In assembling a nucleosome, normally the …(1) histone dimers first combine to form a tetramer, which then further combines with two … (2) histone dimers to form the octamer.

option 1

Explanation:

In assemblying the nucleosome, this reaction occurs in two main steps. the H3 and H4 are recruited first to the DNA in pairs forming the H3/H4 tetramer; meaning two of H3 and two of H4. This gives rise to the nucleosome precursor. Then after this, the dimers of both H2A/H2B are recruited to this precursor, to give rise to the octamer structure around which the DNA is wrapped.  

b - the respond is innate and varies over time.

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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.