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Why can prokaryotes begin translating mRNA before transcription is finished?

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Diagram of a DNA polynucleotide chain showing linked deoxyribose sugars, phosphate groups, and nitrogenous bases (adenine, guanine, cytosine, thymine), with an inset comparing ribose and uracil in RNA. — Nucleic acid A portion of the polynucleotide chain of deoxyribonucleic acid (DNA), with an inset showing the corresponding pentose sugar and pyrimidine base in ribonucleic acid (RNA).

Prokaryotes are able to begin translating messenger RNA (mRNA) while transcription is still ongoing primarily because they lack a nucleus.12

In eukaryotes, the genetic material is sequestered within a membrane-bound nucleus, which physically separates the site of transcription from the ribosomes in the cytoplasm where translation occurs. Furthermore, eukaryotic mRNA must undergo extensive processing, such as splicing and the addition of a cap and polyA tail, before it is considered mature and ready for translation.123

In contrast, prokaryotes do not possess a nucleus, meaning the mRNA is not separated from the cellular machinery required for protein synthesis. Consequently, ribosomes can attach to the mRNA molecule as soon as it is synthesized by RNA polymerase, allowing for the simultaneous coupling of transcription and translation.12