Dna replication involves several steps. first, the enzyme, helicase, unwinds the hydrogen bonds of the dna double helix. then dna polymerase attaches the complementary nucleotides thereby creating two semi-conservative strand of identical dna. semi-conservative replication of dna refers to the idea that dna molecules need to unwind before duplication can begin. each new dna molecule contains two new single rna strands. the identical two strands of dna molecules run in opposite directions. two identical dna strands contain part old and now part new nucleotides.

The kidney filters potentially toxic substances in the blood, and thus “clears” the blood of those substances. this clearance function is dependent upon and proportional to the diffusion gradient of the substance across filtering capillaries, i.e. if the concentration of the substance is doubled, twice as much will be cleared from each ml of blood that is filtered. suppose that the body produces a constant amount of a substance x per unit of time. the kidneys eliminate substance x at a rate directly proportional to the concentration of the substance and the volume of blood cleared each minute (c): elimination = c × [x], where [x] is the steady-state concentration of substance x. imagine an individual with an initial concentration of x equal to [x]0 who develops kidney disease. her baseline clearance c0 drops to one half of the original (½c0). what is the new steady state concentration of x? (for simplicity, assume that substance x is 100% filtered by the kidney).