Cells Revision Notes PDF

**TOPIC 2: CELLS AS THE BASIS OF LIFE** ======================================= 2.1 The Cell Theory ------------------- ### 5 points of the cell theory - All cells come from pre-existing cells - Cells contain genetic information - Cells are the basis for all life - Cells require and us...

**TOPIC 2: CELLS AS THE BASIS OF LIFE** ======================================= 2.1 The Cell Theory ------------------- ### 5 points of the cell theory - All cells come from pre-existing cells - Cells contain genetic information - Cells are the basis for all life - Cells require and use energy - Cells are simplest matter that can live ### Viruses and prions - Both exceptions to the cell theory. Viruses are non-living, thus not cells. Prions are misfolded proteins that can influence the folding of other proteins. ### Cell membrane/fluid mosaic model - Cell membrane surrounds all cells. Separates internal and external environments - Fluid mosaic model is representation of cell membrane. Fluid- ability to move around. Mosaic -- comprised of multiple components. Model -- representation of life. #### Phospholipids - Make up the cell membrane. Cell membrane is a phospholipid bi layer. Have phosphate head and lipid tail. Phosphate head is hydrophilic, lipid tail hydrophobic. #### Glycolipids - lipids with carbohydrates attached. Useful for cellular communication #### Cholesterol - In between phospholipids. Increases fluidity of membrane #### Proteins - Various proteins within and on membrane, all with various functions. Adds to mosaic model. - *Integral proteins* - Proteins within the membrane - *Peripheral proteins* - Proteins on the membrane. Can be temporarily or permanently attached - *Glycoproteins* - Proteins with carbohydrates attached. Used for cellular communication - *Membrane proteins* - *Channel proteins* - Used in facilitated diffusion to allow larger molecules to pass across membrane with the conc grad - *Protein pumps* - Used in active transport to pump molecules across the membrane against conc grad 2.2 Two Types of Cells ---------------------- ### Prokaryotic cells - Bacteria, archea. - Single cell - Simple - No membrane bound organelles - No nucleus, just nuclear region - Undergo asexual reproduction (binary fission) ### Eukaryotic cells - Multicellular organisms - Higher level of complexity and organisation - Membrane bound organelles - Nucleus and nucleolus - Plant and animal cells ### Cytoplasm - Liquid which organelles reside in - Contains other substances - Site of some chemical processes ### Organelles +-----------------------+-----------------------+-----------------------+ | *organelle* | *structure* | *function* | +=======================+=======================+=======================+ | *Nucleus* | Largest organelle. | Controls cell | | | Surrounded by | activity. Contains | | | membrane with pores. | genetic info to guide | | | Contains DNA | the cell. Directs | | | | protein synthesis | +-----------------------+-----------------------+-----------------------+ | *nucleolus* | Within the nucleus. | Where ribosomes are | | | Darker region. | made | | | Compromises proteins | | | | and RNA | | +-----------------------+-----------------------+-----------------------+ | *Mitochondrion* | Double membrane. | Site of aerobic | | | Highly folded inner | respiration. | | | membrane. Has | Synthesis of ATP | | | mitochondrial DNA. | (energy storage). | | | Inner membrane folded | Cristae is site of | | | into CRISTAE -- | ATP production | | | folds, and MATRIX - | | | | inside | | +-----------------------+-----------------------+-----------------------+ | *Chloroplast* | Outer and inner | Site of | | | membrane. Contains | photosynthesis in | | | own DNA (like | plant cells | | | mitochondria). | | | | Thylakoids are | | | | membrane-bound | | | | compartments inside | | | | chloroplasts. Form | | | | stacks -- GRANA, | | | | grana are made up of | | | | disc-shaped THYLAKOID | | | | | | | | STROMA is area inside | | | | chloroplast that | | | | surrounds GRANA | | | | | | | | Green pigment -- | | | | CHLOROPHYLL | | +-----------------------+-----------------------+-----------------------+ | *Vacuole* | Membrane bound fluid | Contain molecules, | | | sacks. | e.g. nutrients and | | | | wastes. Used for | | | Small in animal | transporation in and | | | cells, large in plant | out of cell. IN PLANT | | | cells. | CELLS controls | | | | TURGIDITY | +-----------------------+-----------------------+-----------------------+ | *Golgi Apparatus* | Flattened