Cellular Respiration and Mendelian Genetics

Krebs Cycle

A cyclic series of biochemical redox reactions occurring in the mitochondrial matrix that oxidizes acetyl-CoA into carbon dioxide ($CO_2$) while producing $NADH$, $FADH_2$, and $GTP$ (or $ATP$); also known as the citric acid cycle or tricarboxylic acid cycle.

Hans Krebs

The German-British scientist who discovered the citric acid cycle in the 1930s, after whom the metabolic pathway is named.

Transition Reaction

The intermediate stage between glycolysis and the Krebs cycle wherein pyruvate is oxidized to a two-carbon acetate fragment, decarboxylated to release $CO_2$, coupled with $NAD^+$ reduction to $NADH$, and joined to coenzyme A to form acetyl-CoA.

Pyruvate Dehydrogenase

The multi-enzyme complex that catalyzes the transition reaction, converting pyruvate into acetyl-CoA, carbon dioxide, and $NADH$.

Citrate Synthase

The enzyme that catalyzes the initial condensation reaction of the Krebs cycle, joining two-carbon acetyl-CoA with four-carbon oxaloacetate to yield six-carbon citric acid (citrate).

Aconitase

The enzyme that catalyzes the isomerization of citrate into isocitrate through the rearrangement of its atoms.

Isocitrate Dehydrogenase

The enzyme that catalyzes the oxidative decarboxylation of isocitrate to five-carbon $\alpha$-ketoglutarate, releasing $CO_2$ and reducing $NAD^+$ to $NADH$.

$\alpha$-Ketoglutarate Dehydrogenase

The enzyme that catalyzes the oxidation of $\alpha$-ketoglutarate into succinyl-CoA, releasing a molecule of $CO_2$ and reducing $NAD^+$ to $NADH$.

Succinyl-CoA Synthetase

The enzyme that catalyzes the removal of coenzyme A from succinyl-CoA and couples it with substrate-level phosphorylation to synthesize $GTP$ from $GDP$ and inorganic phosphate, yielding succinate.

Succinic Dehydrogenase

The enzyme that oxidizes four-carbon succinate to four-carbon fumarate while reducing $FAD$ to $FADH_2$; it also functions as Complex II in the electron transport chain.

Fumarase

The enzyme that catalyzes the stereospecific addition of a water molecule ($H_2O$) across the double bond of fumarate to form malate.

Malate Dehydrogenase

The enzyme that catalyzes the final oxidation step of the Krebs cycle, converting malate to oxaloacetate while reducing $NAD^+$ to $NADH$.

Oxaloacetate

A four-carbon molecule that combines with acetyl-CoA to initiate the Krebs cycle and is regenerated at the end of the pathway to allow continued cycles.

Genetics

The branch of biological science that systematically focuses on heredity and genetic variation among living organisms.

Heredity

The biological process, also termed biological inheritance, involving the transmission of traits and genetic information from parent generations to offspring.

Transmission Genetics

The oldest subfield of genetics—often called classical or Mendelian genetics—that investigates the patterns and mechanisms by which traits are transmitted between generations.

Molecular Genetics

The specialized field of genetics concerned with the biochemical nature of $DNA$ as well as the regulation and expression of genes at the molecular level.

Cytogenetics

The subdiscipline of genetics dedicated to the microscopic and molecular analysis of chromosome structure, function, and behavior during meiotic cell division.

Population Genetics

The subdiscipline of genetics that studies how evolutionary forces influence the frequencies and distributions of genes and alleles at the population level.

Gregor Johann Mendel

An Austrian monk born in 1822 in Moravia who discovered the fundamental principles of inheritance through eight years of hybridization experiments with the garden pea.

Pisum sativum

The garden pea; a legume selected by Gregor Mendel as a model organism due to its short life cycle, easily manipulated pollination, and distinct contrasting traits.

Pangenesis

An ancient, debunked theory of heredity proposed by Hippocrates in $400\text{ B.C.E.}$ stating that tiny 'seeds' produced across all bodily organs assemble to form offspring.

Homunculus

A historical, disproven concept developed after the invention of the microscope suggesting that a fully formed 'little man' resided inside individual sperm cells.

Blending Theory of Inheritance

A historical, superseded hypothesis positing that hereditary traits from both parents contribute equally and permanently mix together in offspring, irreversibly diluting variation.

Gene Locus

The specific, defined physical location or position that a given gene occupies along the length of a chromosome.

Allele

An alternative structural or nucleotide sequence variant of a specific gene found at a given chromosomal locus.

Homozygous

The state of possessing two identical alleles at a specific gene locus on homologous chromosomes.

Heterozygous

The condition of possessing two different, non-identical alleles at a specific gene locus on homologous chromosomes.

Genotype

The underlying genetic and allelic constitution of an individual organism at a specific locus or set of loci.

Phenotype

The observable physical, anatomical, physiological, or biochemical manifestation of an organism resulting from the interaction of its genotype.

Principle of Dominance

Mendel's concept stating that in a heterozygous organism