Photosynthesis requires sunlight as a reactant and hence can be considered a light reaction. What you are referring to as 'dark reactions' are cellular respiration reactions and they do take place throughout the life of a plant. So, in reality they are not just dark reactions (since they take place 24 x 7).
Photosynthesis can be chemically represented by the following reaction:
`6CO_2 + 6H_2O + sunlight -> C_6H_12O_6 + 6O_2`
Here, carbon dioxide and water are converted (in the presence of sunlight) to glucose and oxygen.
Cellular respiration is complementary to photosynthesis and uses the products of photosynthesis (that is, glucose and oxygen) as reactants. Cellular respiration can be represented as:
`C_6H_12O_6 + 6O_2 -> 6CO_2 + 6H_2O + ATP`
Here, glucose and oxygen (both generated from photosynthesis) are used and carbon dioxide and water are generated, along with energy molecules (ATP).
Hope this helps.