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A DNA molecule is composed of two strands. The structure of DNA is a double-helix polymer, a spiral consisting of two DNA strands twisted around each other Each strand has a backbone made of alternating sugar the four nitrogenous bases in a dna are  adenine, cytosine, guanine, and thymine,.Complementary pairs of four nitrogenous bases in DNA adenine and thymine, cytosine and guanine,  By a dotted line are specified hydrogen bonds in these pairs, adjacent nucleotides are linked by a phosphodiester bonds

Explanation:

i hope this is right it took me awhile to figure this out

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.