Cell differentiation occurs when various genes are expressed to a different extent in different cells. what does it mean for a gene to be "turned off," or not expressed?

the correct answer is: Transcription into RNA does not occur for that gene.

of those who start drinking before they're 15 will develop alcohol dependence

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.