stacks of | Packing of proteins | | | membrane bound sacs. | and other materials | | | Not linked to one | to leave cell | | | another | | +-----------------------+-----------------------+-----------------------+ | *Rough Endoplasmic | System of tubules | Production of | | Reticulum* | running from nucleus | proteins | | | out to cell membrane. | | | | Embedded with | Intracellular | | | ribosomes (ROUGH) | transport | +-----------------------+-----------------------+-----------------------+ | *Smooth Endoplasmic | System of tubules NOT | Production of lipids | | Reticulum* | embedded with | and metabolism of | | | proteins | carbohydrates | +-----------------------+-----------------------+-----------------------+ | *Ribosome* | Made of rRNA | Protein synthesis | | | | | | | NOT membrane bound | | | | | | | | 2 subunits | | +-----------------------+-----------------------+-----------------------+ | *Lysosome* | Membrane bound | Can digest food | | | vesicle containing | molecules | | | digestive enzymes | | | | | Can breakdown cell | | | | debris | +-----------------------+-----------------------+-----------------------+ | *Cytoskeleton* | Microscopic inner | Cellular structure | | | filaments within | and support | | | cell. Many components | | | | | Spindle fibres pull | | | | sister chromatids | | | | apart in mitosis | +-----------------------+-----------------------+-----------------------+ | *Centriole* | Part of the | Involved in cell | | | cytoskeleton. Small | division | | | microtubule structure | | | | | Formation of cilia | | | | and flagella | +-----------------------+-----------------------+-----------------------+ ### Cytoskeleton - Provides structure to the cell. Made up of proteins (actin and tublin), globular proteins that can be assembled to form long filaments - *Microfilaments* - (actin) -- used in movement. E.g. formation of daughter cells in cytokinesis - *Microtubules* - (tubulin) -- movement of cilia and flagella. Also holds organelles in place - *Intermediate filaments* - (multiple proteins) - strength ### Animal Vs Plant cells - Animal cells do not have cell wall or chloroplasts - Plant cells have larger vacuole - Plants can undergo photosynthesis, hence autotroph. Animals can't - Plants have more rigid shape 2.3 Energy Transformations -------------------------- ### Uses for energy - Energy required for cell to live - Used in synthesis of molecules, movement, cell division, active transport - Energy cannot be created nor destroyed, but can be transformed ### Autotrophs - Make own food through conversion of other energy sources into own energy source - Producers - E.g. plants - Can make own complex organic molecules (i.e. glucose) from external energy sources - Take in CO2 and H2O, releasing O2 and C6H12O6 - Net consumers of CO2 ### Heterotrophs - Opposite to autotrophs - Net producers of CO2 - Cannot synthesis own complex organic molecules, thus must consume others - Gain energy through eating other organisms -- ultimately relying on autotrophs for all nutrition ### Chemosynthesis - Some autotrophs derive organic material using external energy sources of chemicals - Often occurs in harsher environments, w/o sunlight ### Photosynthesis - Occurs in plants, specifically w/in chloroplast - Most living plants have way to successfully photosynthesise - Converts CO2 and H2O into oxygen and glucose by utilising power from the sun - Cells in which photosynthesis occur have many chloroplast -- makes the cells green due to chlorophyll. These cells called mesophyll cells. Can use CO2 and H2o to synthesise carbohydrates - Leaf has specialised structure to maximise rate of photosynthesis - Chloroplasts can move within cells to reach sunlight - EQUATIONS: - water +carbon dioxide sunlight and chlorophyll glucose and oxygen. - 6H­~2~O + 6CO­~2~ sunlight and chlorophyll C~6~H~12~O~6~+6O~2~ - 12 water molecules actually used, but 6 new water molecules are produced, therefore net gain of 6 water molecules, so this is the number shown in equations - LIGHT REACTIONS (photo part) - Occurs in thylakoid, which contain chlorophyll. These are the individual stacks - CALVIN CYCLE (synthesis part) - Occurs in stroma. This is the space around the granum (stacks) ### Energy transformations - Chemical bonds hold atoms together - Energy REQUIRED to BREAK bonds - Energy RELEASED when bonds MADE - Energy absorbed to break bonds, therefore energy taken in, therefore ENDOthermic process (takes in energy, therefore loses heat) - Energy released when new bonds are formed, therefore EXOthermic