In mature Red Blood Cell, nucleus is inactive or dormant. Then how do they survive?
Red blood cells develop from progenitor cells with nuclei but as red blood cells mature, they lose their nucleus completely. Erythrocytes in mammals are anucleate when mature, meaning that they lack a cell nucleus. In comparison, the erythrocytes of other vertebrates have nuclei; the only known exceptions are salamanders of the Batrachoseps genus and fish of the Maurolicus genus with closely related species. The elimination of the nucleus in vertebrate erythrocytes has been offered as an explanation for the subsequent accumulation of non-coding DNA in the genome. The argument runs as follows: Efficient gas transport requires erythrocytes to pass through very narrow capillaries, and this constrains their size. In the absence of nuclear elimination, the accumulation of repeat sequences is constrained by the volume occupied by the nucleus, which increases with genome size. Their function is extremely simple, they act as little more than over-sized vesicles that carry chemicals around in the blood stream the ability to self-replicate is not needed in such cells when the bone marrow can produce them instead. The Nucleus takes up a large portion of the energy a cell uses in its day to day operations, dispensing with this leaves extra room for haemoglobin and reduces energy consumption for the whole organism. Add to this that red blood cells without a nucleus can not become cancerous. If red blood cells had nuclei, cancer could be much more common.