12.1 Cell reproduction  (Page 31/2)

Introduction

DNA was the first three-dimensional Xerox machine.

Non scientists like Boulding , who was an economist, are fond of using machines as metaphors for biological structures and functions. But DNA is a lot more than just a Xerox machine, as you will learn in this section. In fact, the more we learn about DNA, the more remarkable it seems. For DNA not only replicates itself, it directs the development, life, and demise of every organism on the planet.

A human, as well as every sexually reproducing organism, begins life as a fertilized egg (embryo) or zygote. Trillions of cell divisions subsequently occur in a controlled manner to produce a complex, multicellular human. In other words, that original single cell is the ancestor of every other cell in the body. Once a being is fully grown, cell reproduction is still necessary to repair or regenerate tissues. For example, new blood and skin cells are constantly being produced. All multicellular organisms use cell division for growth and the maintenance and repair of cells and tissues. Cell division is tightly regulated, and the occasional failure of regulation can have life-threatening consequences. Single-celled organisms use cell division as their method of reproduction.

So, the continuity of life from one cell to another has its foundation in the reproduction of cells by way of the cell cycle. The cell cycle is an orderly sequence of events that describes the stages of a cell’s life from the division of a single parent cell to the production of two new daughter cells. The mechanisms involved in the cell cycle are highly regulated.

Genomic dna

Before discussing the steps a cell must undertake to replicate, a deeper understanding of the structure and function of a cell’s genetic information is necessary. A cell’s DNA, packaged as a double-stranded DNA molecule, is called its genome . In prokaryotes, the genome is composed of a single, double-stranded DNA molecule in the form of a loop or circle ( [link] ). The region in the cell containing this genetic material is called a nucleoid. Some prokaryotes also have smaller loops of DNA called plasmids that are not essential for normal growth. Bacteria can exchange these plasmids with other bacteria, sometimes receiving beneficial new genes that the recipient can add to their chromosomal DNA. Antibiotic resistance is one trait that often spreads through a bacterial colony through plasmid exchange.