BCG LO2: Explain the roles of sub-cellular components

 Nucleus:

  • Nucleosomes: DNA coiled around histones (positively charged octamer attracts negatively charged DNA), condenses to form chromosomes during replication

  • Chromosomes: short (p) and long (q) arms, two identical chromatids bound at centromere, telomeres (repeat regions which reduce during each replication, regulate cell life)

  • Nucleolus: rRNA transcription, ribosome subunit assembly and pre-rRNA processing, contains dense fibrillar and granular components

 

RNA:

  • rRNA  (translation site of ribosomes, forms ribosomes w/ proteins in nucleolus)

  • tRNA (bound to amino acids, delivered to ribosomes for peptide chain)

  • mRNA allows info from DNA to be read by ribosome - correct tRNA anticodon selected

 

Nuclear membrane:

  • Protects genetic info (DNA never leaves nucleus)

  • Continuous w/ ER and contains nuclear pores

 

Endomembrane system:

Nuclear pores:

  • Protein lined channels closed unless nuclear localisation signals bind to receptors, allowing pores to open.

  • Transcription changes in response to environmental changes rely on nuclear pores

  • 5-15% total surface in mammals

  • Allows mRNA to leave the nucleus for translation

Endoplasmic reticulum:

  • RER - protein synthesis, SER - lipid (+carb) production, liver cells (detoxification), muscle cells (Ca2+ storage)

Golgi apparatus:

  • Cis -> trans face

  • Phosphorylation, saccharide and fatty acid addition / cleavage

Exocytosis:

  • Constitutive - vesicle lined w/ membrane proteins

  • Regulated - active transport by ligand-receptor complexes

Cytosol:

  • 70-80% water

  • Contains proteins, mono- and polysaccharides, amino, fatty and nucleic acids, cations

  • Allows metabolic reactions to occur

 

Mitochondria:

  • Maternally inherited circular DNA - produce ATP synthase and other TCA enzymes

  • Responsible for aerobic respiration (TCA cycle and oxidative phosphorylation)

  • Bacterial conformation (60S) ribosomes

  • Endosymbiotic theory - mitochondria formed when bacteria formed symbiotic relationship w/ Archaea: facilitated differentiation, fitness advantage allowed propagation of eukaryotes

  • Replicate independently of the cell

  • Double membrane - inner membrane is highly folded (cristae)

  • Mitochondrial matrix - contains TCA enzymes

  • Periplasm - intermembrane space, facilitates proton gradient

 

Electron transport chain:

  • Complex I

    • Oxidises NADH, pumps 4 protons, reduces ubiquinone

  • Complex II

    • Oxidises FADH2, does not pump protons, reduces ubiquinone

  • Complex III:

    • Oxidises ubiquinone, pumps 4 protons, reduces cytochrome C

  • Complex IV:

    • Oxidises cytochrome C, pumps 2 protons and reduces O2

 

2H+ + O + 2e-   Oxygen is terminal electron acceptor

 

10 protons pumped for every 2 electrons (Oxidation of NADH and FADH2

Ubiquinone and Cytochrome C act as shuttles

 

Proton motive force powers ATP synthase, in reverse orientation to proton pores

 

 

Ribosomes:

  • Responsible for protein translation

  • Small (40S) subunit holds mRNA binding site

  • Large (60S) subunit has A (aminoacyl), P (peptidyl) and E (exit) sites for tRNA

  • Codon - anticodon binding between tRNA and mRNA

  • Measure of activity (more proteins transcribed, greater ribosomal activity)

  • Free ribosomes produce intracellular proteins

  • Bound ribosomes produce specific proteins (lysosomes, extracellular hormones + enzymes)

  • Highly conserved, phylogenetic marker

 

Cytoskeleton

  • Microfilaments (7nm diameter)

    • Cellular movement - muscle cells use in combination with myosin

    • Chemokines/cytokines attract cells, microfilaments used to move in direction dictated by signalling molecules

    • Actin monomers polymerise - create new binding point at cell 'front'

    • Depolymerisation/hydrolysis at cell 'back' allows movement

  • Intermediate filaments (8-10nm diameter)

    • Structural, filamentous proteins allow cell shape and organelle arrangement to be maintained

  • Microtubules (25nm diameter)

    • Polymerised a and B tubulin dimers, hollow tubes allow vesicle movement, cell division (centrioles attaching to centromeres) and extracellular appendages (cilia + flagella)

  • Centrioles

    • Organisation of short lengths of microtubules

    • Polymerisation (expansion) allows attachment to and movement of chromosomes through centromere attachment

    • 2 perpendicularly attached centrioles = centrosome

 

Peroxisomes and Lysosomes

  • Peroxisomes

    • H2O2  synthesis and degradation

    • Detoxify oxidase reactions

    • Fatty acid degradation for energy release

    • Self-replicate through fission (think binary fission)

