Module 2 (cell structure and function) Flashcards
Cell membrane structure
Arranged as a phospholipid bilayer around cytoplasm; two rows of polar hydrophilic heads (phosphate, face exteriors) and lipid hydrophobic tails (fatty acids, face each other); embedded with proteins; dynamic
Cell membrane functions
Selectively permeable barrier which controls passage of substances into and out of the cell
Cell membrane proteins and functions
Often amphipathic (hydrophobic and hydrophilic regions); can be integral or peripheral; are cell specific and dynamic; allow transport, enzymatic activity, cell to cell recognition, intercellular joining, signal transduction and attachment to cytoskeleton and ECM
Integral proteins
Embedded partially or fully into the membrane (e.g. transmembrane proteins span the entire membrane, contact extracellular and cytoplasmic areas)
Peripheral proteins
Associated with the membrane but are not actually embedded in it
Nucleus description and components
Largest organelle; enclosed by a double lipid bilayer (nuclear envelope) which is continuous with the RER; nuclear pores and nucleolus
Nucleus function
Houses/protects DNA in eukaryotic cells; nuclear pores are tightly regulated channels which allow entry and exit of substances (e.g. protein, mRNA); nucleolus makes RNA and assembles proteins; molecule segregation allows temporal and spatial control of cell function (things can happen fast and be directed to different parts of the cell)
DNA
Deoxyribonucleic acid; double helix made up of nucleotides (nitrogenous base, phosphate group and pentose sugar); forms inherited genetic material
RNA
Ribonucleic acid (mRNA, rRNA and tRNA); relays information from genes to guide synthesis of proteins from amino acids
Fluid mosaic model
Plasma membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids
Cytoplasm
Everything inside the plasma membrane including the organelles (not nucleus); fluid portion is the cytosol which is water and dissolved/suspended substances (e.g. ions, proteins, ATP, lipids)
Endomembrane system
Nucleus, smooth and rough ER, golgi apparatus and lysosomes; along with plasma membrane share membrane structures which allows formation, shipping and movement of compounds around cell
Which major organelles are not a part of the endomembrane system?
Mitochondria, have membranes but don’t share messages between other structures in cells; ribosomes, no membranes
Ribosomes structure
Large and small subunits (rRNA in complex with many proteins) made in nucleolus and leave through nuclear pores and come together in cytoplasm
Ribosomes function
Sites of protein synthesis (translation); some found associated with RER - make non-cytosolic proteins/endomembrane and others found free in cytoplasm making proteins to be used in cytosol (non-endomembrane destinations)
Rough ER structure
Continuous with the nuclear envelope, folded into a series of flattened sacs; outer surface studded with ribosomes
Rough ER function
Produces secreted proteins, membrane proteins and organelle proteins; proteins enter RER for folding; RER surrounds protein to form transport vesicles destined for golgi
Smooth ER structure
Extends from the RER to form a network of membrane tubules and lacks ribosomes;
Smooth ER function
Does not make proteins, but synthesises lipids (including phospholipids and steroids); stores cell-specific proteins (not all cells make all proteins); tissue-type/cell specific
Smooth ER cell-specific examples
Liver cells: houses enzymes for detoxification and glucose release; liver, kidney and intestinal cells: houses enzymes which removes the phosphate group from a glucose molecule so glucose can enter bloodstream; muscle cells: release calcium ions which trigger contraction
Golgi apparatus structure
‘Warehouse’ which is made up of 3-20 flattened membranous sacs with bulging edges (cisternae) stacked on top of each other; more extensive in cells that secrete proteins
Golgi apparatus function
Modifies, sorts, packages and transports proteins received from RER; forms secretory vesicles (proteins for exocytosis), membrane vesicles (PM molecules) and transport vesicles (molecules to other organelles)
Golgi apparatus process
Entry (cis) face faces RER, exit (trans) face faces plasma membrane; each sac/cisternae contains enzymes of different functions where modifications occur; proteins move cis to trans from sac to sac, mature at the exit cisternae and travel to their destination
Lysosomes structure
Contain powerful digestive enzymes; vesicles formed from golgi apparatus
Lysosome function
