During DNA replication, the two strands separate as the hydrogen bonds connecting the parent strands are broken by an enzyme called helicase. In the DNA molecule (double strand) complementary bases are joined by hydrogen bonds; that is; Adenine paired to thyamine and guanine to cytosine; during replication the enzyme helicase separates the double helix by breaking the hydrogen bonds between the complementary bases. 
        
             
        
        
        
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The circulatory system transports blood throughout the body through the veins. The circulatory system works with the respiratory system because it transports oxygen through the body as well. It works with the digestive system because it transports waste so it can be disposed.
Explanation:
joe
 
        
                    
             
        
        
        
The nitrogen cycle. Animal and plant waste decomposes and adds nitrogen to the soil, which bacteria convert into a form that plants use to grow. Animals eat the plants, which completes the cycle.
        
             
        
        
        
<span>The theory of Alkaline Vents theorizes that life originated deep under the sea around hydrothermal vents. This could be likely because these vents provide plentiful methane and ammonia. Critics, however, point out that high temperatures are not conducive to organic life.</span>