  • Lysosomes

    • Highly glycosylated proteins protect membrane from low internal pH

    • Contain digestive enzymes w/ acidic optimum pH

    • Active transport proteins pump H+ into vesicle

    • Bud directly from Golgi apparatus

    • Prevent signalling cascades

    • Autophagy, heterophagy, autolysis/apoptosis, fertilisation

 

Cell Cycle and Cytokinesis

  • Growth, repair and reproduction

  • Binary fission used by bacteria: single chromosome replicated - two identical daughter cells produced

 

Cytokinesis - cell division

Microfilament contraction around cell equator forms cleavage furrow- starting during anaphase and continues in telophase

 

Interphase

  1. G0 Senescence (metabolism halted)

Become quiescent (dormant), can re-enter the cell cycle, senescent cannot (aging or deteriorating), usually due to damaged or defective DNA

Muscle and nerve cells do not divide (continual G0)

 

  1. G1 : Gap1

Visual inactivity, high level of metabolic activity (organelle duplication and protein production)

G1 regulation point causes variation in length

 

  1. S phase - DNA synthesis

Histone production

Phospholipid production

DNA replication, producing

50% of cell cycle

Cyclin production begins

 

  1. G2 : Gap2

Microtubules and other cytokinetic proteins synthesised

Centrosomes cell poles polymerise microtubules to attach to centromeres of chromosomes

Checkpoint repairs any chromosomal errors

 

Mitotic phase

Nuclear division of genetic material - genetically identical daughter cells

  1. Prophase

Nuclear membrane breakdown, chromosomal condense, spindle fibres appear

  1. Prometaphase

Spindle fibres attach to condensed chromosomes

  1. Metaphase

Chromosomes align, metaphase plate

  1. Anaphase

Chromosomes divide, chromatids pulled to opposite poles by depolymerising microtubules

  1. Telophase

Nuclear membrane reforms, chromosomes decondense, spindle fibres disappear

 

Cell Cycle Regulation

Protein concentration changes activated by reactive enzymes

Cyclins, CDKs (Cyclin-independent kinases) and MPF (mitotic promotion factors)

 

  1. Cyclin accumulates, production begins in late S phase, binds to CDK, forms MPF

  2. Cyclin kinase degrades cyclin, releasing CDK

 

P53

  • Prevents DNA replication (G1 -> S) mitosis with unreplicated DNA (disputed)

  • Stops mitosis after induction by damaged DNA (G2 -> M)

  • Stops division of tetraploidy cells (M) - if two replications have occurred without cytokinesis

  • Binds and inhibits cdKs

  • Activation of inhibitor p21

 

Anchorage (planktonic -> sessile) and density (high number of cells) dependence

DNA damage/repair

Growth factors

Kinetochore adhesion (Spindle fibre attachment checkpoint)

 

If cell is forced to stop: senescence can allow DNA repair or apoptosis

If regulatory mechanisms fail, cancer develops

 

Apoptosis - 'clean': Blebs form, nucleus and organelles break apart into apoptotic bodies which can be engulfed by macrophages and reused.

Necoris - 'messy':  release of cellular contents causing immune response  (response to injury - viral infections)

 

 

Meiosis:

Occurs in somatic cells - 4 genetically different daughter cells

Recombination at chiasmata occurs during Prophase I, sister chromatids overlap

Independent assortment, homologous chromosomes align randomly along metaphase plate

Read: Cell Cycle Regulation by Checkpoints

 

Kevin J. Barnum and Matthew J. O'Connell

Enzymes:

  • Catalyse biochemical reactions by lowering the activation energy required:

    • Strains substrate bonds, reducing energy required to break

    • Reduces distance between substrate molecules in catabolic reactions, allowing new bonds to form in a collision

 

  • Substrate molecules complementary (geometrically or electromagnetically) to active site, competitive inhibitors also complementary

    • Vmax maintained - enzyme produces equal product to uninhibited

      • identical L-B y-intercept

    •  Km decreases - less effective affinity for substrate as can also bind competitive inhibitor

      • reduced L-B gradient

      • more negative x-intercept

  • Small conformational changes can occur upon binding

  • Allosteric sites allow binding of non-competitive inhibitors and coenzymes (apoenzyme -> holoenzyme)

    • Vmax decreases, due to reduced product production

      • increases L-B y-intercept

    • Km maintained, as enzyme has identical effective affinity for substrate despite fewer enzymes able to bind substrate

      • steeper L-B gradient

      •  identical x-intercept

  • Uncompetitive inhibitors bind to ES complex, preventing release of products

 

Gibbs' Free Energy

 

G = H - TS

Gibbs' Free Energy must be negative for a reaction to be energetically favourable

H = Enthalpy (heat energy given off/taken in) jmol-1

T = Temperature (Kelvin)

S = Entropy