A weak electrostatic attraction between a hydrogen atom bonded to N, O, or F and another N, O, or F atom. Individual H-bonds are weak (~5% of covalent bond), but collectively they give water its unique properties

The oxygen atom has a stronger electronegativity than hydrogen, creating a polar covalent bond with partial charges (delta- on O, delta+ on H). This polarity makes water an excellent solvent ("universal solvent")

Non-membrane-bound organelles that synthesize proteins. Composed of rRNA and proteins; consists of a small and large subunit. Free ribosomes synthesize proteins for use within the cell; bound ribosomes (on rough ER) synthesize proteins for secretion or membrane insertion

Rough ER: studded with ribosomes; synthesizes and processes proteins destined for secretion or membrane insertion. Smooth ER: lacks ribosomes; synthesizes lipids (including steroids), detoxifies harmful substances, stores calcium. Both are continuous with the nuclear envelope

The specific region of an enzyme where the substrate binds. Formed by amino acid R-groups that create a 3D pocket. The active site has a specific shape and chemical environment complementary to its substrate (lock-and-key or induced fit). Catalytic residues lower activation energy

Enzymes bind specific substrates because of the precise 3D shape and charge distribution of the active site. Determined by the enzyme's primary structure (amino acid sequence). Cofactors (metal ions, vitamins) may assist substrate binding. Different enzymes can act on similar substrates (e.g., digestive proteases)

Direct cell-to-cell contact signaling. A membrane-bound signal on one cell binds to a receptor on an adjacent cell. Requires physical contact between cells. Examples: immune cell interactions, embryonic development (Notch pathway), gap junctions (ions and small molecules pass directly between cells)

Cells release signaling molecules into the extracellular fluid that affect nearby target cells. The signal acts locally over a short distance. Examples: growth factors during development, neurotransmitter diffusion across synapses. Requires no direct cell-cell contact

Meiosis I (reductional division): homologous chromosome pairs separate -> daughter cells are haploid (each with 1 set of chromosomes). Sister chromatids remain attached. Meiosis II (equational division): sister chromatids separate -> produces 4 haploid gametes genetically distinct from each other and from the parent cell

Pairs of chromosomes with the same genes at the same loci (one from each parent). Humans have 23 pairs (22 autosome pairs + 1 sex chromosome pair). Homologs are similar in size and centromere position. They pair during prophase I of meiosis (synapsis), forming a tetrad

Both are 5-carbon pentose sugars. Deoxyribose (DNA): missing the 2'-OH group (has only H at 2' position) -> makes DNA more stable. Ribose (RNA): has a hydroxyl group (OH) at the 2' position -> makes RNA less stable, more reactive, and susceptible to hydrolysis. The 2'-OH is why RNA is single-stranded and DNA is double-stranded

In DNA: adenine (A) pairs with thymine (T) via 2 hydrogen bonds; guanine (G) pairs with cytosine (C) via 3 hydrogen bonds. In RNA: adenine pairs with uracil (U); guanine pairs with cytosine. These rules are complementary: if one strand is 5'-ATGC-3', the other is 3'-TACG-5'. Chargaff's rules: [A]=[T] and [G]=[C] in double-stranded DNA

The theory of evolution by natural selection (Darwin & Wallace, 1858). Key components: individuals in a population vary; some variations are heritable; more offspring are produced than can survive (struggle for existence); individuals with favorable variations survive and reproduce (survival of the fittest); favorable traits become more common over generations (descent with modification)

The number of viable offspring an individual contributes to the next generation. "Fitness" in evolutionary biology = reproductive success, not physical strength. An individual can be "fit" by reproducing more or by helping relatives reproduce (kin selection, inclusive fitness - Hamilton's rule: rB > C). Fitness is relative to the environment and other individuals in the population

Internal biological clocks with a ~24-hour cycle. Regulated by the suprachiasmatic nucleus (SCN) in the hypothalamus. Affect sleep-wake cycles, hormone release, body temperature, metabolism. Entrained (synchronized) by light-dark cycles. Examples: sleep cycles, melatonin secretion, plant leaf movements. Disruption causes jet lag, shift work disorders

The physiological response to the length of day (photoperiod). Many organisms use day length as a cue for seasonal behaviors: flowering in plants (photoperiodism -> long-day/short-day plants), bird migration, animal reproduction cycles, dormancy. Involves phytochrome (plants) and melatonin pathways. Allows organisms to anticipate seasonal changes