process (releases heat) - Larger molecules have more bonds, therefore they have more energy stored within these bonds - Net output of energy occurs when totally energy released when new bonds make is greater than energy required to break original bonds - Aerobic respiration is energy release because amount of energy required to break bonds in oxygen and glucose is less than the energy released to form bonds in water and carbon dioxide - EXOTHERMIC REACTION -- energy RELEASED, when bonds are MADE - ENDOTEHRMIC REACTION -- energy ABSORBED when bonds are BROKEN ### Cellular respiration - Required to transform energy-rich organic molecules into useable energy for the cell - Often energy transfer from bonds in glucose into ATP - 2 types of respiration, anaerobic and aerobic respiration ### Aerobic respiration - Occurs within the mitochondria - Has presence of oxygen - More efficient that anaerobic respiration - EQUATIONS - Glucose + oxygen mitochondria water + carbon dioxide - C~6~H~12~O~6~+6O~2~ mitochondria 6H­~2~O + 6CO­~2~ - Organisms obtain energy by breaking down large macromolecules during cellular respiration - Energy either transferred into ATP (approx. 40%), or lost as heat (60%) - 3 stages consisting of metabolic pathways - *[Glycolysis]* - First stage - Occurs in cytoplasm - Glucose broken down into 2 pyruvic acid molecules - This produces 4 ATP molecules, but 2 are needed for the process, hence net gain of 2 ATP. - *[Krebs cycle]* - Occurs in mitochondria - 2 pyruvic acid molecules broken down into acetyl coenzyme A (acetyl CoA) - Acetyl CoA enters Krebs cycle and is broken down into CO2 and water - *[phosphorylation ]* - also occurs in mitochondria - adds a phosphate group to an ADP molecule, forming ATP - produces a further 34 ATP molecules - glycolis and krebs cycle are energy releasing processes as they break down the glucose - phosphorylation transfers this energy into a usable and storable form, this being ATP - by the end of the process, a total of 36 ATP have been formed from 1 glucose molecule ### ATP and ADP - ATP is source of energy for cell - Stands for adenosine triphosphate - Has an adenosine base, ribose sugar, and 3 phosphate molecules - Phosphate molecuels, particularly the third one, are connected with highly energised hydrogen bonds, which can be broken to release energy -- high energy bond - ADP is adenosine Diphosphate, meaning 2 phosphate molecules - In aerobic respiration, the extra phosphate molecule is added onto ADP using the energy from the reactions - ATP is crucial for cell functioning. All processes either require or produce ATP - Renewable energy source, easily transported around cell ### Anaerobic Respiration (fermentation) - Used to synthesis ATP without the presence of oxygen - Far less efficient - Also produces more harmful by-products - doesnt occur in mitochondria, occurs in cytosol - both only produce 2 ATP, as compared to 36 from aerobic respiration - different in plants vs animals - *PLANTS* - Alcoholic fermentation occurs - Produces ethanol and CO2 - C~6~H~12~O~6­~2C~2~H~5~OH + 2CO~2~ - Glucose ethanol and carbon dioxide - ANIMALS - Lactic acid fermentation - Produces lactic acid - C~6~H~12~O~6­~2C~3~H~6~O~3~ - Glucose lactic acid 2.4 Movement in and out of the cell ----------------------------------- ### Inputs and outputs (differences, examples, definitions) - Inputs are what cells bring in, it is what they require - Outputs is what cells release/remove of, as they are waste or by-products - Autotrophs typically have inputs of CO2 and H2O, with outputs of O2 and glucose - Heterotrophs are the opposite, taking in glucose and oxygen and releasing water and CO2 ### Cell membrane (semi permeable) - Cell membrane is semi-permeable, meaning it can control what enters and leaves the cell - Control is necessary for function and life of cell. No control = death - Molecules can pass across membrane in various ways, depending on the concentration gradient, size and charge of molecules, solubility, direction of travel, etc. ### Diffusion - Passive process, therefore no energy needed - Molecules pass passively through the membrane (between phospholipid molecules) - Goes with the concentration gradient, high low conc. - Aim is to reach equilibrium between intra and extra cellular fluids - Common example: gas exchange in lungs ### Facilitated diffusion - Similar to diffusion in that is a passive process that works with the conc grad to reach equilibrium - Uses channel proteins to facilitate the movement of larger molecules across the cell