Chapter 3: Cells Flashcards
Cell Theory
- The cell is the smallest unit of life
- All organisms are made of one or cells
- Cells only arise from other cells (mitosis) with the exception of sperm and ovum cells which arise by meiosis
The principle of Complementarity of structure and function
The activities of cells are dictated by their shapes, and by the types and relative numbers of the subcellular structures they contain.
-A cells shape reflects its function
3 main parts to a human cell
- Plasma Membrane
- Cytoplasm
- The nucleus
Plasma membrane
- A main part to a human cell (1/3)
- The outer boundary of the cell, which acts as a selectively permeable barrier
Cytoplasm
- A main part to a human cell (2/3)
- The intracellular fluid packed with organelles
The Nucleus
- A main part to a human cell (3/3)
- An organelle that controls cellular activities
Extracellular Materials
Are substances contributing to body mass found outside the cells.
- This includes:
- Extracellular Fluid (ECF)–interstitial fluid; blood plasma; and cerebrospinal fluid
- Cellular Secretions
- Extracellular Matrix
Extracellular Fluid (ECF)
- ECF dissolves and transports substances in the body.
- Includes interstitial fluid and cerebrospinal fluid
Interstitial Fluid
- Is the fluid in tissues that bathes all of our cells, has endless major roles to play
- think about it like a nutritious soup, containing ingredients like: amino acids, sugars, fatty acids, regulatory substances, and wastes
Cellular Secretions
- Extracellular Materials
- These secretions include substances that aid in digestion (intestinal and gastric fluids) and some that act as lubricants (saliva, mucus, and serous fluids)
Extracellular Matrix
- The most abundant extracellular material
- Composed of proteins and polysaccharides (jelly-like substances)
- Secreted by the cells
- These molecules self assemble into an organized mesh in the extracellular space, where they serve as a universal “CELL GLUE” that helps bind body cells together
- Particularly abundant in connective tissues
Plasma Membrane
- Separates 2 of the body’s major fluid compartments:
- intracellular fluid within cells
- extracellular fluid outside cells
The Fluid Mosaic Model
- Depicts the PLASMA MEMBRANE as a very thin structure composed of a bilayer (double layer) of lipid molecules with protein molecules “plugged into” or dispersed in it.
- The proteins, many of which float in the fluid lipid bilayer, form a constantly changing mosaic pattern
Membrane Lipids
Constructed largely of phospholipids and smaller amounts of cholesterol
Phospholipids
- Membrane lipid
- the polar hydrophilic heads (includes phosphate group) of phospholipids are attracted to water. they lie on the inner and outer surfaces of the membrane
-the nonpolar hydrophobic tails (fatty acids) of phospholipids avoid water and line up in the center of the membrane
Cholesterol
- Membrane lipid
- Makes up 20% of membrane lipid
- Has a polar region (hydroxyl group) and a nonpolar region (its fused ring system)
- Wedges its platelike hydrocarbon rings between the phospholipid tails, which stiffens the membrane
- Increases membrane stability and fluidity
Membrane Proteins
-Proteins make up half of the plasma membrane by mass and are responsible for most of the specialized membrane functions Membrane proteins perform many tasks: -Transport -Receptors for signal transduction -enzymatic activity -cell-cell recognition -attachment to the cytoskeleton and extracellular matrix (ECM) -Cell to cell joining slide 12 in ppw
Lipid Rafts
- 20% of the outer membrane surface
- Contain phospholipids, sphingolipids, and cholesterol
- May function as stable platforms for cell-signaling molecules or receptors
Membrane Proteins- TRANSPORT
- A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute
- Some transport proteins hydrolyze ATP as an energy source to actively pump substances across the membrane
Membrane Proteins-Receptors for signal transduction
- A membrane protein exposed to the outside of the cell may have a binding site that fits the shape of a specific chemical messenger, such as a hormone
- when bound, the chemical messenger may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell
Membrane Proteins-enzymatic activity
A membrane protein may be an enzyme with its active site exposed to substances in the adjacent solution
Membrane Proteins-cell-cell recognition
some glycoproteins (proteins bonded to short chains of sugars which help to make up the glycocalyx) serve as ID tags that are specifically recognized by other cells
Membrane Proteins-attachment to the cytoskeleton and extracellular matrix (ECM)
elements of the cytoskeleton (cell’s internal framework) and the extracellular matrix (fibers and other substances outside the cell) may anchor to membrane proteins
-helps maintain cell shape, fixes the location of certain membrane proteins, and plays a role in cell movement
Membrane Proteins-Cell to cell joining
membrane proteins of adjacent cells may be hooked tg in various kinds of intercellular junctions
