The principal difference between procaryotic and eucaryotic cells, and the one that gives the two forms their names, lies in the accommodation of their genetic material. Eucaryotic cells have a true nucleus, surrounded by a nuclear membrane. This is in fact a double membrane; it contains pores, through which messenger RNA leaves the nucleus on its way to the ribosomes during protein synthesis (see Chapter 11).
The organisation of genetic material in eucaryotes is very different from that in procaryotes. Instead of existing as a single closed loop, the DNA of eucaryotes is organised into one or more pairs of chromosomes. The fact that they occur in pairs highlights another important difference from procaryotes: eucaryotes are genetically diploid in at least some part of their life cycle, while pro-caryotes are haploid. The DNA of eucaryotic chromosomes is linear in the sense that it has free ends; however, because there is so much of it, it is highly condensed and wound around proteins called histones. These carry a strong positive charge and associate with the negatively charged phosphate groups on the DNA.
As well as the chromosomes, the nucleus also contains the nucleolus, a discrete structure rich in RNA, where ri-bosomes are assembled. The ribosomes themselves have the same function as their procaryotic counterparts; the differences in size have already been discussed (see Table 3.3). They may be found free in the cytoplasm or associated with the endoplasmic reticulum (see below), depending on the type of protein they synthesise.
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