Membrane proteins pump H+ in to maintain acidic pH; rest of cell protected by membrane; digestion of substances entering the cell, cell components and entire cells
Mitochondria structure
Outer and inner mitchondrial membranes with small fluid-filled space between them (matrix); inner membrane contains cristae (series of folds)
Mitochondria function
Generate ATP through cellular respiration; the more energy a cell requires, the more ATP it must make and so more mitochondria are present
Cytoskeleton parts
Microfilaments, intermediate filaments and microtubules
Microfilaments structure
Thinnest cytoskeleton elements; comprised of actin and myosin molecules assembled in 2 long chains twisted around each other; found around periphery and lining interior of cell; are dynamic
Microfilaments function
Help generate movement, involved in muscle contraction, cell division and cell locomotion; structural support system of cell; bear tension and weight, anchor cytoskeleton to membrane proteins and provide support for microvilli
Intermediate filaments structure
Thicker than microfilaments but thinner than microtubules; comprised of a diverse range of different materials (e.g. keratin); found in cytoplasm or parts of cells subject to mechanical stress; most permanent of cytoskeleton elements
Intermediate filaments function
Bear tension and weight throughout cell; help stabilise and act as scaffold to organelles (e.g. nucleus); help cells attach to one another
Microtubules structure
Largest of cytoskeleton elements; long, unbranched hollow tubes comprised of mainly tubulin protein (coiled); extends from centrosome to periphery of cell; are dynamic
Microtubules function
Help determine and support cell shape and size; guide for movement of organelles (e.g. vesicles from golgi to membrane); chromosome organisation (cell division); support and movement of cilia/flagella
ATP; generation of ATP
Energy currency which powers cellular work; hydrolysis of ATP to ADP and inorganic phosphate releases energy
Steps of cellular respiration
Glycolysis, pyruvate oxidation, citric acid cycle and electron transport train
Glycolysis process
Anaerobic, occurs in cytosol, 2 ATP are invested; glucose is broken down into two pyruvate molecules, 2 net ATP and 2 NADH (formed when NAD+ accepts an H+)
Pyruvate oxidation process
Aerobic, occurs in mitochondrial matrix; pyruvate molecule (3C chain too big to enter citric acid cycle) is converted to Acetyl CoA (2C chain small enough to enter citric acid cycle), no ATP made, 1 NADH produced per pyruvate molecule (2 per glucose molecule) and 1 CO2
Citric acid cycle process
Aerobic, occurs in mitochondrial matrix; 2 Acetyl CoA molecules (one glucose molecule) go through a series of reactions where the product of the first reaction is the substrate for the next to produce 2ATP, 6NADH, 2FADH2 and 4CO2 (NADH and FADH2 are electron donors in electron transport chain; without O2, lactic acid is produced
Electron transport chain process pt1
Aerobic, occurs at proteins within inner membrane (cristae); NADH and FADH2 (from glycolysis and the citric acid cycle) are oxidised to donate 1 or 2 electrons which transfer from protein-to-protein along the chain (4 proteins in total); at each transfer, each electron gives up a small amount of energy which enables H+ ions (in the matrix) to be pumped into the intermembrane space, creating an electrochemical gradient; at last channel, O2 pulls electrons down the chain and is then the final electron acceptor, reduced to water
Electron transport chain process pt2 (chemiosmosis)
H+ ions in the intermembrane space rush down their concentration gradient (chemiosmosis) through ATP synthase, which causes the ‘turbine’ within ATP synthase to turn, enabling the phosphorylation of ADP to generate ATP; produces a total of 26 or 28 ATP
Net total ATP produced per glucose molecule
30-32 (2 from glycolysis, 2 from citric acid cycle and 26-28 from e- transport chain); not all NADH and FADH2 give one or two electrons consistently
Substrate vs oxidative phosphorylation
Substrate: ATP generated by the direct transfer of a phosphate group to ADP (by use of a substrate); oxidative: ATP generated from oxidation of NADH and FADH2 and subsequent transfer of e-s and pumping proteins, more efficient and no requirement for substrate
What can be used for cellular respiration?
Fats, proteins and more complex carbohydrates will generate ATP, along with glucose; they are broken into monomers which will enter glycolysis and the citric acid cycle at different points
How is cellular respiration controlled?
