Genome

The genome is the full set of genes or genetic material carried by a particular organism. The size of a genome is usually measured in numbers of genes or base pairs (a base, or nucleotide, is the building block of the genetic material).

A genomic sequence is the actual order of the nitrogen-containing bases in the DNA nucleotide sequence. The sequence includes regions that encode a product. Each of these regions constitutes a gene. In eukaryotes (organisms whose genetic material is enclosed within a specialized membrane called chromosomes. In prokaryotes such as bacteria, where the genetic material is not enclosed in a nuclear membrane, the genome is not organized into chromosomes.

The size of the genome varies among diffent organisms. Some viruses, which utilize much of a host’s replication machinery to make new copies of themselves, have a genome that may contain fewer than a dozen genes. The human genome contains about 30,000 genes, perhaps a bit humbling when one considers that an earthworm’s genome contains approximately 23,000 genes.

With the explosion of knowledge of the genomic composition of a variety of life, comparison of genes is possible. Genomic libraries, a comprehensive collection of cloned DNA fragments derived from a genome, can be screened for the presence of the sequence of interest by radioactively labeling the DNA (usually between 100 and 500 nucleotides long) and using this as a probe to identify the clone that contains the selected sequence. The clone selected can then be grown in bacteria to produce large amounts of clone DNA, which can be studied. If for example, the sequence of interest was part of a gene, by using this sequence as a probe, the clone containing, hopefully, the whole gene could be isolated. Such genomic comparison has revealed genetic similarities between a number of organisms.