AP Bio U7

Operon

A cluster of functionally related genes can be under coordinated control by a single "on-off switch."

Operator

The DNA segment that acts as the "switch" for an operon, usually located within the promoter.

Repressor

A protein that inhibits gene transcription by binding to the operator and blocking RNA polymerase.

Corepressor

A small molecule that cooperates with a repressor protein to switch an operon off (e.g., Tryptophan).

Inducer

A specific small molecule that inactivates the repressor in an inducible operon (e.g., Allolactose).

Repressible Operon

An operon that is usually "on" but can be inhibited; typically used in anabolic (biosynthetic) pathways.

Inducible Operon

An operon that is usually "off" but can be stimulated; typically used in catabolic (digestive) pathways.

cAMP & CAP

Positive regulation system where cyclic AMP activates Catabolite Activator Protein to "crank up" transcription when glucose is low.

Differential Gene Expression

The expression of different sets of genes by cells with the same genome; the basis for cell specialization.

Histone Acetylation

The attachment of acetyl groups to histone tails, which opens up chromatin and promotes transcription.

DNA Methylation

The addition of methyl groups to certain bases in DNA, usually associated with reduced transcription and gene silencing.

Epigenetic Inheritance

Inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence (e.g., chromatin modifications).

Enhancers

Distal control elements located far from the promoter that bind activator proteins to stimulate high levels of transcription.

Alternative RNA Splicing

A process where different mRNA molecules are produced from the same primary transcript, depending on which segments are treated as exons.

miRNAs (microRNAs)

Small, single-stranded RNA molecules that bind to complementary sequences in mRNA molecules to degrade them or block translation.

siRNAs (small interfering RNAs)

Small RNAs similar to miRNAs that associate with the same proteins; they can block gene expression through RNA interference (RNAi).

RNA Interference (RNAi)

The experimental or natural process of disabling gene expression by using siRNA to target specific mRNA.

Cytoplasmic Determinants

Maternal substances (mRNAs/proteins) in the egg that influence the course of early development by being distributed unevenly during cleavage.

Induction

A process in which signals from neighboring embryonic cells cause changes in target cells, directing them toward a specific developmental path.

Determination

The progressive restriction of developmental potential whereby the cell is "vowed" or committed to its final fate.

MyoD

A "master regulatory gene" that produces a transcription factor that commits a cell to becoming a skeletal muscle cell.

Morphogens

Substance (like the Bicoid protein) that provides positional information in the form of a concentration gradient, establishing an embryo's axes.

Oncogenes

Cancer-causing genes; they are mutated versions of normal genes that stimulate growth.

Proto-oncogenes

Normal cellular genes that code for proteins that stimulate normal cell growth and division.

Tumor-Suppressor Genes

Genes whose normal products inhibit cell division; mutations in these can lead to uncontrolled cell growth.

Ras Protein

A G protein that, when mutated, can trigger a kinase cascade even in the absence of a growth factor, leading to increased cell division.

p53 Gene

The "guardian angel of the genome"; a tumor-suppressor gene that repairs DNA or triggers apoptosis if damage is irreparable.

Acrosomal Reaction

The discharge of hydrolytic enzymes from the sperm's tip that digests the egg's jelly coat to allow penetration.

Cortical Reaction

The release of calcium ions in the egg that triggers the hardening of the fertilization envelope to prevent polyspermy.

Polyspermy

The entry of multiple sperm nuclei into an egg; prevented by fast and slow blocks to ensure a normal diploid embryo.

Cleavage

A period of rapid cell division without growth following fertilization.

Blastula

A hollow ball of cells marking the end of the cleavage stage in many animals.

Gastrulation

The process by which the blastula folds inward, rearranging into a three-layered embryo called a gastrula.

Ectoderm

The outermost germ layer; gives rise to the epidermis and the nervous system.

Mesoderm

The middle germ layer; gives rise to muscles, the skeleton, and the circulatory system.

Endoderm

The innermost germ layer; gives rise to the lining of the digestive tract and organs like the liver and lungs.

Neurulation

The process where the notochord induces the ectoderm to form the neural tube, which becomes the brain and spinal cord.

Ciliary Motion

Movement of tiny hairs in the embryo that generates fluid flow to break symmetry and establish the left-right axis.