A genome is all the DNA in anorganism, including its genes.

•       Comparative Sequence Sizes (Bases)

•       Escherichia coli(bacterium) genome 4.6 Million

•       Entire yeast genome 15 Million

 Entire human genome 3 Billion

Genes  are the basic physical and functional units of heredity.

 “A gene is a specific sequence of nucleotide bases, whose sequences carry the information required for constructing proteins, which provide the structural components of cells and tissues as well as enzymes for essential biochemical reactions”.

Molecular mapping can be of following two types.

1. Genetic maps and

2. Physical maps.

Both genetic and physical maps provide the likely orderof items along a chromosome.

Genetic map, serves to guide a scientist toward a gene, just like an interstate map guides a driver from city to city.

Physical maps mark an estimate of the true distance, in measurements called base pairs,therefore, allow a scientist to more easily home in on the location of a gene.


Physical maps can be divided into three general types:

Chromosomal or cytogeneticmaps,

Radiation hybrid (RH) maps,and

Sequence maps. The different types of maps vary in their degree of resolution, that is, the ability to measure the separation of elements that are close together.


•       Sequence tagged site (STS) mapping is another physical mapping technique. An STS is ashort DNA sequence that has been shown to be unique. To qualify as an STS, the exact location and order of the bases of the sequence must be known, and this sequence may occur only once in the chromosome being studied or in the genomeas a whole if the DNA fragment set covers the entire genome.

Common Sources of STSs

Expressed sequence tags (ESTs)are short sequences obtained by analysis of complementary DNA (cDNA)clones.  They represent the sequences ofthe genes being expressed. An EST can be used as an STS if it comes from aunique gene and not from a member of a gene family in which all of the genes  have the same, or similar, sequences.

Simple sequence lengthpolymorphisms (SSLPs) are arrays of repeat sequences that display length variations.

•       To map a set of STSs, a collection of overlapping DNA fragments from a chromosome is digested into smaller fragmentsusing restriction enzymes, agents that cut up DNA molecules at defined target points and "molecular cloning" is carried out.

•       The data from which the map will be derived are then obtained by noting which fragments contain which STSs. To accomplish this, scientists copy the DNA fragments using a process known

•       Cloning involves the use of a special technology, called recombinant DNA technology, to copy DNA fragments inside a foreign host.

•       First, the DNA fragments are united with a carrier, also called a vector. After introduction into a suitable host, the DNA fragments can then be reproduced along with the host cell DNA, providing unlimited material for experimental study.

•       An unordered set of cloned DNA fragments iscalled a library.

•       Next, the clones, or copies, are assembled in the order they would be found in the original chromosome by determining which clones contain overlapping DNA fragments.

•       This assembly of overlapping clones is called a clonecontig.