Cell Biology Terms Starting With G
Cell Biology Glossary: G
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G Protein
/ jee PROH-teen / · G: GTP-binding + protein
G protein is a guanine nucleotide-binding protein that relays signals from activated cell-surface receptors to intracellular effectors by cycling between inactive GDP-bound and active GTP-bound states.
G proteins act as molecular switches, remaining inactive when bound to GDP and active when bound to GTP. Activated G protein-coupled receptors catalyze GDP release from the alpha subunit, allowing GTP to bind and the alpha subunit to dissociate from the beta-gamma dimer. Both the G?-GTP complex and the free G??
dimer can then activate downstream effectors such as adenylyl cyclase, phospholipase C, or ion channels, depending on the specific G protein subtype. GTPase-activating proteins accelerate the intrinsic GTPase activity of G? subunits, hydrolyzing GTP back to GDP within seconds to minutes and returning the protein to its inactive state.
Alfred Gilman and Martin Rodbell shared the 1994 Nobel Prize in Physiology or Medicine for discovering G proteins and their role in cellular signal transduction. Their work revealed that cholera toxin causes prolonged diarrhea by locking the Gs alpha subunit in its active GTP-bound state, preventing GTPase activity and driving continuous fluid secretion.
G proteins are one single protein type. More than 20 distinct G? subunits exist in humans, each coupling to different receptors and effectors to produce distinct cellular responses.
In rod cells of the eye, the G protein transducin couples activated rhodopsin to a phosphodiesterase that hydrolyzes cyclic GMP, dropping cytoplasmic cGMP levels and closing ion channels within milliseconds to generate a visual signal. A single activated rhodopsin molecule can activate roughly 500 transducin molecules in this amplification cascade, making the pathway sensitive enough to detect a single photon.
G1 Phase
/ jee-WUN fayz / · G: gap + 1 + phase
G1 phase is the first gap stage of the cell cycle, occurring after cell division and before DNA replication, during which the cell grows, synthesizes proteins, and evaluates conditions before committing to division.
G1 phase duration varies from hours in rapidly dividing cells to years in quiescent cells, with length controlled by checkpoint mechanisms monitoring nutrient availability and cell size. Cyclin D accumulates during G1 and activates cyclin-dependent kinases 4 and 6, which phosphorylate the retinoblastoma protein and release E2F transcription factors that drive S phase gene expression. The G1-to-S checkpoint, sometimes called the restriction point in mammalian cells, requires sufficient cell mass, intact mitochondrial function, and the absence of DNA damage signals before committing to DNA replication.
Cells experiencing stress, growth factor withdrawal, or contact inhibition exit into G0 phase, a quiescent state where they maintain normal metabolic functions but do not divide.
Neurons in the adult human brain are among the most extreme examples of G0 arrest, remaining outside the cell cycle for decades. Most mature neurons exit G1 permanently during embryonic development and never re-enter the cycle under normal conditions.
Cell Cycle →G1 is an empty gap with no activity. Cells in G1 actively grow, synthesize proteins, monitor nutrient levels, and respond to external signals that determine whether division proceeds.
In mammalian skin keratinocytes, the G1 checkpoint integrates signals from growth factors such as epidermal growth factor, and cells deprived of EGF arrest in G1 within 8 to 12 hours, halting proliferation until the wound-healing signal is restored. Restriction point passage in human HeLa cells occurs approximately 2 hours before S-phase entry, after which cells complete division even if growth factors are withdrawn. The cyclin D-CDK4/6 complex drives this commitment by phosphorylating the retinoblastoma protein across more than a dozen sites over a 4-hour window.
G2 Phase
/ jee-TOO fayz / · G: gap + 2 + phase
G2 phase is the stage of the cell cycle that follows DNA replication and precedes mitosis, during which the cell verifies replication fidelity, repairs damage, and assembles the machinery needed for division.
G2 phase typically lasts 2 to 5 hours in mammalian cells and involves accumulation of cyclin B-CDK1 complexes that drive mitotic entry through phosphorylation of lamin and condensin proteins. DNA damage checkpoint mechanisms activate ATM and ATR kinases, which phosphorylate p53 and trigger p21 expression to inhibit cyclin B-CDK1 activity and pause the cycle. Centrosome duplication completes during G2, and microtubule nucleation capacity increases through centrosomal accumulation of the NEDD1 protein.
When DNA damage persists or replication errors remain unresolved, cells can arrest indefinitely in G2 or initiate apoptosis through p53-dependent mechanisms.
Some organisms bypass G2 entirely. Early embryos of the fruit fly (Drosophila melanogaster) skip both G1 and G2 during the first 13 rapid cleavage divisions after fertilization, cycling directly between S phase and mitosis to divide the egg into thousands of nuclei within about two hours.
