Melting of DNA means its denaturation.
Melting temperature (Tm) is the midpoint of the temperature range over which DNA is denatured.
Membranes consist of an asymmetrical lipid bilaryer that has lateral fluidity and contains proteins.
Membrane proteins have hydrophobic regions that allow part or all of the protein structure to reside within the membrane; the bonds involved in this association are usually noncovalent.
Metastasis describes the ability of tumor cells to leave their site of origin and migrate to other locations in the body, where a new colony is established.
Micrococcal nuclease is an endonuclease that cleaves DNA; in chromatin, DNA is cleaved preferentially between nucleosomes.
Microsomes are fragmented pieces of endoplasmic reticulum associated with ribosomes.
Microtubules are filaments consisting of dimmers of tubulin; interphase microtubules are reorganized into spindle fibers at mitosis, when they are responsible for chromosome movement.
Microtubule associated proteins (MAPs) are proteins associated with microtubules and responsible for influnencing ther stability and organization.
Microtubule organizing center (MTOC) is a structure from which microtubules may be extended.
Minicell is an anucleate bacterial (E. coli) cell produced by a division that generates cytoplasm without a nucleus.
Minichromosome of SV40 or polyoma is the numcleosomal form of the viral circular DNA.
Mitosis is the division of a eukaryotic somatic cell.
Modification of DNA or RNA includes all changes made to the nucleotides after their initial incorporation into the polynucleotide chain.
Modififed bases are all those except the usual four from which DNA (T, C, A, G) or RNA (U, C, A, G) are synthesized; they result from postsynthetic changes in the nucleic acid.
Monocistronic mRNA condes for one protein.
Monolayer describes the growth of eukaryotic cells in culture as a layer only one cell deep.
Morphogen is a factor that induces development of particular cell types in a manner that depends on its concentration.
MPF (maturaton-or M phase-promoting factor) is a dimeric kinase, containing the p34, catalytic subunit and a cyclin regulatory subunit, whose activation triggers the onset of mitosis.
mtDNA is mitochondrial DNA.
MTOC (microtubule organizing center) is a region from which microtubules emanate. The major MTOCs in a mitotic cell are the centrosomes.
Nyktuciot okasnuds are present in bacteria at amounts greater than one per chromosome.
Multiforked chromosome (in bacterium) has more than one replication fork, because a second initiation has occurred before the first cycle of replication has been completed.
Multimeric proteins consist of more than one subunit.
Mutagens increase the rate of mutation by inducing changes in DNA.
Mutation describes any change in the sequence of genomic DNA.
Mutation frequency is the frequency at which a particular mutant is found in the population.
Mutation rate is the rate at which a particular mutation occurs, usually given as the nuber of events per gene per generation.
Myeloma is a tumor cell line derived from a lymphocyte; usually produces a single type of immunoglobulin.