Phosphofructokinase can be rate limiting for glycolysis; inhibited by abundance of end product (citrate and ATP); stimulated by AMP which accumulates when ADP is not phosphorylated to ATP
Negative feedback
Integral to the control of ATP production (homeostasis usually depends on the mechanisms); more results in less (e.g. blood glucose)
Positive feedback
Can impact homeostasis; more results in more (e.g. blood clotting)
Homeostasis of blood glucose
Increase: beta cells in pancreatic islets secrete insulin, affecting all cells, promoting their glucose uptake and use (and ATP production) and increased conversion of glucose to glycogen; decreases blood sugar levels
Decrease: alpha cells in pancreatic islets secrete glucagon, affecting the liver and skeletal muscle cells; results in an increased breakdown of glycogen to glucose; increases blood sugar levels
Diabetes mellitus
Type 1: body doesn’t produce insulin, as beta cells of pancreas are destroyed; often autoimmune, genetic or through environmental factors
Type 2: body produces insulin but the receptors are non-functional (insulin resistance); can be linked to other pathologies and obesity
The ability to produce or respond to the hormone insulin is impaired; abnormal metabolism of carbohydrates and elevated levels of glucose in the blood
How does insulin work?
Binds to insulin receptor which elicits a change so that a nearby glucose channel will open and all of the high concentration glucose in the bloodstream can rush into the cell (down concentration gradient)
Gene expression and 3 main steps
Protein synthesis; the process of copying DNA to make RNA, which acts as a messenger to allow the information stored in DNA to be used to make proteins that will carry out cellular functions
Transcription, processing and translation
Protein synthesis regulation and importance
Transcription factors need to correctly assemble and DNA needs to be accessible; RNA processing includes capping, extent of polyadenylation, alternate splicing and production of an mRNA able to be translated (all done correctly); specific proteins assist in the export of mRNA; regulatory proteins can block translation, variable mRNA life-spans
To achieve the right thing at the right time in the right place (temporal and spatial control); crucial for cell function, allows cell to produce specific proteins needed and for cellular communication
Steps of transcription
Initiation, elongation and termination
Transcription (initiation)
Assembly of multiple proteins is required; promoter sequence (TATA box and start sequence); several transcription factors bind to DNA , RNA Polymerase II able to bind (along with other transcription factors) to form the transcription initiation complex
Transcription (elongation)
RNA Polymerase II unwinds small portions of the DNA helix and allows RNA nucleotides to H-bond to complimentary bases on the template strand (3’ to 5’); double helix reforms as transcript leaves template strand
Transcription (termination)
After transcription of the polyadenylation signal, the enzyme stops; nuclear enzymes release the pre-mRNA and RNA Polymerase II and transcription factors dissociate from the DNA`
Processing (immature transcript)
Capping: a modified guanine nucleotide is added to the 5’ end
Tailing: 50-250 adenine nucleotides are added to the 3’ end (Poly A tail)
Capping and tailing though to facilitate export, confer stability and facilitate ribosome binding once in cytoplasm
Processing (mature transcript)
Splicing occurs at the spliceosome within nucleus (large complex of proteins and small RNAs
Introns (non-coding regions intervening exons) are removed from the transcript and exons (coding regions including UTRs) are rejoined to make the mature transcript
Alternative splicing
Process where different combinations of exons are joined together; results in the production of multiple forms of mRNA from a single pre-mRNA; allows for multiple gene products from the same gene
UTRs
Untranslated regions at 5’ and 3’ ends that end up in the mature transcript but don’t end up in the protein sequence
Translation (initiation)
Mature mRNA transcripts exit nucleus and is bound by the ribosome; codons are translated into amino acids
Small ribosomal subunit with initiator tRNA (carrying the start codon (met)) already bound binds 5’ cap of mRNA; small ribosomal subunit scans downstream to find translation start site; H bonds form between initiator anticodon and mRNA; large ribosomal subunit then binds and completes the initiation complex
GTP (energy) required for assembly
Translation (elongation)
Codon recognition: base pairs on mRNA bind with complementary anticodons; GTP invested for accuracy/efficiency
Peptide bond formation: large subunit rRNA catalyses peptide bond formation and removes it from tRNA in P site
Translocation: moves tRNA from A to P site, tRNA in P site moves to E and is released; energy (GTP) required
Empty tRNAs are ‘reloaded’ in cytoplasm using aminoacyl-tRNA synthetases
Translation (termination)
Ribosome reaches stop codon on mRNA (bound by release factor to A site), release factor promotes hydrolysis (bond between p-site tRNA and last amino acid is hydrolysed, releasing polypeptide), ribosomal subunits and other components dissociate (hydrolysis of 2 GTP molecules required; ribosome components can be recycled)
Ribosome
Small subunit has an mRNA binding site
Large subunit has 3 binding sites for the tRNA molecules; A site holds “next-in-line” tRNA; P site holds tRNA carrying the growing polypeptide (bond forms); E site is place where tRNAs exit
4 groups of an amino acid
H group, carboxyl group, amino group and side chain
Primary structure of a protein