-some membrane proteins (cell adhesion molecules or CAMS) of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions
Integral Proteins (membrane protein)
- firmly inserted into the lipid bilayer
- most are transmembrane proteins that span the entire membrane and protrude on both sides
- have both hydrophobic and hydrophilic regions
- some transmembrane proteins are involved in transport, form channels, or pores
- Functions as transport proteins (channels & carriers), enzymes or receptors
- small water soluble molecules or ions can move through these pores bypassing the lipid part of the membrane
- others act as carriers that bind to a substance and then moves it through the membrane
- some are enzymes
- others are receptors for hormones or other chemical messengers and relay messages to the cell interior– a process known as signal transduction
Peripheral Proteins (membrane protein)
- not embedded in lipid bilayer
- they either attach loosely to integral proteins or have a hydrophobic region that anchors them into the membrane
- includes a network of filaments that helps support the membrane from its cytoplasmic side
- some peripheral proteins are enzymes
- others are motor proteins involved in mechanical functions such as changing cell shape during cell division and muscle cell contraction
- Functions as enzymes, motor proteins, cell to cell links, provide support on intracellular surface & form part of glycocalyx
Glycolipids and glycoproteins
- Lipids and proteins with sugars attached
- glycolipids have 2 fatty acid tails (like phospholipids) but a carb replaces the phosphate head group
Glycocalyx
- “sugar covering”
- consists of the fuzzy, sticky, carbohydrate-rich area at the cell surface created by the sugars of glycoproteins and glycolipids
- cells are sugar-coated like breakfast cereal
- every cell type has a different pattern of sugars in its glycocalyx, the glycocalyx provides identity molecules - highly specific biological markers by which approaching cells recognize each other
- ex: immune system cells use these markers to determine which cells belong in the body and which are foreign
a carb
Tight Junctions
A series of integral protein molecules in the plasma membranes of adjacent cells fuse tg like a zipper of a ziploc bag.
This forms an impermeable junction that encircles the cell and separates one fluid compartment from another
helps prevent molecules from passing through the extracellular space between adjacent cells and restrict the movements of membrane proteins
ex: keeps digestive enzymes in the stomach
Desmosomes
- serve as anchoring junctions- mechanical couplings scattered like rivets along the sides of adjacent cells to prevent their seperation
- Desmosomes bind neighboring cells together into sheets and also contribute to a continuous internal network of strong fibers that act as “guy-wires”
- guy-wires distribute tension throughout a cellular sheet and reduces the chance of the sheet tearing when it is subjected to pulling forces
- desmosomes are abundant in tissues that are subjected to great mechanical stress such as skin and heart muscle
Gap Junction
- A communicating junction between adjacent cells
- at gap junctions the adjacent plasma membranes are very close , and the cells are connected by hollow cylinders (called connexons) composed of transmembrane proteins.
- different gap junctions are composed of different transmembrane proteins and they determine what can pass through them from one cell to another
- present in electrically excitable tissues such as the heart and smooth muscle
Passive Transport
- no added energy required (uses kinetic energy)
- substances move from high to low concentration (down their concentration gradient)
Active Transport
- Requires added energy (ATP)
- substances can move from low to high concentration (against their concentration gradient)
passive membrane transport processes: diffusion
- simple diffusion
- facilitated diffusion
- osmosis
simple diffusion
- Energy source: Kinetic
- Description: Net movement of molecules down their concentration gradient (from higher to lower)
- Membrane transport protein required?: NO
- Specific and saturable?: NO (passage depends only on small size and lipid solubility)
- examples; lipids, oxygen, carbon dioxide
fat soluble molecules diffuse through the phospholipid bilayer
facilitated diffusion
- Energy source: Kinetic
- Description: Same as simple diffusion, but the diffusing substance is attached to a membrane carrier protein or moves through a channel protein
- Membrane transport protein required?: YES
- Specific and saturable?: YES (specificity depends on shape inside transport protein)
- examples; glucose, Na+, K+
Osmosis
- Energy source: Kinetic
- Description: Diffusion of water through a selectively permeable membrane; can occur directly through the lipid bilayer or via membrane channels (aquaporins)
- Membrane transport protein required?: NO, except for movement through the aquaporins
- Specific and saturable?: NO, except for movement through the aquaporins
- examples; water
Who are the key players of the plasma membrane?
