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).

Mapping of The Human Genome

 The most important objective of human genome project was to construct a series of maps for each chromosome.

1. Cytogenetic map: This is a map of the chromosome in which the active genes respond to a chemical dye and display themselves as bands on the chromosome.

2. Gene linkage map: A chromosome map in which the active genes are identified by locating closely associated marker genes. The most commonly used DNA markers are restriction fragment length polymorphism (RFLP), variable number tandems repeats (VNTRS) and short tandem repeats (STRS). VNTRs are also called as minisatellites while STRs are microsatellites.

3. Restriction fragment map: This consists of the random DNA fragments that have been sequenced.

4. Physical map: This is the ultimate map of the chromosome with highest resolution base sequence. The methods for DNA sequencing are given in next blog page. Physical map depicts the location of the active genes and the number of bases between the active genes.

Terms Used In Tissue Culture

 A selected list of the most commonly used terms in tissue culture are briefly explained

Explant: An excised piece of differentiated tissue or organ is regarded as an explant. The explant may be taken from any part of the plant body e.g., leaf, stem, root.

Callus: The unorganized and undifferentiated mass of plant cells is referred to as callus. Generally, when plant cells are cultured in a suitable medium, they divide to form callus i.e., a mass of parenchymatous cells.

Dedifferentiation: The phenomenon of mature cells reverting to meristematic state to produce callus is dedifferentiation. Dedifferentiation is possible since the non dividing quiescent cells of the explant, when grown in a suitable culture medium revert to meristematic state.

Redifferentiation: The ability of the callus cells to differentiate into a plant organ or a whole plant is regarded as redifferentiation.

Totipotency: The ability of an individual cell to develop into a whole plant is referred to as cellular totipotency. The inherent characteristic features of plant cells namely dedifferentiation and redifferentiation are phenomenon of totipotency.