| The cells of animals, plants, fungi, and a great number | | | | A nucleus in a present-day eukaryotic cell contains lots |
| of single-celled organisms like algae, amoebas, and | | | | of, non-circular chromosomes-the number depends on |
| paramecia are called "eukaryotic' cells. In a eukaryotic | | | | the species. For instance, each fruit fly nucleus |
| cell, a nucleus contains the According to the | | | | contains four pairs of chromosomes, each human |
| endosymbiont theory, certain organelles in eukaryotic | | | | nucleus contains twenty-three pairs. The |
| cells, the chloroplasts and mitochondria, seem to be | | | | chromosomes consist of DNA wrapped around |
| descendants of ancient bacteria. The chloroplasts are | | | | histone proteins like thread wrapped around a spool. |
| very similar to certain photosynthetic bacteria, and they | | | | When genes on this DNA need to be copied into RNA, |
| perform photosynthesis in plant cells. The mitochondria | | | | the DNA containing those genes unwinds. |
| are very similar to certain bacteria highly efficient at | | | | The nucleus itself is enclosed in a double membrane |
| harvesting energy from various energy-rich molecules, | | | | that keeps the nuclear contents separate from the |
| and mitochondria perform the same function in plant | | | | cytoplasm of the rest of the cell. This double |
| and animal cells. | | | | membrane is peppered with pores to allow certain |
| Lots of mysteries remain. Did other organelles descend | | | | molecules through. RNA copies of genes, for instance, |
| from ancient bacteria? If so, what is the connection? If | | | | pass through such pores, out of the nucleus and into |
| not, how did such organelles evolve? Eukaryotic cells | | | | the cytoplasm. There they conduct the business of |
| contain movable skeletal structures, flagella for | | | | producing cell proteins. |
| swimming, packing and shipping structures, digestive | | | | The nucleus also contains apparatus and molecules for |
| organelles-plenty of evolutionary mysteries. But a | | | | duplicating and dividing the chromosomes during |
| major question is Where did the nucleus come from | | | | cell-division, molecules for editing and perfecting copies |
| and how did it come to its present structure? | | | | of DNA and RNA, and much, much more. This |
| According the the endosymbiont theory, somehow the | | | | complex organelle, the nucleus, like the chloroplasts and |
| nucleus, chloroplasts, and mitochondria came together | | | | mitochondria, must have descended from some kind |
| into a permanent symbiotic relationship. We know of | | | | of prokaryotic cell. But is this ancestor still around? If |
| likely bacterial ancestors for the chloroplasts and | | | | so, we haven't found it, though some biologists are |
| mitochondria, but what about the nucleus? | | | | searching hard. |