biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditar ...

, a marker gene may have several meanings. In nuclear biology and molecular biology, a marker gene is a

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

used to determine if a

nucleic acid sequence A nucleic acid sequence is a succession of Nucleobase, bases signified by a series of a set of five different letters that indicate the order of nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule. By convention, sequence ...

has been successfully inserted into an organism's DNA. In particular, there are two sub-types of these marker genes: a selectable marker and a marker for screening. In

metagenomics Metagenomics is the study of genetic material recovered directly from environmental or clinical samples by a method called sequencing. The broad field may also be referred to as environmental genomics, ecogenomics, community genomics or micr ...

and

phylogenetics In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...

, a marker gene is an orthologous gene group which can be used to delineate between taxonomic lineages.

Selectable marker

A selectable marker protects the organism from a ''selective agent'' that would normally kill it or prevent its growth. In a transformation reaction, depending on the

transformation efficiency Transformation efficiency is the efficiency by which cells can take up extracellular DNA and express genes encoded by it. This is based on the competence of the cells. It can be calculated by dividing the number of successful transformants by the ...

, only one in several million to billion cells may take up DNA. Rather than checking every single cell, scientists use a selective agent to kill all cells that do not contain the foreign DNA, leaving only the desired ones.

Antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy, ...

s are the most common selective agents. In bacteria, antibiotics are used almost exclusively. In plants, antibiotics that kill the

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it ...

are often used as well, although tolerance to salts and growth-inhibiting hormones is becoming more popular. In mammals, resistance to antibiotics that would kill the mitochondria is used as a selectable marker.

Screenable marker

A screenable marker will make cells containing the gene look different. There are three types of screening commonly used: *

Green fluorescent protein The green fluorescent protein (GFP) is a protein that exhibits bright green fluorescence when exposed to light in the blue to ultraviolet range. The label ''GFP'' traditionally refers to the protein first isolated from the jellyfish '' Aeq ...

makes cells glow green under UV light. A specialized microscope is required to see individual cells. Yellow and red versions are also available, so scientists can look at multiple genes at once. It is commonly used to measure

gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. ...

.Chalfie, Martin, et al. "Green fluorescent protein as a marker for gene expression." ''Science'' 263.5148 (1994): 802-805. * GUS assay (using β-glucuronidase) is an excellent method for detecting a single cell by staining it blue without using any complicated equipment. The drawback is that the cells are killed in the process. It is particularly common in plant science. * Blue white screen is used in both bacteria and eukaryotic cells. The bacterial lacZ gene encodes a

beta-galactosidase β-Galactosidase (EC 3.2.1.23, lactase, beta-gal or β-gal; systematic name β-D-galactoside galactohydrolase), is a glycoside hydrolase enzyme that catalyzes hydrolysis of terminal non-reducing β-D-galactose residues in β-D-galactosides. β- ...

enzyme. When media containing certain galactosides (e.g. X-gal), cells expressing the enzyme convert the X-gal to a blue product and can be seen with the naked eye. The strategy is therefore to integrate the DNA insert within the lacZ gene and to select the white colored colonies given they will have correctly integrated the insert. Blue colonies on the other hand will be able to convert the X-gal and give rise to the blue precipitate because the DNA insert either didn't integrate at all, or not at the correct location within the

plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; howev ...

Selectable marker

Screenable marker

References

See also