Lipids, carbs, proteins
Phospholipids
- Key player in the plasma membrane (1/3)
- forms the basic structure of the membrane
- hydrophobic tails prevent water-soluble substances from crossing, forming a boundary
Cholesterol:
-stiffens membrane further decreases water solubility of membrane
Proteins
- Key player in the plasma membrane (2/3)
- determines what functions the membrane can perform
- many roles: transport, communication (acting as receptors for signal molecules, and joining cells to each other and to the extracellular matrix
Carbohydrates
- Key player in the plasma membrane (3/3)
- act as identity molecules
- allows cells to recognize “who is who” , e.g during cell development so the cell sort themselves into tissues and organs
- allows immune cells to recognize our own
-are found on the outer surface of the membrane, like the sugar coating on breakfast cereal. this forms a coating called glycocalyx
Speed of diffusion is influenced by 3 factors:
- concentration
- molecular size (smaller molecules diffuse more rapidly)
- Temperature (higher temp, more kinetic energy, increases the speed of molecular movement.
What determines whether a given substance can cross the plasma membrane?
- Lipid solubility- the more lipid soluble, the more readily it will diffuse across
- Size- the smaller the molecule the more readily it will diffuse
A molecule can still cross the barrier if they lack these two things through the use of a carrier molecule such as an ion channel or transport protein
The unassisted diffusion of lipid-soluble or very small particles is called simple diffusion
true
Such substances are usually small non polar molecules that readily dissolve in lipids.
-These include gases such as oxygen and carbon dioxide, steroid hormones, and fatty acids
Assisted diffusion is known as facilitated diffusion
true
Carrier mediated facilitated diffusion
via protein carrier specific for one molecule
- binding of solute causes transport protein to change shape so that it first envelopes a molecule on one side of the plasma membrane, and then releases it on the other side allowing it to bypass the nonpolar regions of the membrane
- lipid-insoluble solutes such as sugars and amino acids can pass through this way
glucose moves down its concentration gradient (high to low), just as in simple diffusion. glucose is higher in concentration in the blood than in the cells. so glucose transport within the body is typically unidirectional, into the cells
-limited by the amount of protein carriers available. if all are being used it said to be saturated
Channel mediated facilitated diffusion through a channel protein
mostly ions selected on basis of size and charge
- Leakage channels: are always open and simply allow ions and water to move according to concentration gradients
- Gated channels: are controlled (open or closed), usually by chemical or electrical signals
- molecules move down the concentration gradient
- channels can become saturated and they tend to be specific
hydrostatic pressure
the backpressure exerted by water against the cell wall
osmotic pressure
the tendency of water to move into the cell by osmosis
-the higher the amount of non diffusible or nonpenetrating solutes in a cell, the higher the osmotic pressure and the greater the hydrostatic pressure must be to resist further net water entry
Tonicity
the ability of a solution to change the shape (or plasma membrane tension) of cells by altering the cells’ internal water volume
Isotonic Solutions
- “The same tonicity”
- cells exposed to isotonic solutions retain their normal shape and exhibit no net loss or gain of water.
Hypertonic Solutions
- have a higher concentration of nonpenetrating solutes than seen in the cell
- cells lose water and shrivel (crenate)
Hypotonic Solutions
- are more dilute (contain a lower concentration of nonpenetrating solutes)
- cells plump up rapidly as water rushes into them
- distilled water represents the most extreme example of hypotonicity
Osmolarity and tonicity are not the same
true
-a solutions osmolarity is based solely on its total solute concentration
- A solutions TONICITY is based on how the solution affects the cell’s volume which depends on:
* solute concentration
* solute permeability of the plasma membrane
How would carbon dioxide diffuse through a membrane ?
through the phospholipid regions of the membrane
-small nonpolar molecules readily dissolve in lipids
Active Transport
- requires transport proteins that combine specifically and reversibly with the transported substances.
- move solutes, most importantly ions, “uphill” against the concentration gradient
Primary active transport
the energy to do work comes directly from the hydrolysis of ATP by transport proteins called PUMPS
- include calcium and hydrogen pumps but most importantly sodium potassium pump.