membrane - E.g. transport of glucose ### Osmosis - Passive transport which works ONLY for water - Travels through aquaporin, proteins in membrane specifically for water - Direction of travel is relevant to solute concentration - Will always travel to reach equilibrium - Hypotonic -- less concentrated - Isotonic -- same concentration - Hypertonic -- more concentrated - If a cell is placed in a hypotonic solution, water will travel into the cell, to reach equilibrium of concentrations. This causes the cell to swell. Plant cells will not burst, but will become more turgid, due to the cell wall maintaining shape. Animal cells do not have a cell wall, thus will burst - A cell placed in an isotonic solution will have no net change in water, thus will not change shape - A cell placed in a hypertonic solution will lose water, as the water will leave the cell to make the concentration of solutes balanced. This will cause the cell to shrivel and lose size/mass - Tonicity -- ability of an extracellular solution to make water move in or out of cell via osmosis - Osmolarity -- concentration of a solution expressed as the total number of solute particles per litre ### Active transport -- e.g. sodium potassium pump - Form of transport which requires energy - Uses protein pumps - Needs energy as molecule usually travelling against the concentration gradient - Transport proteins located in the cell membrane - Specialised to molecules - Needs ATP to operate - Example is sodium potassium pump, which occurs in nerve cells. - Nerve cells need to exchange sodium and potassium ions to communicate - Can run out of these ions, so must be pumped back into cell using AT to recharge cell ### Exocytosis - Type of cytosis. Used to move large substances across the membrane by forming vesicles - Exo = exit, therefore leaving the cell - Often used for removal of waste products - Also used with the golgi body, as the golgi body packages hormones and proteins and such into vesicles which then exit the cell via exocytosis - Vesicles fuse with membrane to release contents ### Endocytosis - Opposite of exocytosis - Way for cells to take in larger molecules and substances - Cell membrane reaches out and engulfs particles, then forms vesicle which brings particles into cell - Two types - PINOCYTOSIS: for liquids - PHAGOCYTOSIS: for solids/food -- used by WBCs to take in pathogens ### SA/V ratio - Refers to the ratio between the surface area of a cell and its volume - Small cells have a higher SA:V, therefore are more efficient - Part of the reason cells divide is to ensure SA:V stays as efficient as possible ### EXAMPLES: - **Small hydrophobic molecules** - Example is CO2 and O2. Able to pass passively through the membrane, e.g. in respiration. Do not need energy or special proteins - **Small hydrophilic molecules** - E.g. is water. Passes through passively, but requires aquaporin to do so. - **Ions** - Have charge, therefore cannot diffuse across membrane without assistance - Use channel protein to move across -- facilitated diffusion - **Large hydrophilic molecules** - Large molecules e.g. glucose and amino acids cannot pass through the cell membrane without assistance - Require channel proteins - Facilitated diffusion, or active transport (depends on conc grad) 2.5 Cell Metabolism ------------------- ### Cell metabolism - Cell metabolism refers to the biochemical processes that occur in a cell - Critical to survival of cells - All influenced by enzymes - Metabolic pathways exist, with specific steps which rely on enzymes - Many processes occur in structures that are membrane bound - Membranes usually contain enzymes which facilitate the metabolic pathway ### Internal membrane of mitochondria - Inner membrane is highly folded to form cristae. Membrane contains enzymes which assist in ATP synthesis -- ATP synthase. Folds of membrane increase SA for enzyme attachment ### Internal membrane of chloroplast - Chloroplast has inner system of membranous flattened sacs -- thylakoids, stacks called grana. Chlorophyll is contained within these membranes, which is needed in light stage of photosynthesis ### Energy pathways - Energy pathways are used in the breakdown and synthesis of molecules - If process were to occur all at once, amount of energy required or released would be immense, thus this is not feasible - Lots of energy would be lost as heat - This is particularly important in relation to food and energy - If all energy stored in food was released at once, it would be a violent reaction, with most energy being lost as