Define crossing over. in which stage does it occur?

crossing over is take place in methapace

in 1972 the fluid—mosaic membrane model of membrane structure was proposed based on thermodynamic principals of organization of membrane lipids and proteins and available evidence of asymmetry and lateral mobility within the membrane matrix [s. j. singer and g. l. nicolson, science 175 (1972) 720–731].

answer; ///(b) it is composed of four protein chains and four heme groups;