Cell Communication 10 21 2013 Cell to cell communication is essential for multicellular organisms The combined effects of multiple signals determine cell response Signal transduction pathway a series of steps by which a signal on a cell s surface is converted into a specific cellular response Local and Long Distance Signaling Cells receiving signals go through three processes Cells in a multicellular organism communicate by chemical messengers Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent In local signaling animal cells may communicate by direct contact or cell cell recognition In many other cases animal cells communicate using messenger molecules that travel only In long distance signaling plants and animals use chemicals called hormones cells short distances Reception the binding between a signal molecule ligand and receptor is highly specific o The receptor changes shape which is often the initial transduction of the signal o Most signal receptors are plasma membrane proteins o Most water soluble polar signal molecules bind to specific sites on receptor proteins in the plasma membrane These ligands cannot cross the cell membrane o There are three main types of membrane receptors G protein coupled receptors a plasma membrane receptor that works with the help of a G protein The 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 tyrosines Can trigger signal transduction pathways at once Ligand gated ion channel acts as a gate when the receptor changes When 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 Ion channel receptors shape receptor Intracellular receptors Small or hydrophobic ligands have intracellular receptors found in the cytosol or nucleus of target cells Examples of hydrophobic messengers are the steroid and thyroid hormones of animals An activated hormone receptor complex can act as a transcription Transduction usually involves multiple steps factor turning on specific genes o Can amplify a signal a few molecules can produce a large cellular response o Can provide more opportunities for regulation of cellular response o Like falling dominoes the receptor activates another protein which activates another and so on until the protein producing the response is activated o At each step the signal is transduced into a different form usually a shape change in o In many pathways the signal is transmitted by a cascade of protein a protein phosphorylations o Protein kinases transfer phosphates from ATP to protein a process called phosphorylation Turns the protein ON o Protein phosphatases remove the phosphates from proteins a process called de phosphorylation Turns the protein OFF o The extracellular signal molecule ligand that binds to the receptor is a pathways o Second messengers small non protein water soluble molecues or ions that spread first messenger throughout a cell by diffusion o Cyclic AMP and calcium ions are common second messengers Cyclic AMP cAMP is one of the most widely used second messengers Adenylyl cyclase an enzyme in the plasma membrane converts ATP to cAMP in response to an extracellular signal Calcium ions Ca2 act as a second messenger in many pathways Calcium is an important second messenger because cells can regulate its concentration Response Nuclear and Cytoplasmic responses o Ultimately a signal transduction pathway leads to regulation of one or more cellular activities o The response may occur in the cytoplasm or may involve action in the nucleus The Cell Cycle 10 21 2013 Multicellular organisms depend on cell division for Development from a zygote Growth Repair Cell division is an integral part of the cell cycle the life of a cell from formation to its own division Most cell division mitosis results in daughter cells with identical DNA Meiosis produces non identical daughter cells gametes sperm and egg cells All the DNA in a cell constitutes the cell s genome DNA molecules in a cell are packaged into chromosomes Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus Somatic cells non reproductive cells have two sets of chromosomes one from each Gametes reproductive cells sperm and eggs have half as many chromosomes as somatic Eukaryotic chromosomes consist of chromatin a complex DNA and protein that condenses In preparation for cell division DNA is replicated and the chromosomes condense Each replicated chromosome has two sister chromatids which separate during cell division The centromere of the replicated chromosome is where the two chromatids are most parent cells curing cell division closely attached Eukaryotic cell division consists of Mitosis the division of the nucleus Cytokinesis the division of the cytoplasm The cell cycle consists of Mitotic M phase the dividing phase mitosis and cytokinesis o Mitosis is conventionally divided into five phases Prophase Assembly of spindle microtubules begins in the centrosome the microtubule organizing center The centrosome replicates forming two centrosomes that migrate to opposite ends of the cell as spindle microtubules grow out from Some spindle mictrotubules attach to the kinetochores of the chromosomes and begin to move the chromosomes The chromosomes are all lined up at the metaphase plate the midway point between the spindle s two poles Sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell Non kinetochore microtubules from opposite poles overlap and push against each other elongating the cell Genetically identical daughter nuclei form at opposite ends of the them Prometaphase Metaphase Anaphase Telophase cell during mitosis furrow cells o Mitotic spindle apparatus of microtubules that controls chromosome movement o Cytokinesis is well underway by late telophase Animal cells occurs by a process known as cleavage forming a cleavage Plant cells a cell plate forms which will form the cells walls of the daughter Interphase the non dividing phase cell growth and copying of chromosomes in preparation for cell division o Interphase about 90 of the cell cyle can be divided into subphases G1 phase first gap S phase synthesis G2 phase second gap o The cell grows during all three phases but chromosomes are
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