Final answer:
In prokaryotes, translation can begin while transcription is still in progress, because there is no nucleus and the processes occur in the same compartment, allowing for rapid protein synthesis. In contrast, eukaryotes require that transcription be completed in the nucleus and the mRNA modified before translation can occur in the cytoplasm.
Step-by-step explanation:
In prokaryotes, ribosomes can attach to the mRNA and begin translation even though transcription has not been completed. This coupling of transcription and translation is possible because in prokaryotic cells, there is no nucleus to separate the two processes, allowing them to occur concurrently in the cytoplasm. Thus, a prokaryotic mRNA can be used to begin synthesis of a polypeptide by a ribosome before transcriptional termination of that mRNA molecule. By contrast, in eukaryotes, the transcription occurs in the nucleus, and the mRNA must undergo processing and then be transported to the cytoplasm before translation can begin. These prerequisites in eukaryotes result in transcription and translation being distinct and separate processes, precluding their simultaneity.
The simultaneous processes in prokaryotes facilitate a rapid response to environmental signals that require new proteins because multiple transcription and translation events can occur concurrently on the same DNA template. This results in the formation of polyribosomes or polysomes, where a single mRNA is translated by multiple ribosomes at the same time. In contrast, eukaryotic cells can only form polyribosomes after the mRNA has been fully synthesized, processed, and exported from the nucleus to the cytoplasm.