Topic 2 - Cells / AQA A-level Biology 3.2, Study notes of Biology

Download Topic 2 - Cells / AQA A-level Biology 3.2 and more Study notes Biology in PDF only on Docsity! EUKARYOTIC CELLS Eukaryotes are inclusive of unicellular and multicellular organisms. The defining property of eukaryotes is the presence of a membrane-enclosed nuclear body, NUCLEUS Nuclear body is most prominent feature of cell, constitutes 8% of cell volume. Nucleoplasm and cytoplasm separated by two concentric membranes which comprise nuclear envelope. ▪ Membrane is continuous with the endoplasmic reticulum, extends into endomembrane system ▪ distinct nuclear body maintained by bilayer ⇢ compartmentalization allows the cell to prevent translation of unspliced Mrna + protect DNA from damage ⇢ Nuclear pores present on nuclear membrane allow selective passage of molecules such as mature mRNA ▪ Chromatin fibres contain genome in association with histone fproteins.▪ Nucleolus contains genes for ribosomal RNAS, direct synthesis of machinery ▪ FUNCTIONS Preserve integrity of genome by maintaining separate body▪ Site of ribosome synthesis and location of ribosomal RNA manufacture▪ Site of transcription ▪ RNA post-transcriptional modifications (splicing etc) occurs in spliceosome complexes suspended in nucleosome ▪ MITOCHONDRIA Energy-generative function, synthesising ATP through Kreb's cycle + oxidative phosphorylation. Aerobic respiratory pathway relies on mitochondria as primary site Inner mitochondrial membrane is site of electron transport chain, where successive redox reactions are coupled with H+ transport which generates proton gradient necessary to drive atp synthesis ○ cristae infoldings extend surface area available for occupation by redox complexes embedded in membrane ○ • Originated via endosymbiosis of bacteria, retains degree of autonomy through having own mitochondrial DNA, ribosomes and membrane • Mitochondrial matrix contains concentrated mixture of enzymes distinct from chemical composition of cytosol • RIBOSOMES Ribosome complexes found embedded in membrane of RER, free ribosomes are suspended in cytosol. rRNA synthesis + ribosomal assembly occurs within nucleolus, exported via nucleoporins. ▪ mRNA transported to ribosome for translation, binds to form polyribosome complex. ▪ ⇢ Enzymatic component catalyses addition of amino acids to growing polypeptide chain as directed by mRNA sequence. EUKARYOTIC 80s PROKARYOTIC MITOCHONDRIAL 70s CHLOROPLAST Membrane-enclosed hydrolytic vesicles containing concentrated mixture of enzymes responsible for breaking down molecules (autophagy and efferocytosis) in intracellular environments ▪ Enzymes optimised for function in acidic conditions pH ~5 maintained within the lysosome▪ LYSOSOMES 3.2.1- CELL STRUCTURE 15 March 2022 10:20 3.2.1 - CELL STRUCTURE Page 1 GOLGI BODY Consists of stack of flattened, membranous stacks (cisternae). In close association with the ER, forms complex cytoskeletal network with adjacent membranous tubes. Site of post-translational modifications and export of synthesised proteins from trans face to intended intra/extracellular destinations ▪ Glycosylation, oligosaccharide chains added to proteins during synthesis of glycoproteins. ▪ Vesicle buds from trans Golgi network fuse with plasma membrane via exocytosis. Secretory role as pathway allows continual supply of newly synthesised lipids and protein. ▪ Proteins destinations regulated ⇢ sorting, packing, exported ▪ ROUGH ENDOPLASAMIC RETICULUM irregular body of interconnected membranous sacs which occupies cytoplasm, extending out from nuclear membrane ▪ Lumen maintained separate from cytosol by single membrane▪ ⇢ provides a specialized local environment that favors the modificationolding, and assembly of a selected subset of the cell’s proteins. Rough portion studded with ribosomes▪ membrane provides a large surface area to which many ribosomes attach ▪ SMOOTH RETICULUM interconnecting system of membranous