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A substrate-recognition component of a ubiquitin ligase that recognizes phosphorylated substrates, including Sic1.

A component of the active eukaryotic CMG replicative helicase.

A licensing factor that helps load MCM2-7 onto origin DNA before helicase activation.

The cell cortex is a peripheral layer of cytoplasm beneath the cell membrane, containing the actomyosin ring during typical animal cytokinesis.

A specialized contact connecting neighbouring biological cells or connecting a cell to its surroundings.

The account that organisms are organized from biological cells and that cellular continuity is fundamental to their growth and reproduction. Its nineteenth-century formulation developed alongside changing accounts of cell division.

Cell differentiation is the acquisition of a specialized cell state, involving changes in structure, function and gene expression. A neuron and a red blood cell illustrate different outcomes: signaling and oxygen transport. Different functions usually do not require a different inherited DNA sequence for each function. The shared-genome principle concerns nucleated precursors and most nucleated descendants: a mature mammalian red blood cell has lost its cell nucleus and mitochondria, so it is not an example of two mature biological cells both retaining the same nuclear DNA. ^[ref]

The puzzle is how a shared set of genes supports different jobs. Gene expression changes which RNA and proteins a biological cell produces, and in what amounts. A transcription factor can regulate several genes; different combinations of regulators can interpret the same signal differently. Stable regulatory patterns can persist through the cell cycle, providing memory of developmental history. Specialization therefore usually changes use of inherited information, rather than replacing the genome wholesale. ^[ref]

A small counting example separates sequence from activity. Suppose the same three toy genes are present in every nucleated biological cell, and independently assign each gene an on/off state. There are $2^3=8$ possible expression profiles with no DNA replacement. The figure shows only two: H and A on versus H and B on. Real gene expression is graded and coordinated, so this count illustrates possible descriptions, not a prediction of eight actual cell types. The interesting consequence is that a stable cell state can depend on how a shared record is used.

Stem cells combine self-renewal with production of differentiated descendants; progenitor cells generally have more restricted developmental options and renewal capacity. Neither term means that a biological cell must give rise to every cell type. Terminal differentiation denotes advanced specialization within a lineage, often accompanied by exit from the cell cycle. Cell fate concerns the developmental outcome of a biological cell or its descendants, whereas cell state describes its current properties. A temporary change in gene expression after signal transduction is not, by itself, proof of a new cell fate. ^[ref]

A microtubule-organizing structure in relevant animal cells. Positioning centrosomes relative to a niche can prepare the orientation of a later mitotic spindle. Budding yeast instead has a spindle pole body.

Methylation in the symmetric CG sequence context.

Chemiosmosis couples transmembrane ion movement down an electrochemical gradient to a chemical transformation. For mitochondrial ATP synthesis, protons return from the intermembrane space to the mitochondrial matrix through ATP synthase.^[ref]

An original thermodynamic example separates the electrical and chemical contributions. At fixed temperature $T$, use ideal dilute thermodynamic activities $a$ and the electric potential $\psi$ on each side. The molar electrochemical potential difference for inward transfer of a singly charged proton isHere $R$ is the molar gas constant, $F$ the Faraday constant, and $\ln$ the natural logarithm. The sign convention is destination minus origin: a negative difference makes inward transfer favourable. Both lower proton activity and a more negative electric potential inside favour inward movement.

Stipulate $T=300\mathrm{K}$, $a_{\mathrm{matrix}}/a_{\mathrm{outside}}=0.1$, ${\psi }_{\mathrm{matrix}}-{\psi }_{\mathrm{outside}}=-0.12\mathrm{V}$, $R=8.314\mathrm{J}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-1}$ and $F=96485\mathrm{C}\mathrm{mo}{\mathrm{l}}^{-1}$. These illustrative inputs give approximately $-5.743$ and $-11.578\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ for the chemical and electrical terms, hence $-17.321\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$ per transferred proton. For a hypothetical device coupling three inward protons per ATP to an ATP-formation cost of $45\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$,The sign follows by adding changes in the state function Gibbs free energy. It establishes thermodynamic feasibility for this chosen model, not its reaction rate. Three is a stipulated coupling count, not a universal ATP synthase stoichiometry; transport costs and leaks can also consume the available free energy.

The space between the two chloroplast envelope membranes.

A cellular projection whose structure and functions differ from the bacterial flagellar apparatus. Motile cilia can move fluid; a transport estimate must compare advection with diffusion.

A mutation affecting a house mouse circadian clock.

A crista is a fold of the inner mitochondrial membrane into the mitochondrial matrix. Its membrane remains continuous with the surrounding inner mitochondrial membrane; it is not a separately inserted sac like a thylakoid.^[ref]

The diagram idealizes access through the fold's opening. Connectivity alone does not imply instant mixing or identical concentrations throughout the intermembrane space. Narrow geometry and molecular interactions can matter to transport. Increasing surface area also does not, by itself, prove a proportional increase in ATP production: the supply of substrates, available oxygen and the state of the coupling machinery must also support the flux.

An electron carrier protein in the mitochondrial respiratory chain.