|MadSci Network: Cell Biology|
Actually, nuclear pores do allow and facilitate the transport of proteins into the nucleus. Proteins containing a nuclear localization signal within their amino acid sequence are efficiently translocated from the cytoplasm into the nucleus. For example, signalling hormones bind to and activate hormone receptors in the cytoplasm. When the receptor is activated, a chaperone protein is released from the receptor to expose the nuclear localization signal. The hormone receptor is then translocated into the nucleus where it binds to DNA and stimulates transcription of specific genes. To answer your question as to whether cytosolic proteins touch DNA during mitosis, the answer is yes. To give just one example, the centrosome which resides outside the nucleus is a complex of many proteins. This structure nucleates microtubuleswhich extend and contact the chromosomes once when the nuclear envelope breaks down. These microtubules will then shorten and pull the chromosomes apart. Alberts, et al, Molecular Biology of the Cell, 3rd Ed. Garland Publishing NY, 1994, pages 566-567 and 911-933.
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