During de novo pyrimidine synthesis, the pyrimidine ring is assembled and then joined to phosphoribosyl pyrophosphate (PRPP), forming orotidine monophosphate. Orotidine monophosphate is next converted to uridylate, or UMP. UMP can then be converted to another pyrimidine ribonucleotide, CTP. Identify the source of each atom in the pyrimidine ring.

nuclear fission produces elements that are heavier than helium.

the elements that are used in nuclear fission and their products are much heavier than helium.

it is the minimum amount of material needed to sustain a fission reaction.

critical mass is the minimum amount of fissionable material that must be assembled for a sustained fission reaction to take place. although two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction. if the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.   at the point where the chain reaction can become self-sustaining, this is referred to as critical mass.