Читать книгу Cell Biology - Stephen R. Bolsover - Страница 40

An animal cell's own DNA should remain in the nucleus, except for the tiny amount that is within mitochondria. DNA in the cytosol will likely belong to a pathogen such as an invading virus. Cells therefore contain active DNAses in the cytosol that rapidly destroy DNA, while leaving RNA intact. It is to evade this defense mechanism that many viruses use RNA as their genetic material, even though RNA is a much less stable molecule than is DNA.

Unlike DNA, RNA is also found in the cytoplasm associated with particles called ribosomes whose function is to make proteins. Ribosomes are made in the nucleus, in specialized regions called nucleoli that form at specific nucleolar organizer region sites on the DNA. These contain blocks of genes that code for the ribosomal RNA. Nuclear pores allow ribosomal subunits to exit the nucleus.

It should be stressed that the appearance of the nucleus we have described thus far relates to the cell in interphase, the period between successive rounds of cell division. As the cell enters mitosis (Chapter 14) the organization of the nucleus changes dramatically. The DNA becomes more and more tightly packed and is revealed as a number of separate rods called chromosomes, of which there are usually 46 in human cells. The nucleolus disperses, and the nuclear envelope fragments. Upon completion of mitosis, these structural rearrangements are reversed and the nucleus resumes its typical interphase organization.