2.1Cell Growth and DNA Replication
Cell Growth - the Most Basic Cell Function
All organisms on earth have one thing in common: they are made of cells. Surrounded by a phospholipid bilayer, cells contain proteins as functional polymers and have DNA as genetic information that dictates cell structures and functions. As the smallest units of organisms, all cells divide and multiply to create progeny cells. Although multicellular organisms produce offspring as a function on the level of individual organisms, this function is supported by the multiplication of component cells, and cell multiplication is a basic function of the process by which a fertilized egg develops into an individual organism. Cell multiplication is the most basic and common of cell functions, and has survived a long process of evolution.
Special Characteristics of DNA Replication
Cells multiply by binary fission. Before cell division, all cellular components must be doubled (Fig. 2-1). However, cellular components consist of a great number of molecules, and the concept of “doubling” here is applied loosely; components are not necessarily equally divided and distributed precisely into two cells (i.e., daughter cells).
DNA, which consists of genetic information, is quite different. Prokaryotes have only one DNA molecule per cell. Although eukaryotes have a structure slightly more complex than that of prokaryotes, they also have only one molecule of the same DNA type per cell. Since DNA molecules contain all the genetic information of the organism to which they belong, a DNA molecule identical to that of the parent cell must be replicated during cell multiplication, and the two resulting identical copies of the DNA must be equally distributed to the two daughter cells. This phenomenon, which involves individual molecules, presents a rigorous condition not found in other cellular molecules. First, let’s start by looking at exactly what DNA is.