The living material that lines the inside of one's cheek is mucous membrane
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They use a hard shell, but not hard enough though because we humans can get through them hehehe
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Nitrogen is in the soil under our feet, in the water we drink, and in the air we breathe. In fact, nitrogen is the most abundant element in Earth's atmosphere: approximately 78% of the atmosphere is nitrogen! Nitrogen is important to all living things, including us. Nitrogen is a naturally occurring element that is essential for growth and reproduction in both plants and animals. It is found in amino acids that make up proteins, in nucleic acids, that comprise the hereditary material and life's blueprint for all cells, and in many other organic and inorganic compounds.
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DNA replication needs to occur because existing cells divide to produce new cells. Each cell needs a full instruction manual to operate properly. So the DNA needs to be copied before cell division so that each new cell receives a full set of instructions!