Sequence of amino acids bonded in a polypeptide chain; N-terminus (amino group at the end) at 5’ end which comes out first, C-terminus (carboxyl group at the end) which comes out at the end; primary structure is very temporary
Secondary structure of a protein
Occurs straight away, as soon as there is H-bonding able to interact with neighbouring H-bonds; held by weak H-bonds to form alpha helices or beta pleated-sheets
Tertiary structure of a protein
3D shape stabilised by side chain interactions of H-bonds; more globular, smaller and collapses into itself by tightly coiling; may be fully functional
Quaternary structure of a protein
Multiple proteins associate together to form a functional protein; not all proteins do this
How mRNA gets from ribosome to ER (6 steps)
- Polypeptide synthesis begins
- SRP (signal recognition particle) binds to signal peptide (N terminus of protein)
- SRP binds to receptor protein
- SRP detaches and polypeptide synthesis continues
- Signal-cleaving enzyme cuts off signal peptide
- Completed polypeptide folds into final conformation; secretory proteins are solubilised in lumen, while membrane proteins remains anchored to membrane; both then go into Golgi via vesicles for further maturation
Post-translational modification common method and importance
Phosphorylation (addition of a phosphate group); some occur in Golgi, others occur in cytosol; errors in modification could lead to non-functional proteins
Can confer activity; able to regulate it’s function
Cytosol proteins
Made by free ribosomes and modified by other free proteins in the cytosol
Mutations and effects of DNA sequence changes
Mutations can affect the structure and function of a protein; altered DNA sequence can be germ line (can affect many cells and be catastrophic) or local (during cell division, not whole body); chromosomal rearrangements
Point mutations: substitutions (one base replaced with another, can have major or minimal effect); insertions/deletions (can cause frameshift (can have major effect if within coding sequence)
Cell communication
Sending molecules or chemicals, either locally or widely, to elicit a response; need to be able to respond as a cell and as part of a whole tissue; often chemical signals, but can also be from the sensory system (light, taste, smell, etc.)
Local vs long distance cell signalling and examples
Local: signals act on nearby target cells - growth factors (paracrine, where cell releases signals to very nearby cells); neurotransmitters (synaptic, where neurotransmitters are released by exocytosis into space between axon and target cell, binds to receptors and may induce the opening of a channel, allowing for the influx of ions to enter)
Long distance: signals act from a distance; hormones produced by specialised cells travel via circulatory system to act on specific cells
Cell communication 3 main steps
Reception, transduction and response
Cell signalling: reception
Signalling protein (primary messenger/ligand) binds to a receptor protein which causes a shape and/or chemical change within the receptor protein; allows or causes the activation of a protein (e.g. G-protein/adenylyl cyclase)
Cell signalling: transduction
Activation of a protein (often an enzyme), relay molecules (second messengers) and/or other proteins may cause a relay of changes (signal transduction pathway) often via phosphorylation (known as a phosphorylation cascade)
Cell signalling: response
All of the activated proteins cause one or more functions to occur within the cell; the cell will do something
Two main types of receptors
Intracellular receptors; inside cell, often in cytosol, primary messenger is generally hydrophobic and/or small (lipid soluble, can cross PM); e.g. testosterone and estrogen
Membrane-bound/cell surface receptors: primary messenger is generally hydrophilic and/or large (needs help to cross PM); e.g. G protein coupled receptor, receptor tyrosine kinase and ligand-gated ion channel
Specificity of receptors
Human body will simultaneously send out many different chemicals/molecules aimed at eliciting certain responses but only target receptor on target cell will interact with that signal (ligand) and use it to activate signal transduction pathways; only certain cells at certain times will have particular receptors - while signal may be widespread, transmission of signal occurs only where it is needed (temporal and spatial control)
GPCRs
G-protein coupled receptors; transmembrane proteins which pass PM 7 times; have many different ligands and diverse functions; couple with G-proteins (molecular switches that are on or off depending on whether GDP or GTP are bound
How does reception work with GPCRs
- At rest, receptor is unbound and G-protein is bound to GDP; enzyme is in an inactive state
- Ligand binds to receptor (which causes a shape change) and binds the G protein; GTP replaces GDP so G-protein is active; the enzyme is still inactive
- Activated G-protein dissociates from receptor and moves along PM where it binds to and activates an enzyme (which undergoes a conformational change) to elicit a cellular response (via a phosphorylation cascade or secondary messenger)
- G-protein has GTPase activity (release of GTP), promoting its release (dephosphorylation), reverting back to resting state
Ligand-gated ion channels
Channel receptor which contain a gate; binding of ligand at specific site on receptor elicits change in shape and channel opens/closes so that ions can pass through the channel
The nervous system: released neurotransmitters bind as ligands to ion channels on target cells to propagate action potentials
How does reception work with ligand-gated ion channels?