- the sodium-potassium pump drives 3 Na ions out of the cell and pumps 2 K ions back in
- ions diffuse according to electrochemical gradients
Secondary active transport (Cotransport)
transport is driven by energy stored in concentration gradients of ions created by primary active transport pumps, usually by the sodium-potassium pump
- always moves more than one substance at a time using a cotransport protein
- uses a cotransport protein to couple the “downhill” (down its concentration gradient) movement of one solute to the “uphill” (against its concentration gradient) movement of another solute.
-
Symport System
2 transported substances move in the same direction
Antiport System
the transported substances “wave to each other” as they cross the membrane in opposite directions
Vesicular transport
fluids containing large particles and macromolecules are transported across cellular membranes inside bubble-like membranous sacs called vesicles.
- also used for combination processes such as transcytosis and vesicular trafficking
- like active transport, vesicular transport moves substances into the cell (endocytosis) and out of the cell (exocytosis)
transcytosis
- part of vesicular transport
- moves substances into, across, and then out of the cell
- common in endothelial cells lining blood vessels
vesicular trafficking
moves substances from one area (or membraneous organelle) in the cell to another
- can be thought of as Fedex
- energized by ATP
endocytosis
vesicles provide the main route for bringing bulk solids, most macromolecules, and fluids into the cell (or transporting them across a cell via transcytosis)
- begins with a coated pit: an infolding the membrane
- 3 types of endocytosis: phagocytosis, pinocytosis, receptor-mediated endocytosis
phagocytosis
- “cell-eating”
- cell engulfs relatively large or solid material such as clump of bacteria, cell debris, etc.
- when a particle binds to receptor on cells surface, cytoplasmic extensions called pseudopods form and flow around the particle
- macrophages and certain white blood cells are experts at this, known as phagocytes
- most phagocytes move by “amoeboid motion”
- The phagosome combines with a lysosome and its contents are digested
Pinocytosis
“Cell drinking”
- Also known as fluid-phase endocytosis
- The droplet that enters the cell fuses with a sorting vesicle called an endosome
- routine activity in cells
- no receptors are used so this process is nonspecific
Receptor-Mediated endocytosis
- Main mechanism for the specific endocytosis and transcytosis of most macromolecules by body cells.
- Receptors for this: plasma membrane proteins
- very selective
- substances taken up by this form of endocytosis include enzymes, insulin (and other hormones), low density lipoproteins, iron. Viruses and bacteria also jack this route
- extracellular substances bind to specific receptor proteins, enabling the cell to ingest and concentrate specific substances in protein coated vesicles.
Exocytosis
- Vesicular transport processes that eject substances from the cell interior into the extracellular fluid.
- Stimulated by a cell surface signal (such as, binding of a hormone to a receptor or change in membrane voltage)
- accounts for hormone secretion, neurotransmitter release, mucus secretion, and in some cases, ejection of wastes.
- substance to removed from the cell is enclosed in a secretory vesicle
- transmembrane proteins on the vesicles, called v-SNAREs (v for vesicle), recognize certain plasma proteins, called t-SNAREs (t for target), and bind with them
Membrane potential
-voltage across the membrane
-all cells have a resting membrane potential for this reason, all cells are said to be electrically polarized
-typical ranges -50 to -90 mV
-the negative in front of the numbers indicate that inside of the cell is negative compared to the outside
-this difference in charge only occurs at the membrane
-
Diffusion causes ionic imbalances that polarize the membrane, and active transport processes maintain that membrane potential
True
The resting membrane potential is mainly determined by the concentration gradient of potassium (K+) and by the differential permeability of the plasma membrane to K+ and other ions
true
K+ and protein anions predominate inside the cell while Na predominates in the extracellular fluid which is balanced by Cl-
Roles of Cell Adhesion Molecules (CAMs)
- Play a key role in embryonic development, wound repair, and immunity
- The molecular velcro that cells use to anchor themselves to molecules in the extracellular space and to each other
- The “arms” that migrating cells use to haul themselves past one another
- SOS signal sticking out from the blood vessel lining that rally protective white blood cells to a nearby infected or injured area
- mechanical sensors that transmit info about changes in the extracellular matrix into the cell, bringing about a variety of cell responses such as cell migration, proliferation (growth by cell division), and specialization
Membrane receptors
- Huge group of integral proteins that serve as binding sites
- Most are glycoproteins
Contact Signaling
Cells come together and touch, the means by which cells recognize each other
Chemical Signaling
The process in which a ligand, the chemical messenger, binds a specific receptor and initiates a response
- Include:
- Neurotransmitters (nervous system signals)
- hormones (endocrine system signals)
- Paracrines (chemicals that act locally and are rapidly destroyed)
-When a ligand binds to a membrane receptor, the receptors structure changes and cell proteins are altered in some way.
