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Modern Genetic Analysis Chapter 2. The Structure of Genes and Genomes
Summary

The phenotype of an organism is determined by the phenotype of its cells; therefore, to understand how genes dictate the characteristics of species it is necessary to understand the nature of genomes at the cellular level.

Virtually all genomes are made of DNA, a double-helical molecule composed of two intertwined, antiparallel nucleotide chains. There are always four nucleotides represented: A, T, G, and C. An A in one strand always pairs with T in the other because of complementary hydrogen bond formation; similarly G in one strand always pairs with C in the other. The nucleotide sequence in DNA represents a genetically encoded set of information ultimately necessary for specifying biological form.

Prokaryotes (bacteria) have one circular chromosome present in one to several copies. Eukaryotes have either one or two nuclear genomes depending on whether they are haploid or diploid. The eukaryotic nuclear genome is composed of several to many linear chromosomes. Eukaryote cells also contain smaller chromosomes in their mitochondria and chloroplasts; these are circular chromosomes that are present in many copies. Viruses, although not organisms, also have small genomes. A gene is a functional unit along the DNA of the chromosome. A gene can be defined as a region of DNA capable of making a functional RNA transcript at the appropriate time and place and in the proper amounts to permit normal development. Each eukaryotic chromosome contains one DNA molecule. Eukaryotic chromosomes are efficiently packed by coiling and supercoiling using spool-like structures composed of histone proteins and a scaffold-like central core.

© 1999 by W. H. Freeman and Company. All rights reserved.