heat rather than transferred into ATP - This is why smaller steps are taken in energy pathways -- controlled intermediate steps #### E.g. Glycolysis - First stage of aerobic respiration that occurs in the cytosol - Converts glucose into pyruvic acid and 2 atp - has 6 steps, each step is controlled by a specific enzyme -- allows cell to control process and ensure it doesn't happen too quickly - maximised amount of energy transferred into ATP, minimises heat loss ### Exergonic reactions - RELEASE of energy -- exer=exit=release - Products have LESS energy than the reactants - Aerobic respiration is example. Takes larger molecules and breaks down into smaller ones, RELEASING energy ### Endergonic reactions - ABSORBS energy -- ender=enter=absorb - Joins smaller molecules into larger ones - Products have MORE energy than reactants - Photosynthesis is example as takes smaller molecules and forms glucose (bigger molecule) ### Regulation of metabolic pathways - Metabolic pathways are controlled by enzymes, thus affected by same factors of heat, pH, inhibitors, etc. ### Chemicals in relation to pathways - Many chemicals inhibit metabolic pathways by disrupting/inhibiting enzymes - E.g. cyanide disables enzyme in respiration metabolic pathway 2.6 Binary Fission and Mitosis ------------------------------ ### Cell division (definition, reasons, uses) - Cell division is the replication of cells by splitting into two - Used for reproduction, repairs, growth - Different depending on cell type - Each have different steps ### Somatic cells, haploid vs diploid, homologous pairs vs sister chromatids - Somatic cells are all body cells - Have diploid number of chromosomes (2 copies of each chromosome, 1 maternal, 1 paternal) - Human cells have 46 chromosomes, with 23 homologous pairs - Sex cells have half the number of diploid cells, and are called haploid -HALF-HAPLOID ### Binary fission - Occurs in prokaryotic cells - Form of asexual reproduction - Daughter cells are genetically identical to the parent - STEPS: - Circular DNA is replicated - Attaches to the cell membrane using proteins - Cell elongates - DNA is drawn to either end of cell - Cell membrane pinches in - 2 identical daughter cells are formed ### Asexual reproduction - Formation of an offspring from a single parent - Doesn't require fertilisation to occur - Only source of genetic variation is mutations ### Stages of cell cycle/interphase - Only occurs in eukaryotic cells - Distinct stages - First stage is interphase - This is where DNA and organelles replicate and cell grows larger - DNA exists as chromatin -- long strands, spaghetti like. When DNA replicates it condenses, which are visible under a light microscope ### Mitosis - Form of cell division - 4 stages - Cretes two genetically identical diploid daughter cells\`\`\`\`\`\`\`\`\`\`\`\`\`\`\`\`\`\` #### Prophase - First stage - Prophase -- PAIR - Chromatids condense - Replicated sister chromatids find their pair and pair up -- join at centromere - Nuclear envelope dissolves - Spindle apparatus forms - Centrioles divide and form 2 pairs, moving to opposite ends of cell #### Metaphase - Second stage - Metaphase -- MIDDLE - Sister chromatid pairs line up along cell equator -- metaphase plate - Have specific order in which they do so (e.g. chromosome 1 from mum is followed by 1 from dad, then 2M, then 2F, etc.) - Spindle fibres attach to proteins at centromere - Sister chromatids face opposite poles #### Anaphase - Spindle fibres retract - Sister chromatids pulled apart to opposite sides of cell #### Telophase - Chromatids at each pole - Spindle fibres dissolve - Nuclear envelope reforms, remaking nucleus - Cytoplasm begins to pinch in - Chromatids uncoil, returning to chromatin form. ### Cytokinesis - Last major stage of the cell cycle - Last stage of mitosis - Cell membrane pinches in to form 2 separate genetically identical diploid daughter cells ### Kinetochore - Relevant to the centromere - Centromere is the central region of chromatids by which they attach - Kinetochore is a disc-shaped protein that attaches on either side of the centromere -- this attachment occurs due to the base sequence of the centromere - This is where the spindle fibres attach to ### Binary fission vs mitosis - Binary fission occurs in prokaryotic cells - Has less steps as doesn't need to separate multiple chromosomes - Mitosis occurs in eukaryotic cells, and has multiple stages to separate chromatids - Both processes result in 2 daughter cells that are identical to the parent cells. - Binary fission