Why is down syndrome called trisomy 21

 "Trisomy" basically means that instead of there being two copies of a chromosome there are three (tri=3). (All normal chromosomes have only two copies- they're all pairs, as you will notice if you look at any picture of a chromosome).  

Trisomy + 21 means that the 21st chromosome has an extra copy, which is essentially the definition of Down Syndrome.  

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explanation:

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step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.