Since the DNA is transcribed into mRNA in the nucleus, and protein synthesis takes place in the cytoplasm, the mRNA has to exit the nucleus to the cytoplasm. The environment in the nucleus differs in many ways from that of the cytoplasm. To separate these two environments from each other the nucleus is enclosed by a double membrane, and the only connection to the surrounding cytoplasm is through channels called the nuclear pore complex (NPC).
When it is time for the mRNA to leave the nucleus, the mRNA is believed to be "tagged" by proteins which serve as export signals, directing the mRNA to the nuclear pore complex that the mRNA is to leave. The mRNA and it's bound export proteins then attaches to export receptors and the whole complex (RNA, export signal proteins and export receptors) is translocated through the nuclear pore complex. The mRNA is released into the cytoplasm and is immediately ready for the next step: translation»

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