Cell-to-cell communication is essential for multicellular organismsThe combined effects of multiple signals determine cell responseSignal transduction pathway: a series of steps by which a signal on a cell’s surface is converted into a specific cellular responseLocal and Long Distance SignalingCells in a multicellular organism communicate by chemical messengersAnimal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cellsIn local signaling, animal cells may communicate by direct contact or cell-cell recognitionIn many other cases, animal cells communicate using messenger molecules that travel only short distancesIn long distance signaling, plants and animals use chemicals called hormonesCells receiving signals go through three processes:Reception: the binding between a signal molecule (ligand) and receptor is highly specificThe receptor changes shape, which is often the initial transduction of the signalMost signal receptors are plasma membrane proteinsMost water-soluble (polar) signal molecules bind to specific sites on receptor proteins in the plasma membrane. These ligands cannot cross the cell membraneThere are three main types of membrane receptors:G protein coupled receptors: a plasma membrane receptor that works with the help of a G proteinThe G protein acts as an on/off switch: If GDP is bound to the G protein, the G protein is inactive. If GTP is bound to the G protein, the G protein is active.Receptor tyrosine kinases: membrane receptors that attach phosphates to amino acids called tyrosinesCan trigger signal transduction pathways at onceIon channel receptorsLigand-gated ion channel: acts as a gate when the receptor changes shapeWhen a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptorIntracellular receptorsSmall or hydrophobic ligands have intracellular receptors, found in the cytosol or nucleus of target cellsExamples of hydrophobic messengers are the steroid and thyroid hormones of animalsAn activated hormone-receptor complex can act as a transcription factor, turning on specific genesTransduction: usually involves multiple stepsCan amplify a signal: a few molecules can produce a large cellular responseCan provide more opportunities for regulation of cellular responseLike falling dominoes, the receptor activates another protein, which activates another, and so on, until the protein producing the response is activatedAt each step the signal is transduced into a different form, usually a shape change in a proteinIn many pathways, the signal is transmitted by a cascade of protein phosphorylationsProtein kinases: transfer phosphates from ATP to protein, a process called phosphorylation. Turns the protein ONProtein phosphatases: remove the phosphates from proteins, a process called de-phosphorylation. Turns the protein OFFThe extracellular signal molecule (ligand) that binds to the receptor is a pathways “first messenger”Second messengers: small, non-protein, water-soluble molecues or ions that spread throughout a cell by diffusionCyclic AMP and calcium ions are common second messengersCyclic AMP (cAMP) is one of the most widely used second messengersAdenylyl cyclase: an enzyme in the plasma membrane, converts ATP to cAMP in response to an extracellular signalCalcium ions (Ca2+) act as a second messenger in many pathwaysCalcium is an important second messenger because cells can regulate its concentrationResponse: Nuclear and Cytoplasmic responsesUltimately a signal transduction pathway leads to regulation of one or more cellular activitiesThe response may occur in the cytoplasm or may involve action in the nucleusMulticellular organisms depend on cell division for:Development from a zygoteGrowthRepairCell division is an integral part of the cell cycle, the life of a cell from formation to its own divisionMost cell division (mitosis) results in daughter cells with identical DNAMeiosis produces non-identical daughter cells (gametes: sperm and egg cells)All the DNA in a cell constitutes the cell’s genomeDNA molecules in a cell are packaged into chromosomesEvery eukaryotic species has a characteristic number of chromosomes in each cell nucleusSomatic cells (non-reproductive cells) have two sets of chromosomes (one from each parent)Gametes (reproductive cells: sperm and eggs): have half as many chromosomes as somatic cellsEukaryotic chromosomes consist of chromatin, a complex DNA and protein that condenses curing cell divisionIn preparation for cell division, DNA is replicated and the chromosomes condenseEach replicated chromosome has two sister chromatids, which separate during cell divisionThe centromere of the replicated chromosome is where the two chromatids are most closely attachedEukaryotic cell division consists of:Mitosis: the division of the nucleusCytokinesis: the division of the cytoplasmThe cell cycle consists of:Mitotic (M) phase: the dividing phase (mitosis and cytokinesis)Mitosis is conventionally divided into five phases:ProphaseAssembly of spindle microtubules begins in the centrosome, the microtubule organizing centerThe centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out from themPrometaphaseSome spindle mictrotubules attach to the kinetochores of the chromosomes and begin to move the chromosomesMetaphaseThe chromosomes are all lined up at the metaphase plate (the midway point between the spindle’s two poles)AnaphaseSister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cellNon-kinetochore microtubules from opposite poles overlap and push against each other, elongating the cellTelophaseGenetically identical daughter nuclei form at opposite ends of the cellMitotic spindle: apparatus of microtubules that controls chromosome movement during mitosisCytokinesis is well underway by late telophaseAnimal cells: occurs by a process known as cleavage, forming a cleavage furrowPlant cells: a cell plate forms, which will form the cells walls of the daughter cellsInterphase: the non-dividing phase (cell growth and copying of chromosomes in preparation for cell division)Interphase (about 90% of the cell cyle) can be divided into subphases:G1 phase: “first gap”S phase: “synthesis”G2 phase: “second gap”The cell grows during all three phases, but chromosomes are replicated only during the S phaseBinary
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