Why Transcription and Translation are Coupled in Prokaryotes
Transcription means formation of mRNA from DNA and mRNA carries the information which is required for protein synthesis. Translation is the process of synthesis of protein from mRNA.
The mRNA which is formed by transcription process is transported out of the nucleus into the cytoplasm for translation process. mRNA is transported to the ribosome which is called as cell protein synthesis machine. Ribosomes bind to the mRNA and protein synthesis starts. Thus, in eukaryotes tanscription and translation occur in different cellular compartments that are separated by a membrane barrier such as transcription occurs inside the nucleus and translation occurs in the cytoplasm.
In prokaryotes, protein synthesis which is called translation starts while the mRNA is still being synthesized which is called transcription. This is because there is no nucleus in prokaryotes that separates the transcription and translation process. Therefore, when bacterial genes are transcribed then transcripts begin to translate immediately. Prokaryotic transcription occurs in the cytoplasm alongside translation and both processes occur simultaneously.
Prokaryotes unlike eukaryotes has no nucleus. Thus, prokaryotic genomic DNA is present in the cytoplasm. When prokaryotic DNA is transcribed then ribosomes attach to the mRNA and transcription and translation are coupled. But in eukaryotes, the transcription process occurs in the nucleus, when the RNA is made, it is processed (splicing, polyA etc. ) and transported to the cytoplasm where it is translated. So in eukayotes transcription and translation are not coupled and occurs in different compartments.