pipelines traversing the cytoplasm, lacks associated ribosomes ▪ Site where steroid hormones in the endocrine cells of the gonad and adrenal cortex are synthesised ▪ Calcium ion mediated cell function involves regulated release of Ca 2+ from the SER (⇢ Muscle contraction + nerve signalling) ▪ produce new phospholipids from free fatty acids and insert them into the cytosolic monolayer to construct membranes ▪ Newly synthesized membrane proteins, as well as secretory and lysosomal proteins, leave the ER and enter the Golgi complex at its cis face and then pass across the stack to the trans face. PLANT-SPECIFIC ORGANELLES CELL WALL Protective role, confers resistance to cell against damage from mechanical abrasion and pathogens Present in prokaryotes, fungi + plants, absent in animals▪ structural polysaccharides starch, peptidoglycan and chitin comprise main structure Organisation into rod-like microfibrils that confer rigidity- Hydrogen bonds provide high resistance to tensile forces- ▪ Mediate intracellular interaction▪ Structural reinforcement of plant▪ VACUOLE Occupies as much as 90% of cell volume▪ Turgidity conferred by vacuole supports cell and maintains integrity of plant ▪ Storage/isolation of toxic compounds, functions as site of temporary storage of metabolites or materials ▪ Hydrolases sequestered within vacuole ▪ Tonoplast membrane dominated by active transport pumps which maintain osmotic gradient so that water enters vacuole via osmosis. ▪ Structurally simpler prokaryotic cells inclusive of bacteria– Structurally complex eukaryotes incude protoctista, fungi, plants and animals– ENDOMEMBRANE SYSTEM PROKARYOTIC VS EUKARYOTIC CELLS FEATURE PROKARYOTES EUKARYOTES nucleus No distinct nuclear body, genetic information not separate from cell body Distinct compartmentalisation of genetic information inside dense nuclear body and cytoplasm endomembrane Lack of endomembrane system complex membranous cytoplasmic organelles respiration Electron transport chain uses diverse substrates as terminal acceptor Oxygen is final recipient of electrons in the redox series reproduction Division via binary fission in asexual reproduction Sexual reproduction, involving meiotic division and fertilisation offspring Produces genetically identical daughter cells Sexual reproduction leads to generation of unique gametes cell wall Peptidoglycan cell wall Cell wall comprised of structural polysaccharides such as cellulose or chitin (if present) chromosomes Circular chromosome loop (in nucleoid), dissociated and free floating in cytosol Linearly arranged chromosomes associated with histone proteins, compact genome size Contain fewer base pairs, less genetic information Highly complex genome with intron/non-coding regions 3.2.1 - CELL STRUCTURE Page 2 Each cell passes through a series of 4 defined stages which constitute the cell cycle. Two major phases of cellular activity differentiated, INTERPHASE + M-PHASE MITOSIS Process of cellular division in which replicated DNA is segregated into two distinct nuclear bodies. This is accompanied by cytokinesis - division of cytoskeleton and larger cell body PROPHASE duplicated chromosomes are prepared for segregation and the mitotic machinery is assembled1. Chromosome compaction, chromosomes converted into condensed visible structures ▪ ⇢ condensed form chromosomes consist of two sister chromatid attatched at the centromere Mitotic spindle formation initiated.