M13
A filamentous, single-stranded DNA phage that infects E. coli. The M13 DNA replicates as a circular double-stranded intermediate (called a replicative form) then single-stranded DNA produced by rolling circle replication is packaged into phage particles. Derivatives of M13 are used as cloning vectors.

MacConkey plates
MacConkey medium contains pH indicators that can be used to differentiate colonies that can ferment a sugar from colonies unable to ferment a sugar. For example, on MacConkey-Lactose plates Lac+ colonies ferment lactose and the acid turns the pH indicator red colored, but Lac- colonies cannot ferment lactose so they remain white colored.

Male-specific phage
Phages that only adsorb to receptor sites on the F-pilus.

Map unit
An arbitrary unit representing the distance between genes, usually derived from the percentage of recombination, but also defined by the time at which the gene is transferred during conjugation.

Marker
A genetic trait of which one allelic form is selected or screened for following recombination.

Marker exchange
Use of recombination to switch allelic forms of a gene with a detectable phenotype. For example, to move a mutation from a plasmid or phage to the chromosome.

Marker rescue
Repair of a mutational defect by recombination. For example, when a cell is co-infected with a mutant phage that is unable to replicate and a wild-type phage, recombination between the two phage can repair the replication defect and allow the recombinant derivative of the mutant phage to reproduce.

Mating
See Conjugation.

Meiosis
The process of nuclear division associated with the formation of gametes or of haploid cells from a diploid.

Merodiploid
A partially diploid bacterium, carrying both its own chromosome and a chromosome fragment introduced by conjugation, transformation or transduction.

Messenger RNA (mRNA)
The transcript of a segment of chromosomal DNA which is a template for protein synthesis.

Methyl transferase
Enzymes that catalyze the transfer of methyl groups from one molecule to another.

Microarrays
Ordered sets of DNA fragments fixed to solid surfaces. The DNA fragments may represent all the open reading frames in a genome, a particular gene family, or any other subset of genes. Microarrays can be probed with RNA or cDNA derived from an organism to determine which transcripts are expressed under certain conditions. Microarrays are sometimes called gene chips.

Microinjection
A method of introducing new DNA into a cell by injecting it directly into the nucleus.

Minimal essential pairing sequence (MEPS)
The shortest length of DNA homology required for the initiation of homologous recombination.

Minimal medium
A defined medium that provides only the minimum number of different nutrients needed for growth of a particular bacterium.

Mismatch
A defect in the pairing of two complementary DNA sequences where a base in one strand is different from that expected according to complementarity with the other.

Mismatch repair
A mechanism that corrects mismatched base pairs that have escaped correction by the proofreading activities of the DNA polymerases.

Mispairing
Improper alignment of two nucleic acid strands.

Missense mutation
A mutation that changes a codon for one amino acid to a codon for a different amino acid, resulting in an amino acid substitution in the protein product.

Mitosis
The normal process of nuclear division in a eukaryote, whereby nuclear division occurs on a spindle structure without reduction in the chromosome number in the daughter nuclei.

Mobile element
A sequence of DNA that is able to promote its own transposition. An insertion sequence or a transposon.

Mobilizable plasmid
A plasmid that is not self-transmissible byt can be co-transferred with a self-transmissible plasmid, either independantly or as a cointegrate.

Modification (of DNA)
The specific addition of secondary residues (methyl, hydroxymethyl, glucosyl) to deoxyribonucleotides by an organism to differentiate its own DNA from other DNA or for regulatory purposes.

Monocistronic
An mRNA that only encodes a single gene product.

Mosaic
The substitution of portions of a gene with sequences acquired from another site or another organism via genetic recombination. Comparative sequence analysis indicates that many genes are composed of a "patchwork" of sequences derived from multiple recombination events.

Motif
A small portion of a protein (typically less than 20 amino acids) that is homologous to regions in other proteins that perform a similar function.

MOI
See Multipilicity Of Infection.

Moron
A short region of DNA that is inserted between a pair of genes in one phage genome but the genes of this same pair are adjacent in a related phage genome. The term (coined by Roger Hendrix) refers to the observation that one phage contains "more" DNA than the other phage at this particular spot. Many virulence factors of pathogenic bacteria are encoded in morons of integrated prophages.

mRNA
An RNA molecule that includes the coding region(s) and the translation signals for a gene or operon. An intermediate that specifies the amino acid sequence of the encoded polypeptide(s) during translation.

Mu
A bacteriophage that reproduces by transposition. Mu infects a variety of enteric bacteria.

Mud
A defective derivative of phage Mu. Mud insertions are commonly used to construct operon and gene fusions in enteric bacteria.

Multicopy plasmid
A plasmid with a high copy number.

Multifactorial
A phenotype caused by two or more genes.

Multigene family
A number of identical or related genes present in the same organism, usually coding for a family of related polypeptides.

Multiplicity of infection (MOI)
The ratio of pathogen to host cells during an infection. For example, the ratio of phage P22 to Salmonella cells, or the ratio of Salmonella to cultured macrophages.

Mutagen
A chemical or physical agent that increases the frequency of mutation, usually by directly damaging the DNA.

Mutagenesis
The formation of mutations.

Mutant
An organism with an altered base sequence in one or several genes. Usually refers to an organism with a mutation that causes a phenotypic difference from the wild-type.

Mutation
Any heritable alteration in the base sequence of the genetic material. See Types of mutations

Mutation rate
The number of mutations per cell division. The mutation rate can be determined from a Luria-Dulbruck fluctuation analysis. Sometimes the mutant frequency is described instead of mutation rate. The mutant frequency is simply the ratio of mutants / total cells in the population. The mutant frequency is much easier to measure but may show large fluctuations depending upon when the first mutation appeared in the population.

Mutator gene
A mutant gene which increases the frequency of mutation in other genes. Mutations in genes responsible for DNA repair typically have a mutator phenotype. For example, mutations in the genes responsible for methyl-directed mismatch repair, designated mutS, mutL, and mutH, increase the spontaneous mutation frequency about 1000-fold. See Mutator strains.