins that can induce or inhibit transcription by acting on promoter elements. Our project focuses on using DNA as logic gates. The process is illustrated in Figures 4 and 5. Given two binary inputs A and B, their sum can be computed by passing these inputs through various logic gates. A list of relevant logic gates is as follows:įor the following explanations, A and B represent statements that can be either true or false. Also, it is often useful to think of of 0 as representing "false", and 1 as representing "true". Usually the inputs and outputs are binary, meaning they each have a value of either 0 or 1.
#FULL ADDER VS HALF ADDER TRUTH TABLE FULL#
See Figure 2 for a full list of possibilities.Ī logic gate performs a logical operation on one or more logic inputs and produces a logic output. Such a chain of full-adders is called a ripple carry adder. The full-adder will output the resulting sum bit and carry bit, and the process will continue until all the digits have been added. Each full-adder is responsible for adding one pair of corresponding digits from the two numbers to be added, and it must add to that the carry bit from the previous full-adder. A full list of the possibilities is shown in Figure 1.Ī full-adder is merely a half-adder that accepts an extra input namely, the carry bit from another full-adder. To add 1 + 1, the two inputs would each be 1, the sum bit would be 0, and the carry bit would be 1. In terms of implementation, a half-adder accepts two inputs (the two digits to be summed) and returns two outputs (the "sum bit" and the "carry bit"). A half-adder can only add together two single digits, whereas a full-adder is needed to add two numbers consisting of more than one digit.įor example, a half-adder could perform the addition 1 + 0 = 1, or 1 + 1 = 10, but it would take a full-adder to be able to perform 1100101 + 100101. Any construct designed to add two numbers will either be a half-adder or a full-adder.