Size, Structure, Organization, and Complexity of Genomes

Except for certain viruses, DNA is the genetic material for all organisms and self-replicating units, including viruses and such intracellular organelles as chloroplasts (in plants), kinetoplasts (in protozoa), and mitochondria (in most eukaryotes). GenomicDNAis double helical (except for the genomes of certain bacterial viruses), and its size is related to the complexity of the organism (Table I). In subcellular organelles, viruses, and plasmids, the genome often exists as a circular molecule consisting of up to several thousand base pairs. The genome of bacteria, such as that of the widely studied enteric strain E. coli, is present as a single, circular, double-stranded molecule containing about 4.7 million base pairs. By and large, the genome of many small self-replicating entities is circular DNA, without any terminus in the unbranched polymeric chain.

In contrast, the large nuclear genomes of more complex organisms (from lower eukaryotes such as unicellular yeast with a genome size only an order of magnitude larger than that of E. coli, to mammals with genomes larger by three orders of magnitude) consist of multiple, distinct, linear subunits organized in chromosomes. Depending on the stage of the cell cycle, the structure of chromosomes (collectively called chromatin) varies from the highly extended and amorphous state occurring in much of the (interphase) nucleus to highly compacted, linear, organized chromosomes (metaphase) after completion of DNA duplication followed by cell division (mitosis). This complex organization of eukaryotic genomes is a distinctive feature which separates them from the prokaryotes.

TABLE I Genomic DNA Characterized in Biology^a

Organism	Structure	Total size (bp)	Number of genes	Sequence
Bacteriophage	Linear, circular	5∼200×103	10∼100	Completed for many species
Virus		Up to 2×105	10∼100	Completed for many species
Bacteria E. coli	circular	4.6×106	∼4300	Completed
Eukaryote yeast (S. cerevisiae)	Linear	1.4×107	∼6000	Completed
Drosophila	Linear	1.4×108	1.4×104	Partially completed
Human	Linear	3×109	4×104 to 1×105	Partially completed
aAs of Feb 2001 the data are to be renewed continuously and are available at the website http://ncbi.nlm. nih.org/entrez.