Begative complementation occurs when interallelic complementation allow a mutant subuntit to suppress the activity of a wild-type subunit in a multimeric protein.
Negative regulators function by switching of transcription or translation.
Negative regulators function by switching off transcription or translation.
Negative supercoiling comprises the twisting of a duplex of DNA in space in the opposite sense to the turns of the strands in the double helix.
Neutral substitutions in a protein are those changes of amino acids that do not affect activity.
Nick in duplex DNA is the absence of a phosphodiester bond between two adjacent nucleotides on one strand.
Nick trandslation describes the ability of E. Coli DNA polymerase I to use a nick as a starting point from which one strand of a duplex DNA and be degraded and replaced by resynthesis of new material; is used to introduce radioactively labeled nucleotides into DNA in vitro.
Nonautonomous controlling elements are defective transposons that can transpose only when assisted by an autonomous controlling element of the same type.
Nondisjunction describes failure of chromatids (duplicate chromosomes) to move to opposite poles during mitosis or meiosis.
Nonpermissive conditions do not allow conditional lethal muants to survive.
Nonrepetitive DNA shows reassociation kinetics expected of unique sequences.
Nornreplicative transposition describes the movement of a transposon that leave a donor site (usually generating a double strand break) and moves to a new site.
Nosense codon is any one of three triplets (UAG, UAA, UGA) that cause termination of protein synthesis. (UAG is know as amber; UAA as ochre.)
Nonsense mutation is any change in DNA that causes a (termination) codon to replace a codon representing an amino acid.
Nonsense suppressor is a gene coding for a mutant tRNA able to respond to one of more of the termination codons.
Nontranscribed spacer is the region between transcription units in a tandem gene cluster.
Northern blotting is a technique for transferring RNA from an agarose gel to a nitrocellulose filter on which it can be hybridized to a complementary DNA.
Nuclear envelope is a layer of two membranes surrounding the nucleus. It is penetrated by nuclear pores and bounded on the interior by the nuclear laminim.
Nuclear lamina consists of a proteiaceous layer on the inside of the nuclear envelope. It consists of (upto) three lamin proteins.
Nuclear matrix is a network of fibers surrounding and penetrating the nucleus.
Nuclear pores represent holes in the nuclear envelope and are presumed to be used for transport of macromolecules.
Nucleolar organizer is the region of a chromosome carrying genes coding for rRNA.
Nucleoid is the compact body that contains the genome in a bacterium.
Nucleoid is a discrete region of the nucleus crdeated by the transcription of rRNA genes.
Nucleosome is the basic structural subunit of chromatin, consisting of ～200 bp of DNA and an octamer of histone proteins.
Nucleolytic reactions involve the hydrolysis of a phosphodiester bond in a nucleic acid.
Null mutation completely eliminates the function of a gene, usually because it has been physically deleted.

Ochre codon is the triplet UAA, one of three codons that cause termination of protein synthesis.
Ochre mutation is any change in DNA that creates a USS condon at a site previously occupied by another codon.
Ochre suppressor is a gene coding for a mutant tRNA able to respond to the UAA codon to allow continuation of protein synthesis; ochre suppressors also suppress amber codons.
Okazaki fragments are the short stretches of 1000-2000 bases produced during discontinuous replication; they are later joined into a covalently intact strand.
Oncogenes are genes whose products have the ability to transform eukaryotic cells so that they grow in a manner analogous to tumor cells. Oncogenes carried by retroviruses have names of the form v-onc. See also proto-oncogene.
Open reading frame (ORF) contains a series of triplets coding for amino acids without any termination codons; sequence is (potentially) translatable into protein.
Operator is the site on DNA at which a repressor protein binds to preven transcription from initiating at the adjacent promoter.
Operon is a unit of bacterial gene expression and regulation, including structural genes and control elements in DNA recognized by regulator gene product(s).
Organelles is a unit of bacterial gene expression and regulation, including structural genes and control elements in DNA recognized by regulator gene product(s).
Organelles are compartments located in the cytoplasm and surrounded by a membrane.
Origin (ori) is a sequence of DNA at which replications is initiated.
Orphons are isolated individual genes found in isolated locations, but related to members of a gene cluster.
Overwinding of DNA is caused by positive supercoiling (which applies further tension in the direction of winding of the two strands about each other in the duplex).

Packing ratio is the ratio of the length of DNA to the unit length of the fiber containing it.
Pairing of chromosomes –see synapsis.
Palindrome is a sequence of DNA that is the same when one strand is read left to right or the other is read right to left; consists of adjacent inverted repeats.
Palindrome is a sequence of DNA that is the same when one strand is read left to right or the other is read right to left; consists of adjacent inverted repeats.
Papovaviruses are a class of animal viruses with small genomes, including SV40 and polyoma.
Parnemic joint describes a region in which two complementary sequences of DNA are associated side by side instead of being intertwined in a double helical structure.
pBR322 is one of the standard plasmid cloning vectors.
PCR (polymerase chain reaction) describes a tenchnique in which cycles of denaturation, annealing with primer, and extension with DNA polymerase, are used to amplify the number of copies of a target DNA sequence by >106 times.
Perinuclear space lies between the inner and outer membranes of the nuclear envelope.
Periodicity of DNA is the number of base pairs per turn of the double helix.
Permissive conditions allow conditional lethal mutants to survive.
Petite strains of yeast lack mitochondrial function.