Cell division and sexual reproduction

a) Explain how cell division and the sexual reproduction help in genetic variability of organism.
b) How do both the meiosis and sexual reproduction (fertilization) generate offspring which differ genetically from parents?
c) Describe two attributes of the enzymes in catalyzing the chemical reactions and in metabolic pathways like photosynthesis and aerobic cellular respiration.
d) Both photosynthesis and the aerobic cellular respiration are examples of the complex metabolic pathways, containing of several associated chemical reactions that need enzymes to function. Explain two features of enzymes in catalyzing chemical reactions and in metabolic pathways in detail.