Genetics Chapter 7

Point mutation

a mutation of one or a few base pairs, or a small deletion or insertion in DNA.

Allele

alternative forms of a single gene.

Forward mutation

a mutation that changes a wild-type allele of a gene to a different allele.

Reverse mutation

a mutation that causes a mutant to change back to wild type. Synonymous with reversion.

Germline mutation

a genetic change in a reproductive cell (egg or sperm) that is passed from a parent to a child during conception and becomes present in every cell of the offspring's body.

Somatic mutation

any genetic alteration that occurs in a non-germline cell (any cell of the body except sperm and egg cells) after fertilization

Tautomer

similar chemical forms of molecules, such as bases in DNA, that interconvert.

Transition

A base substitution mutation in which a purine is replaced by another purine (A $\leftrightarrow$ G), or a pyrimidine is replaced by another pyrimidine (C $\leftrightarrow$ T).

Transversion

A base substitution mutation in which a purine is replaced by a pyrimidine, or a pyrimidine is replaced by a purine (i.e., A or G $\leftrightarrow$ C or T).

Slipped mispairing

slippage of one DNA strand relative to the other during DNA replication through repeat regions. This process, also called stuttering, can result in expansion or contraction of SSR repeat numbers.

Trinucleotide repeat disease

A hereditary disorder caused by the unstable expansion of a three-nucleotide repeat sequence beyond its normal threshold, which disrupts normal gene expression or function.

Intercalating agent

A class of chemical mutagens composed of flat, planar molecules that can sandwich themselves between successive base pairs, disrupt the machinery of DNA replication, and thereby cause mutation (typically insertions or deletions leading to frameshifts).

Methyl-directed mismatch repair

a DNA repair mechanism that corrects mistakes in replication, discriminating between newly synthesized and parental DNA by the methyl groups on the parental strand.

Thymine dimers

covalent linkage between adjacent thymine residues in DNA that can cause mutation.

Base excision repair

A homology-dependent DNA repair mechanism that removes damaged or inappropriate single bases (such as uracil produced by cytosine deamination, or oxidized bases like 8-oxoguanine).

Nucleotide excision repair

A DNA repair pathway that removes bulky lesions and helix-distorting damage—such as UV-induced thymine dimers—by cleaving the damaged strand on both sides of the lesion, excising an oligonucleotide fragment containing the damage, and resynthesizing the missing DNA using the undamaged complementary strand as a template.

Mismatch repair

A DNA repair pathway that recognizes and corrects base-pair mispairings and small insertion/deletion loops that escape the 3′-to-5′ proofreading activity of DNA polymerase during DNA replication. It specifically identifies and excises the mistake on the newly synthesized daughter strand using the parental strand as the accurate template.

Nonhomologous end joining

mechanism for stitching back together ends formed by double-strand breaks. NHEJ relies on proteins that bind to the ends of the broken DNA strands and bring them close together.

Double strand break repair

Cellular mechanisms that rejoin broken DNA molecules after both strands of the double helix have been severed, either through: Homologous recombination (HR) or Nonhomologous end-joining (NHEJ).

Bacterial colony

A mound of genetically identical cells that all descend from a single cell.

Minimal media

A simple, chemically defined growth medium containing only the basic inorganic salts, a simple carbon source (such as glucose), and water. It provides only the bare minimum nutrients required for wild-type organisms (prototrophs) to survive, and cannot support the growth of auxotrophic mutants unless specifically supplemented.

Complete media

A rich, nutrient-filled growth medium that supplies all the nutritional requirements (such as amino acids, vitamins, and nucleotides) needed for both wild-type strains and auxotrophic mutants to grow.

Auxotroph

a mutant microorganism that can grow on minimal medium only if it has been supplemented with one or more nutrients not required by wild-type strains.

Prototroph

a microorganism (usually wild type) that can grow on minimal medium.

Homologous recombination

or crossing-over; during meiosis, the breaking and rejoining of nonsister chromatids, resulting in the exchange of corresponding sections of DNA. Also during repair of broken chromosomes, sister chromatids can cross over.

Revertant

An organism, cell, or strain in which a mutant phenotype has been reversed back to the wild-type phenotype, typically through a secondary mutation (either a true back-mutation at the original site or a compensatory suppressor mutation).

Substitution

a mutation that occurs when a base pair in a DNA molecule is replaced by one of the other three base pairs.

Insertion

the addition to a DNA molecule of one or more nucleotide pairs.

Deletion

the loss of a block of one or more nucleotide pairs from a DNA molecule.

Endonuclease

n enzyme that cleaves the phosphodiester bonds within a polynucleotide chain (DNA or RNA), as opposed to chewing in from the ends like an exonuclease.

Exonuclease

enzyme that removes nucleotides from an end of a DNA molecule.

Fluctuation test

The Luria-Delbrück experiment to determine the origin of bacterial resistance. Fluctuations in the numbers of resistant colonies growing in different petri plates showed that resistance is not caused by exposure to bactericides.

Ames test

screen for chemicals that cause mutations in bacterial cells.

Pre-mutation allele

a normally functioning allele of an unstable trinucleotide repeat gene with a repeat region expanded to the point where further expansion to a disease allele occurs at high frequency.

Carcinogens

a substance that causes cancer; a mutagen.

Genetic screen

a process whereby researchers examine a large number of organisms and identify rare individuals with a mutant phenotype of interest.

Genetic selection

a process that establishes conditions in which only the desired mutant will grow.