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Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, James D. Watson - The Cell Nucleus: Molecular Biology of the Cell, Second Edition
There seems to be a large excess of DNA in the genomes of higher organisms. Long before it was possible to examine the nucleotide sequences of chromosomal DNA directly, it was evident that the relative amounts of DNA in the haploid genomes of different organisms has no systematic relationship to the complexity of the organism: human cells, for example, contain about 700 times more DNA than the bacterium E. coli, while some amphibian and plant cells contain 30 times more DNA than human cells.

Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, James D. Watson - How Cells Are Studied: Molecular Biology of the Cell, Second Edition
A typical animal cell is 10 to 20 mm in diameter, or about five times small than the smallest visible particle. It was, therefore, not until good light microscopes became available, in the early part of the nineteenth century, that all plant and animal tissues were discovered to be aggregates of individual cells. This discovery, proposed as the cell doctrine by Schleiden and Schwann in 1838, marks the formal birth of cell biology.

Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, James D. Watson - How Cells Are Studied: Molecular Biology of the Cell, Second Edition
Cells are small and complex. It is hard to see their structure, hard to discover their molecular composition, and harder still to find out how their various components function. What we can learn about cells depends on the tools at our disposal, and major advances have frequently spring from the introduction of new techniques. To understand contemporary cell biology, therefore, it is necessary to know something of its methods.