Cell Cycle →Cells enter mitosis immediately after copying DNA. The G2 checkpoint actively monitors replication completeness and DNA integrity, and can pause or permanently arrest the cycle when problems are detected.
In dividing sea urchin (Strongylocentrotus purpuratus) embryos, G2 is extremely brief during early cleavage divisions, lasting under 15 minutes, because maternal cyclin B stockpiles activate CDK1 without requiring new transcription. Later in development, after the mid-blastula transition, G2 lengthens to several hours as the embryo switches to zygotic transcription and begins activating the DNA damage checkpoint machinery. Irradiating sea urchin embryos with 100 Gray of X-rays at this stage extends G2 by over 60 minutes, measurable by flow cytometry.
Gap Junction
/ GAP JUNK-shun / · Old English gap, opening; Latin junctio, joining
Gap junction is a protein channel complex that directly connects the cytoplasm of two adjacent animal cells, allowing ions and molecules smaller than about 1,000 daltons to pass between them without entering the extracellular space.
Gap junctions form when connexin protein subunits from two neighboring cells each assemble into a hexameric hemichannel called a connexon; the two connexons from adjacent cells dock end-to-end to create a continuous pore roughly 1.5 nanometers wide. This pore diameter permits passage of ions such as potassium and calcium, as well as small signaling molecules like inositol 1,4,5-trisphosphate, while excluding larger proteins and nucleic acids. Channel conductivity is regulated by voltage changes, phosphorylation events, and pH shifts that cause conformational changes in the connexin subunits, opening or closing the pore within milliseconds.
The human genome encodes at least 21 distinct connexin proteins, and mutations in the gene encoding connexin 26 are among the most common causes of hereditary hearing loss worldwide.
Invertebrates form functionally equivalent cell-cell channels using an entirely different protein family called innexins, and a related family called pannexins is expressed in vertebrate cells as well. Despite performing similar communication roles, innexins and connexins share no significant sequence similarity, representing a striking case of convergent molecular evolution.
Differences Between Plant and Animal Cells →Gap junctions are not simple open pores between cells. Each connexon channel is gated by voltage, cytoplasmic calcium concentration, and pH, closing within milliseconds when intracellular calcium rises above roughly 500 nanomolar , a mechanism that electrically isolates damaged cells from their healthy neighbors.
In cardiac muscle tissue, connexin 43 channels at intercalated discs allow electrical depolarization to spread from cell to cell at approximately 0.3 to 1 meter per second, enabling the coordinated contraction that pumps roughly 70 milliliters of blood per beat. Each intercalated disc cluster contains hundreds to thousands of connexin 43 channels grouped in plaques averaging 0.4 micrometers in diameter. Knockout mice lacking connexin 43 develop severe cardiac arrhythmias and die within hours of birth, demonstrating that gap-junction-mediated synchrony is essential for cardiac function.
Golgi Apparatus
/ GOL-jee ap-uh-RAY-tus / · Named for Camillo Golgi (1898)
Golgi apparatus is a stack of flattened membrane-bound sacs in eukaryotic cells that receives proteins and lipids from the endoplasmic reticulum, chemically modifies them, and sorts them for delivery to their correct destinations.
The Golgi apparatus contains 4 to 8 flattened cisternae arranged in a cis-to-trans polarity, receiving cargo from the endoplasmic reticulum in vesicles approximately 50 to 100 nanometers in diameter. Resident enzymes including sulfotransferases, galactosyltransferases, and proteases modify proteins and lipids with stage-specific precision as cargo moves through successive cisternae. Clathrin-coated vesicles and COPI vesicles mediate anterograde transport toward the trans-Golgi network and retrograde recycling of Golgi-resident proteins back to earlier cisternae.
At the trans-Golgi network, modified proteins are sorted into constitutive secretory vesicles, regulated secretory granules, or endosomal compartments based on sequence signals and protein modifications.
Plant cells contain a dispersed Golgi system rather than a single perinuclear stack. A typical plant cell, such as one from the root tip of Arabidopsis thaliana, can harbor several hundred individual Golgi stacks scattered throughout the cytoplasm, each operating independently to process and sort cargo.
The Golgi makes all proteins from scratch. Proteins are synthesized by ribosomes on the endoplasmic reticulum, and the Golgi then processes and sorts them rather than producing them.
In mucus-secreting goblet cells lining the human intestine, the Golgi apparatus packages heavily glycosylated mucin glycoproteins into secretory vesicles at a rate sufficient to replace the entire mucus layer every 4 to 6 hours. Each goblet cell Golgi stack processes roughly 10 million mucin molecules per hour, adding O-linked oligosaccharide chains averaging 50 sugar residues each through a series of glycosyltransferases distributed across the medial and trans cisternae. The trans-Golgi network then sorts these bulky cargo molecules into condensing vesicles that fuse with the apical membrane upon appropriate stimulation.
Goblet Cells →