▪ microtubule elongation from centrioles located at opposite poles▪ Nuclear envelope degradation, disassembly of nuclear body▪ METAPHASE2. Microtubule attachment to kinetochore of centromere▪ Moving via congression to centre of cell apparatus▪ Alignment along metaphase plane, tensile pulling by opposite spindle fibres orients chromosomes ▪ ANAPHASE3. Cleavage of cohesion triggers the separation of sister chromatids ▪ shortening of chromosomal microtubules causes poleward migration. ▪ TELOPHASE 4. mitotic spindle apparatus disassembles▪ nuclear envelope reformation▪ Chromosomes decondense and disperse▪ Cytokinesis - partitioning of the cytoplasm ▪ ⇢ (abscission, surfaces of the cleavage furrow fuse splitting the cell Asexual form of reproduction▪ Utilised by prokaryotic organisms such as bacteria + amoeba▪ BINARY FISSION Differs from eukaryotic mitosis in: do not undergo karyokinesis • No spindle apparatus• Produces 2 identical daughter cells, no variation is introduced • No histone proteins must be dissasociated • STAGES Replication of circular DNA begins at origin of replication and occurs bidirectionally 1. Cell elongation initiated2. Duplicated chromosomes separate, migration towards opposite poles. 3. Majority of cycle dominated by interphase: Interphase consists of G1, S, G2 • Involves normal metabolism, organelle duplication, DNA replication and growth • M phase involves successive sequence of mitotic division followed by cytokinesis Duplicated chromosomes segregate• Migration to opposite poles• division into 2 distinct cell bodies• maintains the chromosome number and generates new cells for the growth and maintenance of an organism.▪ STAGES Daughter cells return to the interphase conditions Chromatids of each chromosome are split apart and separate into two daughter nuclei in a single division. Septum formation initiated at midpoint of cell. extending gradually from the periphery toward the centre of the cell. Cell wall pinches 4. each origin point moves away from cell wall attachment toward opposite ends of cell.5. 3.2.2 - CELL DIVISION 22 April 2022 11:06 3.2.2 - ALL CELLS ARISE FROM OTHER CELLS ^M MEIOSIS Page 1 MEIOSIS Process by which chromatids of a pair of replicated homologous chromosomes are distributed among four daughter nuclei. As they are paired, homologous chromosomes engage in a process of genetic recombination that produces chromosomes with novel combinations of maternal and paternal alleles. Variation introduced through: Independent assortment, maternal and paternal chromosomes become shuffled during formation of gametes• Recombination of chromosome loci which have synapsed, crossing over introduced novel combinations• Consist of two sequential divisions without an intervening round of DNA replication Forms haploid daughter cells which fuse during sexual reproduction• ⇢ doubling of the chromosome number at fertilization is compensated by an equivalent reduction in chromosome number at a stage prior to formation of the gametes synapsis, pairing between homologs▪ Bivalent (tetrad) forms between pair of synapsed homologous chromosomes, ▪ Chiasmata formed by covalent junctions between sites on non-sister chromatids▪ Genetic variation introduced, novel allele pairs generated - mix of paternal + maternal alleles▪ MEIOSIS II ▪ MEIOSIS I ` microtubules extend from opposite poles▪ Sister chromatids attach to same spindle fibre▪ Alignment of chromosomes of each bivalent on the metaphase I plate is random - consequence is random assortment of chromosomes (⇢ obeys mendels law) ▪ chromosomes form tetrad▪ Synapsis, chiasmata formation▪ recombinatio▪ pulling by spindle apparatus separates a pair of recombinant homologous chromosomes into two separate cells ▪ Dissolution of chiasmata, cohesion between arms of homologs lost ▪ 2n (diploid) cells result▪ chromatids are held together at centromere, with microtubules from opposite poles attached to 2 kinetochores▪ kinetochores of sister chromatids face opposite poles, beco▪ synchronous splitting of the centromeres holding sister chromatids together, allowing them to move toward opposite poles of the cell Genetic information enclosed in distinct body by reformation of the nuclear envelope. products of meiosis are haploid cells (1n) ⇢ contain only one member of each pair of homologous chromosomes 3.2.2 - ALL CELLS ARISE FROM OTHER CELLS ^M MEIOSIS Page 2 ▪ me attached to opposing sets of spindle fibres. ▪ 3.2.2 - ALL CELLS ARISE FROM OTHER CELLS ^M MEIOSIS Page 3