About three billion years ago, single-celled organisms called cyanobacteria lived in earth’s oceans. they thrived on the ocean’s abundance of , which they used during the process of photosynthesis. much later, this process allowed organisms, such as fish, to survive by extracting dissolved from ocean waters.

oxygen

carbon dioxide

nitrogen

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About three billion years ago, single-celled organisms called cyanobacteria lived in Earth’s oceans. They thrived on the ocean’s abundance of carbon-di-oxide , which they used during the process of photosynthesis. Much later, this process allowed organisms, such as fish, to survive by extracting dissolved oxygen from ocean waters.

Explanation:

Cyanobacteria is the bacteria that is primitive form of bacteria that used to be first organisms even before the present day bacteria came into existence. They were able to do photosynthesis.

Cyanobacteria used the huge concentration of carbon dioxide that was mixed with the sea water. This carbon dioxide was picked up by millions of cyanobacteria that converted the carbon dioxide into useful glucose and other products and valuable oxygen. Thus oxygen came in earth and that was used by future organisms for respiration.  

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.

c) 50% there are four chromosomes. after crossing over, 2 of the four have been changed. that means any gamete has a 2 in 4 chance or 50% chance of receiving the mutated chromosome.