-G-protein coupled receptors exert there effect indirectly through a G protein, a regulatory molecule that acts as the middleman or relay to activate (or inactivate) a membrane bound enzyme or ion channel.
This generates intracellular chemical signals, known as second messengers, which connect plasma membrane events to the internal metabolic machinery of the cell. 2 types of second messengers: Cyclic AMP and ionic calcium both activate protein kinase enzymes
this pathway is involved in neurotransmission, smell, vision, and hormone action.
Cytoplasm
“cell forming material”
- cellular material between the plasma membrane and the nucleus
- consists of 3 major elements:
- cytosol
- organelles
- inclusions
Cytosol
The vicious, semi-transparent fluid in which the other cytoplasmic elements are suspended.
Complex mixture with properties of both a colloid and a true solution
Inclusions
Chemical substances that may or may not be present, depending on the cell type
-Example include stored nutrients, such as the glycogen granules in liver and muscle cells; lipid droplets in fat cells ; pigment (melanin) granules in certain skin and hair cells
Mitochondria
- Powerplants of a cell, providing most of the ATP supply
- Enclosed by 2 membranes :
- Outer membrane: smooth and featureless
- Inner membrane folds inward, forming shelflike cristae, that protrude into the matrix, the gel-like substance within the mitochondrion
- Aerobic cellular respiration: as metabolites are broken down and oxidized, some of the energy released is captured and used to attach phosphate groups to ADP molecules to form ATP. Requires Oxygen.
- Contain their own DNA, RNA, ribosomes, and are able to reproduce themselves
- theory: mitochondria arose from bacteria
Ribosomes
- Small dark-staining granules composed of proteins and a variety of RNAs called ribosomal RNAs
- Sites of protein synthesis
Free ribosomes
Free ribosomes float freely in the cytosol. They make soluble proteins that function in the cytosol, as well as those imported into mitochondria and some other organelles
Membrane-bound ribosomes
Attached to membranes forming a complex called the rough ER. They synthesize proteins destined either for incorporation into cell membranes or lysosomes or for export from the cell.
Endoplasmic Reticulum
“Network within the cytoplasm”
- An extensive system of interconnected tubes and parallel sacs called cisterns
- fluid filled interior
- continous with outer nuclear membrane and accounts for half of the cells membranes
Rough ER
Proteins assembled on these ribosomes thread their way into the fluid-filled interior of the ER cisterns
- When complete, the newly made proteins are enclosed in vesicles for their journey to the Golgi apparatus for further processing
- Rough ER functions:
- Its ribosomes manufacture all proteins secreted from cells
Smooth ER
- Its enzymes (all integral proteins integrated into its membranes) play no role in protein synthesis
- Instead, enzymes catalyze reactions such as :
- metabolize lipids, synthesize cholesterol and phospholipids, synthesize the lipid components of lipoproteins (in liver cells)
- synthesize steroid based hormones such as sex hormones
- Detoxification
- break down stored glycogen to form free glucose
- store calcium in most cell types
Golgi apparatus
-Principal “traffic director” for cellular proteins
“post office”
Major function is to modify, concentrate, and package the proteins and lipids made at the rough ER and destined for export from the cell
Three types of packaged products leave Golgi:
- plasma membrane renewal
- secretions to be discharged from cell
- enzymes for cytosol contained in lysosomes
Peroxisomes
Resembling small lysosomes, they are spherical membranous sacs containing a variety of powerful enzymes, the most important of which are OXIDASES and CATALASES
-Oxidases use oxygen to detoxify harmful substances, including alcohol and formaldehyde. Their most important function is to neutralize free radicals. Oxidases convert free radicals to hydrogen peroxide and catalase quickly convert this to water.
- Numerous in liver and kidney cells
- Play a role in energy metabolism
Lysosomes
“demolition crew”
Membraneous bags containing digestive enzymes
Digest ingested bacteria, viruses, and toxins
Degrade nonfunctional organelles
Break down and release glycogen
Break down bone to release Ca2+
Destroy cells in injured or nonuseful tissue (autophagy) webbed fingers and toes in fetus
Tay-Sachs disease – genetic disease where lysomes lack an enzymes, lipids overaccumulate in NN cells
Cytoskeleton
“Cell skeleton”
-Elaborate series of rods throughout cytosol for strength and flexibility. Acts a cells bones, muscles, and ligaments by supporting cell structures.
