CELL BIOLOGY Essay Example | Topics and Well Written Essays - 500 words

CELL BIOLOGY - Essay Example In the first set, all hybrids with thiolase residue 100 intact have been imported into the peroxisome, and those lacking residue 100 or more remain in the cytosol. In the second set, all hybrids with residue 125 or greater have been imported into the peroxisome, and those lacking residue 125 or lesser remain in the cytosol. 3. In the absence of hormones, protein B binds to protein A in the cytosol (immediately after translation of protein A in the cytosol). This binding prevents access to the nuclear transport machinery. This can happen in many ways, two of which are described here: protein B may mask the nuclear localisation signal (as in the case of the hsp90 - steroid hormone system), or protein B may anchor protein A in the cytosol. In each case, the binding of the hormone to either protein causes dissociation of the two proteins, and protein A has access to the nuclear transport machinery. Thus, in normal cells, protein B is always found in the cytosolic extract, whereas the presence of hormone causes protein A to be found in the nucleus, rather than the cytosolic extract, where it is seen in the absence of hormone. 4. A. The protein enters the ER and goes through the default pathway (ER->Golgi->Cell Surface). This is because ER import is co-translational, whereas nuclear import is post-translational. Thus, before the nuclear import signal can be recognized, the protein is already within the ER lumen. B. The protein is imported into the mitochondria. This is because the nuclear export signal is different from the nuclear import signal, therefore the protein never enters the nucleus, and the mitchondrial import signal is recognized in the cytosol ii) The chances of fusion are increased, whenever the same protein (either Y or Z) is present on both donor and target membranes. Thus, when none of the two are present on both, the chances of fusion is 50% of the maximal, when either Y or Z is present on both membranes, chances of fusion increases