- At rest, ligand is unbound and gate is closed
- Upon ligand binding, gate opens and specific ions can flow into cell; ions act to initiate cell response due to their electrical signals
- Following ligand dissociation, gate closes and receptor is at rest again
Signal transduction pathways
Signals can be relayed from receptors to target molecules within the cell via a ‘cascade’ of molecular interactions
A typical phosphorylation cascade: protein kinases (enzymes) transfer a phosphate group from ATP to another protein, which typically activates it; series of protein kinases each adding a phosphate to the next kinase; phosphatases (enzymes) dephosphorylate the protein, rendering it inactive but recyclable
Secondary messengers
Small molecules (that aren’t proteins) sometimes included in the cascade which transfer the signal from the enzyme to the cascade or gate; e.g. cAMP and calcium ions
cAMP as a secondary messenger
cAMP in GPCR signalling: activated enzyme is adenylyl cyclase that converts ATP to cAMP (cyclic AMP) which acts as a second messenger and activates downstream protein, signal progresses from there
IP3 as a secondary messenger
Ca2+ and IP3 in GPCR signalling: activated protein is phospholipase C which then cleaves PIP2 (phospholipid) into DAG and IP3; IP3 diffuses through cytosol and binds to gated channel in ER; Ca2+ ions flow out of ER down concentration gradient and activate other proteins toward a cellular response
Which body process uses Ca2+ ions as secondary messengers?
Muscle contraction
Importance of multiple steps in the signal cascade
Amplifies the cellular response, provides multiple control points, allows for specificity of response despite molecules in common (temporal and spatial control), allows for coordination with other signalling pathways
Cellular response examples
Gene expression; alteration of protein function to gain/lose activity; open/close of ion channel; alteration of cellular metabolism; regulation of cellular organelles or organisation; rearrangement/movement of cytoskeleton; any combination of these
Termination of signalling
All signals are for a limited time; activation/start of cellular response usually promotes the start of deactivation so that signalling is of short period of time (ensuring homeostatic equilibrium); cell is ready to respond again if required
Adrenaline stimulation of glycogen breakdown
Reception: binding of epinephrine to G protein-coupled receptor
Transduction: adrenaline acts through a GPCR, activates cAMP and 2 protein kinases in a phosphorylation cascade
Response: results in an active glycogen phosphorylase which converts glycogen to glucose 1-phosphate; amplification means that 1 adrenaline molecule can result in 10^8 glucose 1-phosphate molecules
Glucose 1-phosphate converted to glucose 6-phosphate which can be used in glycolysis to generate ATP
Somatic cell division
Mitosis; diploid 2n to diploid 2n (cell makes 2 identical cells); growth and development, tissue renewal
Parts of the cell cycle
G1, S and G2 (interphase) and M
G0 phase
Stage where cell is not dividing; some cells which don’t divide, spend majority of life or have opted out of cell cycle are in this stage and do not progress past G1; e.g. nerve cells
G1 phase
Growth or gap phase 1 (active cell); most cellular activities are occurring here; duration is variable (cell type specific); organelle doubling occurs in preparation
S phase
Synthesis of DNA; DNA replication occurs; strands are separated at H-bonds holding nucleotides together and new strand of DNA is synthesised by DNA polymerase opposite each of the old strands (from 5’ to 3’ by okazaki fragments)
G2 phase
Growth or gap phase 2; checks for correct DNA synthesis; prepares for Mitotic phase (synthesis of proteins and enzymes required, gathering of reactants); replication of centrosomes is completed
M phase
Mitotic phase (mitosis and cytokinesis)
Prophase: mitotic spindle fibres (microtubules) form, chromatin condenses to chromosomes held together by centromeres, nuclear membrane dissolves
Metaphase: spindle fibres are fully formed and attach to centrosomes of condensed chromosomes, arranging them to cell equator
Anaphase: spindle fibres contract and separate identical sister chromatids to opposite poles of the cell
Telophase: chromosome reverts to chromatin and nuclear membrane reforms; cytokinesis occurs where cytosol splits, cleavage furrow appears and plasma membrane pinches off to form two identical cells
