What Is a Marker Molecule?

Definition of a Molecular Marker

• Molecular markers are fragments of DNA which are associated with a particular region of the genome. Molecular markers can take the form of short DNA sequences, such as a sequence surrounding a single nucleotide polymorphism, where a single base-pair change occurs. They can also take the form of longer DNA sequences, such as microsatellites, which are 10 to 60 base pairs long.

Classes of Molecular Marker

• Restriction fragment length polymorphisms are molecular markers that are used to follow a particular DNA sequence as it passes between cells. It is one of the most common types of molecular markers and is based on the hybridization of cloned DNA to DNA fragments. They are specific to a single clone or restriction enzyme combination. Randomly amplified polymorphic DNA molecular markers are generally used in plant breeding and are based on the polymer chain reaction gene cloning of random locations of a plants genome. Isozyme molecular markers are used to mark proteins. They are designed to identify enzymes that differ in amino acid sequences but catalyze the same amino acid reaction.

Uses of Molecular Markers

• Molecular markers are used by geneticists to examine the relationship between hereditary diseases and their causes. They can indicate the location of a specific mutation of a gene that may result in an impaired protein and have been used to identify such diseases as sickle cell anemia and Huntington's disease. Molecular markers can also have agricultural applications, such as in marker assisted breeding, in paternity tests and in plant variety identification, by identifying a plant's identity, purity and stability.

Genetic Engineering

• Molecular markers are used in genetic engineering to mark a where a defective, mutated proteins have been replaced by correctly functioning ones. This is carried out by replacing the damaged DNA sequence with an identical but correctly functioning sequence transplanted from elsewhere. Less than 1 percent of cells generally take up the vector, so a molecular marker is essential to distinguish the cells which have been transformed.