(i) Rudolf Virchow (1855) first explained that the cells divide and new cells are formed from the pre-existing cells.
(ii) Schleiden and Schwann observed the cells and cell membranes for the first time. They proposed that animal and plant cells are composed of cells and products of cells.

The DNA content of the nucleus is constant for a given species. The haploid organisms have half the content of DNA than that of their diploid parents. The amount of DNA remains constant in all the members of a given species, On the other hand, the extrachromosomal DNA which occurs either in the mitochondria or in chloroplast is found to be variable among the members of a population. In the case of a highly active organism, the DNA is found to be more as compared to less active ones.

The body of a multicellular organism has cell as a basic structural unit. The cells are organised to form tissues such as blood, bone, etc. The tissue are organised to form organs such as heart, kidney, etc. The organs are then organised to form organ systems such as digestive system, reproductive system respiratory system, etc. The various organ system of organism gets arranged to form a complete individual.

(a). Histones are the basic proteins found in the chromatin of eukaryotes.
(b). They help in packaging the long DNA molecules into nucleosomes on the chromosomes.

The nucleolus is a ribosome factory. It combines RNAs with proteins to produce ribosome subunits. After their formation, the ribosome subunits pass out and get established in the cytoplasm. It also receives and stores ribosomal proteins formed in the cytoplasm.