Most human cells contain 23 pairs of chromosomes. The 3 billion base pairs in the human genome therefore are distributed along 22 autosomes and the X and Y sex chromosomes. Mapping and sequencing Several types of map are possible. Cytogenetic maps mark features of chromosomes that are visible under the microscope. Genetic maps show the locations of genetic markers, such as genes for inherited diseases or genes that determine blood type.
Physical maps show the actual distances between markers.
Whereas mapping determines the locations of genes or other markers, sequencing examines the DNA in much greater detail to reveal the order of its nucleotide bases. So constructing a map is like marking the locations of cities on a road map, whereas sequencing is like identifying the locations of houses within the cities and the features that lie en route. Cloning Before sequencing can begin, researchers must obtain enough material to study. One way is to clone the DNA by inserting it into a rapidly growing organism that is tricked into copying the foreign DNA along with its own. Small pieces of DNA can be cloned in the bacterium E. coli , which contains rings of DNA called plasmids in addition to its chromosome.