Mitotic cell checkpoints occurrence
Near end of G1 phase (G1 checkpoint), between G2/M phases (G2 checkpoint) and after M phase (M checkpoint)
If cell hits a checkpoint, but fails checks, the process will halt
G1 checkpoint
Check: if the DNA is damaged, if the cell size and nutrition is OK, if the appropriate signals are present, if energy stores are appropriate
If yes, cell will go into S stage; if no, cell will exit to G0 or continue in G1
M checkpoints
Check: if all chromosomes are attached to spindles; final point prior to anaphase and telophase
If yes, cell will undergo mitosis; if not, stop signal is received and production of cyclin will be delayed or cancelled until cell repairs or dies
Importance of checkpoints
Uncontrolled cell growth of dysfunctional cells
Key regulatory molecules for G2 checkpoint
Cyclin: protein that fluctuates throughout cell cycle, produced in late S phase and early G2 phase
Cyclin dependent kinase (Cdk): normally inactive kinase (phosphorylates/activates proteins) that is activated when attached to a cyclin
M-phase promoting factor (MPF): a cyclin and Cdk complex which is a key for G2 checkpoint; phosphorylates many other proteins, allowing mitosis to commence
Both MPF activity and cyclin concentration spike during M phase
Once mitosis is initiated, cyclin is degraded (to prevent continuous promotion of cell division) and MPF reverts back to Cdk to be recycled
Mutations in cell cycle
DNA changes can be small scale (point) or gain/loss/translocation of chromosomes/genes; can be the result of acquired changes (effects specific cells; e.g. viruses, UV damage, drugs and treatment) or inherited changes (effects all cells; e.g. susceptibility genes)
In both types, altered protein function may occur which can result in loss of cell cycle control
Cell-division promotor
Proto-oncogene
Cell-division inhibitor
Tumor suppressor gene
Cancer definition and process
Loss of the control of the cell cycle; deactivation of tumor-suppressor gene (TSG) and activation of proto-oncogenes (to form oncogenes); multiple mutations must occur
Cell/tumor level: series of accumulated mutations as a results of damage to the DNA
Systemic: born with mutated DNA
Proto-oncogene mutation (3 types) and examples
Mutation within gene results in a hyperactive, growth-stimulating protein, oncogene, (enables excessive growth of cell); can be larger-scale chromosomal rearrangements where there are multiple copies of the gene, which results in a higher chance of unwanted stimulation of the cell-cycle; can be moved to new DNA position under new controls (new promoter) which could result in multiple copies of the gene and a higher chance of unwanted cell-cycle stimulation
e.g. ras (a GTPase) and Myc (a transcription factor)
Tumor suppressor gene mutation and examples
Small-scale or large-scale mutation results in a defective, nonfunctioning protein; cell division not under control resulting in uncontrolled cell growth (cancer)
e.g. TP53 (P53), BRCA1, BRCA2
Reproductive cell division
Meiosis; diploid 2n to haploid n, fertilisation restores diploid number of chromosomes; produces 4 genetically different cells; meiosis 1 and meiosis 2
Meiosis I
Prophase I: two sister chromatids of each pair of homologous chromosomes pair up; non-sister chromatids within tetrads may cross over (recombination) (4 chromatids = tetrad)
Metaphase I: pairs of homologous chromosomes (tetrads) randomly line up along cell equator and spindle fibres attach to centromeres
Anaphase I: sister chromatids remain attached as homologous pairs are randomly pulled apart to opposite poles of cell
Telophase: cleavage furrow appears and cytokinesis takes place
Meiosis II
Same steps as mitosis (PMAT), although DNA replication has not occurred prior to cell division; sister chromatids are separated during anaphase II; haploid daughter cells (4) are formed
Differences between mitosis and meiosis
Individual chromosomes with 3 spindle fibres (mitosis) vs pairs of homologous chromosomes/tetrads (meiosis; synapsis during prophase I) line up along equator; cross over between tetrads can occur during meiosis; 2 genetically identical cells produced (mitosis) vs 4 genetically different cells produced (meiosis)
Sources of genetic variation in meiosis
Independent assortment at metaphase; crossing over at prophase I; fusion between two gametes (sexual reproduction)
