21.   The scientists said that in each case an insoluble ball of protein formed in the cell nucleus and killed it.

22.   The scientists said that in each case an insoluble ball of protein forms in the cell nucleus and kills it.

23.   Ultraviolet radiation, which is highly energetic, bombards the basic units of DNA inside skin cell nuclei, causing many of them to fuse into nonfunctional fragments.

24.   When a receptor is activated, a series of biochemical events take place that carry messages to the cell nucleus to turn on other genes.

25.   When leptin binds to a receptor and sends a signal to the cell nucleus, it turns on other genes, which are not yet known.

26.   While homeobox genes produce transcription factors, which work within the cell nucleus, BMP molecules are used to send signals from cell to cell.

27.   While homeobox genes produce transcription factors, which work within the cell nucleus, B.M.P. mole- cules are used to send signals from cell to cell.

28.   But unlike the cell nucleus, which gets DNA from each parent, mitochondria are inherited only from the mother.

29.   In contrast, the inheritance genes in the cell nucleus come from both mother and father and contain the fundamental blueprint for the whole individual.

30.   Nuclear matrix proteins, or NMPs, help form the skeleton of cell nuclei as cells reproduce.