Genes control the genetic traits, and genes are DNA, which is organized into chromosomes. Both prokaryotes and eukaryotes have chromosomes, although the organization level is different. A gene is a region of DNA segment that controls certain trait of inheritance, while chromosome is the basic inheritance unit in cells. Genes are arranged on chromosomes linearly.
Prokaryotes do not have nucleus but they have their own chromosomes, which contain either linear or circular DNA molecules. DNA is naked in prokaryotes, which means they do not associate with histones. Prokaryotes usually only have one copy of chromosome, therefore they are haploid. Virus and organelles in an eukaryotes also have chromosomes similar to chromosomes in prokaryotes.
Eukaryotes usually have large, linear and multiple chromosomes within a cell nucleus. DNA forms highly packed structure and wrapped with histones. These chromosomes are visible under light microscope when a cell is in metaphase of mitosis.
Genome organization in eukaryotes
Genes are arranged linearly on chromosomes. Usually a gene has a regulatory region which is called promoter and a coding sequence which determines the amino acid sequence of the gene. For majority of the genes in mammalian cells, the coding sequence is interrupted by non-coding sequence called introns (the coding sequence are called exons).
Proposed by Beadle and Tatum in 1942, this hypothesis linked the enzyme function to genes. Based on the hypothesis, a mutation outcome can be predicted if genes are within a biochemical pathway.