doesn't use spindle fibres ### DEFINITIONS - *Cancer -*uncontrolled cell growth - *Centrioles ­*-- microtubules in animal cells that duplicate to form spindle fibres - *Centromere --* middle section of chromosomes which is where sister chromatids join - *Centrosome* -- both centrioles together - *Chromatid* -- duplicated chromosome. Forms part of sister chromatid which are separated in mitosis - *Chromatin* -- non-condensed form of chromosomes. State which DNA is usually in - *Chromosome* -- condensed strands of DNA and proteins that carry genetic info - *Cytokinesis* -- last stage in cell cycle where cytoplasm splits - *Cytoplasm* -- the entire inner part of a cell, including the cytosol and organelles - *Cytosol* -- just the liquid of the cytoplasm - *Cristae* -- folded inner membrane of a mitochondria - 2.7 Sexual Reproduction and Meiosis ----------------------------------- ### Somatic vs germline/gametes - Somatic cells are all body cells -- have diploid number of chromosomes - Germline cells are sex cells/gametes (egg and sperm) -- have haploid number (half) - This is for when zygotes are formed, so that the cell has the right number of chromosomes once fertilised ### Homologous chromosomes - Homologous chromosomes are chromosomes of the same type -- e.g. chromosome 1 from m and f - Also have 2 copies of each gene due to this -- alleles -- dominant and recessive ### Sexual reproduction vs asexual reproduction - Sexual reproduction requires the combination of 2 different cells (usually from different parents) to form a new organism - asexual reproduction doesn't require this - sexual reproduction needs fertilisation - exception of the 2 parent organism rule is with some plants, who can self-fertilise. These are 1 parent organism, but still need 2 cells to reproduce - has 2 stages: - meiosis -- forms gametes/sex cells - fertilisation -- fusion of sex cells to form zygote ### Meiosis - Meiosis is division of cell to form haploid cell with half the number of chromosomes - Results in 4 genetically non-identical daughter cells - In animals, occurs in gonads - Has 2 stages, meiosis 1 -- the reduction stage, and meiosis 2 -- identical to mitosis - Meiosis 1 halves the number of chromosomes, meiosis 2 separates them ### Meiosis I #### Prophase I - Chromosomes condense and become visible - Sister chromatids pair - Homologous chromosomes also pair -- process is called SYNAPSIS -- structures formed are called bivalents (4 chromatids together) - Nuclear membrane dissolves - Spindle fibres form - Genetic information is exchanged between homologous pairs through process of crossing over. -- position at which exchange occurs is chiasmata - Centrioles move to poles #### Metaphase I - Homologous pars line up together along the metaphase plate - Has process of random assortment to increase genetic variability. - Spindle fibres attach to centromeres #### Anaphase I - Spindle fibres retract, pulling homologous pairs apart, separating the paris into replicated chromatids, but NOT separating the chromatids - Referred to as reduction division as it reduces the diploid number (halves) #### Telophase I - Haploid sets are pulled to either end of the pole - Spindle apparatus dissolves - Cytoplasm pinches in and forms 2 separate daughter cells -- CYTOKINESIS - Chromosomes do not revert to chromatin, staying as paired sister chromatids - Nucleus does NOT reform ### Meiosis II #### Prophase II - New spindle is formed - Chromatids already paired, nucleus already dissolved #### Metaphase II - Sister chromatids line up along cell equator/metaphase plate - Spindle fibres attach to centromere #### Anaphase II - Spindle fibres retract, separating the sister chromatids #### Telophase II - Chromatids pulled to opposite ends of pole - Revert back to chromatin - Nucleus reforms - Cytoplasm pinches and separates - Spindles dissolve - 4 non-genetically identical haploid daughter cells are formed ### Fertilisation - Process by which gametes combine to restore the diploid number of a cell, to form offspring - E.g. sperm fertilises the egg, combining genetic material from parent organisms ### Variation - Due to differences between meiosis and mitosis, the daughter cells produced from meiosis are NOT genetically identical - This is beneficial as it creates genetic variety within a population -- important in evolution - Daughter cells are genetically SIMILAR, but not identical to parent and sisters #### Crossing over - Occurs during prophase 1 when bivalent is formed - Is the swapping of genetic material between homologous