Which statement best predicts why a cell's progression through the cell cycle might be halted by the G1/S checkpoint?

The missing options are:

A. Spindle fibers have not correctly attached to chromosomes.

B. Damage occurred to DNA when it was being copied in G1.

C. There are not enough nucleotides available to construct new DNA.

D. Proteins necessary for M phase of the cell cycle have not been produced.

The correct option is C.There are not enough nucleotides available to construct new DNA.

Explanation:

During the G1 phase, the cell grows in size since it prepares its organelles and molecule combinations for the next phase of the cell cycle, the S phase. Before the cell transitions to the S phase, it has to pass through a checkpoint that makes sure that there are no mutations, it has grown enough, and has the necessary molecules for the next phase, which is the synthesis of DNA. If there are not enough nucleotides, the cell will not be able to synthesize new DNA in the S phase, so the checkpoint stops the cells from moving to this phase till it has all the necessary things to do it.

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.