These rods include:
- Microfilaments
- Intermediate Filaments
- Microtubules
Microfilaments - cytoskeleton
- Thinnest elements of the cytoskeleton, semi-flexible strands of the protein ACTIN
- Involved in cell motility or changes in cell shape
- actin forms the cleavage furrow that pinches one cell into two during cell division
- Microfilaments attached to cell adhesion molecules are responsible for the crawling movements of amoeboid motion, and for membrane changes that accompany endocytosis and exocytosis
Intermediate Filaments- cytoskeleton
- Tough, insoluble protein fibers that resemble woven ropes. The most stable and permanent of the cytoskeleton elements
- strongly resist tension
- attach to desmosomes and their main job is to act as internal cables to resist pulling forces exerted on the cell
Microtubules- cytoskeleton
- Elements with the largest diameter
- near the nucleus in region called centrosome
- determine the overall shape of the cell and distribution of cellular organelles
tony protein machines called motor proetins continually move and reposition along the microtubules
organelles attach to this like Christmas ornaments on tree
Centrosome
“Cell center” near nucleus
Generates microtubules; organizes mitotic spindle
Contains paired centrioles: Small tube formed by microtubules
Which of the following would accurately track a glycoprotein from its site of initial synthesis to its arrival at the plasma membrane
ER-> transport vesicle->golgo ap. -> secretory vesicle->plasma membrane
Cilia
Whip like motile extensions on exposed surfaces of certain cells
- move substances in 1 direction across cell surfaces
- when a cell is about to form cilia, the centrioles multiply and line up beneath the plasma membrane at the cells exposed surface
Flagella
Projections that are also formed by centrioles but are longer than cilia
-the only flagellated cell in the human body is sperm
Cilia propel other substances across a cell surface while flagellum propel the cell itself
true
Microvilli
tiny, fingerlike extensions of the plasma membrane that project from an exposed cell surface
- increase plasma membrane surface area
- mostly found on absorbative cells such as intestinal and kidney tubule cells
- have a core bundled of actin filaments
skeletal muscle cells, bone destruction cells, and some liver cells are multinucleate
true
Anucleate
Mature red blood cells
- cannot reproduce
- w/o a nucleus a cell cannot produce mRNA to make proteins and when enzymes and cell structures start to break down they cannot be replaced.
Nuclear envelope
- Nuclues bounded by nuclear envelope
- double membrane barrier
- Inner nuclear membrane lined by the nuclear lamina (rod shaped proteins that assemble to form intermediate filaments) that maintains the shape of the nucleus and acts as a scaffold to organize DNA in the nucleus
- envelope is punctured by nuclear pores. complex of proteins called a nuclear pore complex, lines each pore, forming an aqueous transport channel and regulating entry and exit of molecules (mRNAs) and large particles into and out of the nucleus.
- selectively permeable (more relaxed than other membranes, substances usually have no trouble entering). this process is energy dependent and guided by soluble transport proteins
Nucleoli
- Within the nucleus. Dark-staining spherical bodies where the ribosomal subunits are assembled
- not membrane bound
Chromatin
-System of bumpy threads weaving through the nucleoplasm
Composed of :
*30% DNA
*60% globular histone proteins, which package and regulate the DNA
*10 % RNA chains
- Fundamental units of chromatin is NUCLEOSOMES, which consist of flattened disc shaped clusters of 8 histone proteins connected like beads on a string by DNA molecule. The DNA winds twice around each nucleosome and continues on to the next cluster via linker DNA segments
- Histones provide a physical means for packing the very long DNA molecules, in a compact way. They also play a role in gene regulation
Cell cycle
the series of changes the cell goes through from the time it is formed until it reproduces
Interphase
-The period from cell formation to cell division
-metabolic phase or growth phase or resting phase. basically no dividing of cells
-Divided into 3 subphases :
*G1
*S
*G2
(the Gs stand for gaps before and after the S phase, and the S phase is for synthetic)
-In all 3 phases the cell grows by producing proteins and organelles
-chromatin only produced during the S phase
Interphase- G1
- The cell is metabollicaly active, synthesizing proteins rapidly and growing vigorously
- Most variable phase in terms of length
- virtually no activities relating to cell division occur
- towards the end of this phase, centrioles start to replicate in preparation for cell division
Interphase-G0
cells that permanently stop dividing
Interphase-S
- DNA replication takes place
- new histones are made and assembled into chromatin
- without a correct S phase there could be no correct mitotic phase
Interphase- G2
enzymes and proteins needed for division are synthesized and moved to their proper sites
- by the end of this phase centriole replication is complete (begun in G1)
- At the end of this phase is the G2/M checkpoint where the cell ensures that all DNA is replicated and damaged DNA has been repaired
Most cells of nervous tissue, skeletal muscle, and heart muscle lose their ability to divide when they are fully mature and repairs are made with scar tissue
true
Mitosis
The division of the nucleus. is the series of events that parcels out the replicated DNA of the parent cell to 2 daughter cells. 4 phases: prophase metaphase anaphase telaphase
Cytokinesis
The division of cytoplasm. Begins during late anaphase and is completed after mitosis ends
- A contractile ring made of actin filaments draws the plasma membrane inward to form a cleavage furrow over the center of the cell.