chromosome pairs - Place at which swap occurs is chiasmata - When correspondin2g regions of DNA of different chromosomes touch, enzymes can cut and swap these sections between chromatids, forming recombinant DNA, which combines maternal and paternal DNA, increasing genetic variation - Completely random, occurring 2-3 times per chromosome #### Independent assortment - Occurs during metaphase 1 - Happens when chromosomes line up along metaphase plate in homologous pairs - It is random which pole the maternal chromosome will face and which pole the paternal chromosome with face. This increases variation in daughter cells when homologous pairs are pulled apart in anaphase - Characteristics from M and P chromosomes are randomly sorted between gametes #### Fertilisation - Fertilisation is a random process. - Cannot determine which egg fuses with which sperm - This process also increases genetic variability - Also combines DNA from different parents, increasing gene pool #### Mutations - Can occur in sexually and asexually reproducing organisms - Rare, but do occur - Are the ONLY source of variation in asexually reproducing organisms (e.g. bacteria)? - 2.8 Control of Cell Division ---------------------------- ### Cell division - Cell division is the process by which cells split and duplicate - Happens via mitosis, binary fission, or meiosis - Must happen for growth, repair, reproduction, etc. - When cell divides, one cell remains in cell cycle, one splits off and differentiates to become a specialist - Specialised cells do not divide (if they do, do so at a greatly reduced rate) ### Cell cycle - Cell cycle is the life cycle of a cell - Covers growth and division of cell - Cell division must be highly regulated process to ensure errors do not pass through to daughter cells, and that cells do not divide exponentially - Enlargement must occur prior to division to ensure that daughter cells do not become too small - Cell enlarges by taking in water and synthesising new components - Process of enlargement and division is the cell cycle - Cell cycle has 3 main stages: interphase, mitosis, and cytokinesis - Interphase is the largest phase, and is where the cell spends most of its time - 3 checkpoints throughout to ensure cell is dividing properly ### Interphase - Stage where cell spends most of its life - Thought of to be the "dormant" phase, as the cell is not actively dividing, but this is incorrect, actually a very active phase - Where the cell enlarges and duplicates components to prepare for division - Has 3 main subsections and 2 checkpoints #### Gap 1/ growth phase - First stage of interphase - Organelles ready for duplication - Cell accumulates energy - Ribosomes are created, which create more proteins - Cell takes in water and nutrients to grow - *Checkpoint 1:* checks DNA to ensure ready for replication. If cell is of satisfactory size and growth factor is received, moves through checkpoint to phase 2 #### S phase - Stage where DNA is replicated. - No checkpoint moves straight into next phase #### Gap 2/ growth phase 2 - Last phase in interphase - Organelles replicated - Cell continues to develop and readies organelles for division - *Checkpoint 2:* DNA is checked to ensure chromosomes are duplicated correctly. [ ] Needs mitosis promoting factor (MPF) before continuing ### Mitosis - Process in which cell divides - Needs MPF to begin - MPF formed when enzyme Cdk (cyclin dependent kinase = enzyme) combines with protein cyclin -- happens at end of checkpoint two - Cyclin accumulates over the process of interphase - Cdk remains at constant level - As cyclin binds with Cdk, MPF is formed. Increase in MPF leads to mitosis - *Checkpoint 3:* occurs during metaphase, ensures that spindle fibres are attached properly so that they are separated correctly in anaphase. Anaphase is triggered by a decrease in MPF. ### Cytokinesis - Final phase in cell cycle - It is the separation of cells to form 2 separate daughter cells. - One daughter cell remains unspecialised, remaining in the cell cycle to continue replicating - Other daughter cell leaves the cell cycle to differentiate into a specialised cell ### G0 - Cell cycle arrest - The phase which cells that have left the cell cycle are said to be in - Do not replicate, but remain functioning - Can still replicate if stimulated to do so, but the process is slow ### Checkpoints (and purpose of each) - Purpose of checkpoint is to act as safety net/quality control of the cell cycle, ensuring that the cell is dividing properly - Checkpoints regulate