Both the number of base pairs and the number of genes vary widely from one species to another, and there is only a rough correlation between the two (an observation is known as the C-value paradox).At present, the highest known number of genes is around 60,000, for the protozoan causing trichomoniasis (see List of sequenced eukaryotic genomes), almost three times as many as in the human genome.

The halving of the genetic material in gametes is accomplished by the segregation of homologous chromosomes during meiosis.

In haploid organisms, including cells of bacteria, archaea, and in organelles including mitochondria and chloroplasts, or viruses, that similarly contain genes, the single or set of circular or linear chains of DNA (or RNA for some viruses), likewise constitute the genome.

In such circumstances then, "genome" describes all of the genes and information on non-coding DNA that have the potential to be present.

In eukaryotes such as plants, protozoa and animals, however, "genome" carries the typical connotation of only information on chromosomal DNA.

When talking about genome composition, one should distinguish between prokaryotes and eukaryotes as there are significant differences with contents structure.

In prokaryotes, most of the genome (85–90%) is non-repetitive DNA, which means coding DNA mainly forms it, while non-coding regions only take a small part.An analogy to the human genome stored on DNA is that of instructions stored in a book: In 1976, Walter Fiers at the University of Ghent (Belgium) was the first to establish the complete nucleotide sequence of a viral RNA-genome (Bacteriophage MS2).The next year Fred Sanger completed the first DNA-genome sequence: Phage Φ-X174, of 5386 base pairs.Typically, when it is said that the genome of a sexually reproducing species has been "sequenced", it refers to a determination of the sequences of one set of autosomes and one of each type of sex chromosome, which together represent both of the possible sexes.Even in species that exist in only one sex, what is described as a "genome sequence" may be a composite read from the chromosomes of various individuals.The first complete genome sequences among all three domains of life were released within a short period during the mid-1990s: The first bacterial genome to be sequenced was that of Haemophilus influenzae, completed by a team at The Institute for Genomic Research in 1995.