- Furrow deepens until 2 daughter cells are made
Control of cell divsion
Factors:
- The ratio of cell surface area to cell volume
- chemical signals such as growth factors and hormones released by other cells
- The availability of space. Normal cells stop proliferating when they begin touching, a phenomenon known as contact inhibition
Cyclins and cyclin-dependent kinases (Cdks)
- These are 2 groups of proteins that are crucial to the cells ability to finish the S phase and enter mitosis
- Joining of specific Cdk and cyclin proteins initiates enzymatic cascades needed for cell division
Prophase-early phase
Chromosomes become visible, each with two chromatids joined at a centromere
Centrosomes separate and migrate toward opposite poles
Mitotic spindles and asters form
Prophase-late phase
Nuclear envelope fragments
Kinetochore microtubules attach to centromeres and draw them toward the equator of the cell
Metaphase
-“middle”
-Centromeres of chromosomes are aligned at the equator
This plane midway between the poles is called the metaphase plate
Anaphase
“A for away”
- Shortest phase
- Centromeres of chromosomes split simultaneously—each chromatid now becomes a chromosome
- Chromosomes (V shaped) are pulled toward poles by motor proteins of kinetochores
Telophase
- Begins when chromosome movement stops
- The two sets of chromosomes uncoil to form chromatin
- New nuclear membrane forms around each chromatin mass
- Nucleoli reappear
- Spindle disappears
DNA is said to specify only the structure of protein molecules, including the enzymes that catalyze the synthesis of all classes of biological molecules
true
Gene
A segment of DNA molecule that carries instructions for creating one polypeptide chain
RNA
- single stranded
- has sugar ribose instead of deoxyribose
- has base uracil instead of thymine
- 3 forms of RNA typically act together to carry out DNAs instructions for polypeptide synthesis:
- Messenger RNA (mRNA)
- Ribosomal RNA (rRNA)
- Transfer RNA (tRNA)
Messenger RNA (mRNA)
carries coded info to the cytoplasm, where protein synthesis occurs
Ribosomal RNA (rRNA)
along with proteins, forms the ribosomes, which consist of 2 subunits - one large and one small
-the 2 subunits combine to form functional ribosomes, which are sites for protein synthesis
Transfer RNA (tRNA)
small, roughly L shaped molecules that ferry amino acids to the ribosomes
-There they decode mRNAs message for the amino acid sequence in the polypeptide to be built
Becasue rRNA and tRNA do not transport codes for synthesizing other molecules, they are the final products of the genes that code for them. They act together to translate the message carried by mRNA
True
Polypeptide synthesis involves 2 major steps
- Transcription: DNAs information is encoded in mRNA
- Translation in which the info carried by mRNA is decoded and used to assemble polypeptides. Takes place in the cells cytosol
autophagy
“self eating”
- sweeps up bits of cytoplasm and needed organelles into double-membrane vesicles called autophagosomes
- they are then delivered to lysosomes for digestion of the contents, which the cell resuses
Has 3 roles:
*allows cells to dispose of clumps of unneded proteins, cytoplasmic oragnelles, such as mitochondria, when they are worn out
- in times of stress (starvation) it allows cells to cannibalize parts of themselves in order to survive
- helps restructure cells during development
problems with autophagy is linked to neurodegenerative disorders such as Alzheimers and Parkinsons
Ubiquitin-Proteasome pathway
handles individual proteins that are misfolded, damaged, or needed and must be disposed of.
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