Eukaryotes use mRNA to carry the sequence for a protein to the ribosomes outside of the nucleus. Why then don't prokaryotes translate DNA directly, rather than use mRNA, which in eukaryotes is the code that passes from the interior of the nucleus to the site of protein synthesis?

Explanation:

Eukaryotes use mRNA to carry the sequence for protein synthesis, to the ribosomes outside of the nucleus.

In the eukaryotes, mRNA is processed (capping, poly-adenylation and RNA splicing) before translation. But these processes are absent in prokaryotes.

Prokaryotes don't translate DNA directly because control of transcription of mRNA is a major method of controlling gene expression

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Here in case of prokaryotes, If proteins were made directly from DNA, then the prokaryotes would always express every gene in their genome.

Explanation:

Prokaryotes translate DNA directly, rather than use mRNA, which in eukaryotes is the code that passes from the interior of the nucleus to the site of protein synthesis because control of transcription of mRNA is a major method of controlling gene expression        

Gene expression is a multi-step process, ultimately produce either functional RNA or functional protein

If proteins were made directly from DNA, prokaryotes would always express every gene in their genome      

a physical change is being made.

it would be heredity