the cell cycle, ensuring growth is controlled and errors are not made - DNA damage is assessed before (G1 checkpoint) and after (G2 checkpoint) replication to correct any errors - Spindle attachment and alignment is checked in the M checkpoint to ensure that chromatids are separated correctly ### External factors that regulate cell cycle - Cell must meet requirements before S phase begins (DNA replication) - NUTRIENTS: must have particularly nutrients in extracellular fluid - ACHORAGE: some cells need to be anchored to something (substrate or surface) before dividing - DENSITY: cells will not divide if too crowded/dense - LARGE CELL SIZE: cell must be of appropriate size before dividing - CHEMICAL FACTORS: e.g. hormones also control cell division/cycle. Hormones can bind to specific target cell receptors and stimulate release of growth factors, which stimulate DNA replication in S phase. CYTOKININS stimulate cell division ### Internal factors that regulate cell cycle - Gene products can regulate cell cycle - Cyclin is regulatory protein -- levels can fluctuate - Important as increase in cyclin leads to increase in MPF when it binds with Cdk enzyme (which has constant levels). Increase in MPF stimulates mitosis to begin, and a drop in MPF initiates anaphase (checkpoint 3) ### Cancer - Uncontrolled cell growth which leads to tumour formation - Cancer cells divide and produce daughter cells which both stay as stem cells, neither divides off and enters G0 phase - This means that both daughter cells replicate and form 2 more cells. This growth is uncontrolled and thus exponential, which leads to tumours - Change in DNA passes on to all daughter cells - Cancer cells have abnormal cell cycles, which work to bypass checkpoints - Either can have faulty checkpoints which do not need growth factors to pass through - Or can produce own growth factors so progression isn't restricted - Cancer is linked to chemical carcinogens, radiation, and viruses. All which disrupt the DNA sequence, altering it. ### Genes associated with cancer - Some genes when altered are directly associated with cancer - *Proto-oncogenes:* help cells grow. If faulty, become ONCOGENES, which accelerate division and growth - *Tumour suppressor gene:* produces p53 protein which acts as tumour suppressor, working to prevent production and replication of cells with faulty DNA. If DNA is damaged, the p53 protein blocks cell cycle in G1 allowing for cell to repair the DNA, or die if damage too severe. More than ½ all human cancers are caused by fault in this gene - *DNA repair gene*: produces products which repair DNA. If damaged DNA not repaired. ### Metastasis - Cancer can begin anywhere if tumour cells evade destruction - Lump of non-invasive tissue (tumour) forms first, but if cells from this tumour break off and are transported elsewhere in the body, this is when it becomes a major problem, and the tumour is said to have metastasised. ### Cell culturing (uses, needs, process, HeLa) - Cell culturing is process by which cells are taken and grown in a lab - Millions of cells can be grown in a small area, with hundreds of generations being able to be studied - HeLa cells are example of this, coming from cervical cancer samples from Henrietta Lacks. She (nor family) consented for these cells to be studied for so long, presenting ethical issue. However, these cells helped with many developments in modern medicine. #### Animal Cell Culturing - Needs: suitable medium (with correct nutrients), appropriate temperature, correct osmotic balance, suitable pH, sterile environment - Process: tissue of interest taken and dissected to expose cells. Protein digesting enzymes added to remove intercellular matrix leaving just cells. Cells placed in culture medium. Antibiotics can be added to prevent unwanted microbe growth. Kept at optimum temperatures and allowed to grow - Uses: testing, production of tissues and medicines, Ames test (tests for carcinogens) #### Plant cell culturing - Plant cells cut out, washed to remove microbes, placed in solution containing minerals, plant growth hormones, and nutrients (e.g. glucose). Can be controlled what the cells differentiate into. Can also produce hybrid plants by dissolving cell walls and allowing cells to fuse #### Benefits and limitations - B: research, manufacturing of vaccines/other products, growth for testing - L: many unknown areas, ethical limitations. All cells produced are identical. Most cells tend to stop dividing after about 50 divisions due to telomere breakdown prompting apoptosis