Molecular Marker

Molecular Marker 

A molecular marker is a DNA sequence in the genome which can be located and identified. As a result of genetic alterations (mutations, insertions, deletions), the base composition at a particular location of the genome may be different in different plants. These differences, collectively called as polymorphisms can be mapped and identified. Plant breeders always prefer to detect the gene as the molecular marker, although this is not always possible. The alternative is to have markers which are closely associated with genes and inherited together.

The molecular markers are highly reliable and advantageous in plant breeding programmes.

• Molecular markers provide a true representations of the genetic make up at the DNA level.
• They are consistent and not affected by environmental factors. 
• Molecular markers can be detected much before development of plants occur.
• A large number of markers can be generated as per the needs.

Basic principle of molecular marker detection

Let us assume that there are two plants of the same species-one with disease sensitivity and the other with disease resistance. If there is DNA marker that can identify these two alleles, then the genome can be extracted, digested by restriction enzymes, and separated by gel electrophoresis. The DNA fragments can be detected by their separation. For instance, the disease resistant plant may have a shorter DNA fragment while the disease-sensitive plant may have a longer DNA fragment . Molecular markers are of two types.

1. Based on nucleic acid (DNA) hybridization (non-PCR based approaches). 
2. Based on PCR amplification (PCR-based approaches).