Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

I
PART
II
III IV V
Basic Genetic MechaniSms
DNA, Chromosomes,
and Genomes
CHAPTER
4
Life depends on the ability of cells to store, retrieve, and translate the genetic
instructions required to make and maintain a living organism. This hereditary
information is passed on from a cell to its daughter cells at cell division, and from
one generation of an organism to the next through the organism’s reproductive
cells. The instructions are stored within every living cell as its genes, the information-containing
elements that determine the characteristics of a species as a
whole and of the individuals within it.
As soon as genetics emerged as a science at the beginning of the twentieth century,
scientists became intrigued by the chemical structure of genes. The information
in genes is copied and transmitted from cell to daughter cell millions of times
during the life of a multicellular organism, and it survives the process essentially
unchanged. What form of molecule could be capable of such accurate and almost
unlimited replication and also be able to exert precise control, directing multicellular
development as well as the daily life of every cell? What kind of instructions
does the genetic information contain? And how can the enormous amount
of information required for the development and maintenance of an organism fit
within the tiny space of a cell?
The answers to several of these questions began to emerge in the 1940s. At
this time researchers discovered, from studies in simple fungi, that genetic information
consists largely of instructions for making proteins. Proteins are phenomenally
versatile macromolecules that perform most cell functions. As we saw in
Chapter 3, they serve as building blocks for cell structures and form the enzymes
that catalyze most of the cell’s chemical reactions. They also regulate gene expression
(Chapter 7), and they enable cells to communicate with each other (Chapter
15) and to move (Chapter 16). The properties and functions of cells and organisms
are determined to a great extent by the proteins that they are able to make.
Painstaking observations of cells and embryos in the late nineteenth century
had led to the recognition that the hereditary information is carried on chromosomes—threadlike
structures in the nucleus of a eukaryotic cell that become
visible by light microscopy as the cell begins to divide (Figure 4–1). Later, when
biochemical analysis became possible, chromosomes were found to consist of
deoxyribonucleic acid (DNA) and protein, with both being present in roughly the
same amounts. For many decades, the DNA was thought to be merely a structural
In This Chapter
THE STRUCTURE AND
FUNCTION OF DNA
Chromosomal DNA and
Its Packaging in the
Chromatin Fiber
CHROMATIN STRUCTURE AND
FUNCTION
THE GLOBAL STRUCTURE OF
CHROMOSOMES
How Genomes Evolve