Cito Logie
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Transcript of Cito Logie
Learning objectives
• To describe the basic structure of a eukaryotic cell
• To understand the cellular processes occurring within the cell
The Eukaryotic cell
• The biological building unit of all multicellular organisms
• Contain a membrane enclosed nucleus and can replicate independently.
• The cell membrane is made up of a phospholipid bilayer.
• Fluid mosaic model (1972, Singer)
• Barrier
• Transporter
• Maintains – Membrane potential
– Cell volume
– Homeostasis
• Anchoring of receptors, enzymes and cytoskeleton
• Connection and communication between cells
Cell membrane
hydrophobic
hydrophillic
Permeability
• The most important property of the cell membrane is its selective permeability, • Small and nonpolar (hydrophobic) molecules can freely pass through the membrane,
but charged ions and large molecules such as proteins and sugars are barred passage.
Cell junctions
Nature Reviews Gastroenterology & Hepatology 10, 90-100 (February 2013)
Tight junction
Adherent junction
Desmosomes
Gap junctions
• Intracellular channel consists of two hemi-channels, connexons that are docked end-to-end
• Connexons, translated at 4 to 8-cell stage only begin to traffic to the plasma membrane after compaction.
• Main action – CELL SHARING
– allow electrical current, small molecules (less than 1kDa), metabolites or ions to travel between cells, including but not limited to cAMP, ATP, IP3, glucose, glutamate, Ca2+ and K+ .
– Role in homeostasis and cell signalling (via phosphoryltaion of C-terminus).
Chatterjea, BluSci
The cytoplasm consists of – cytosol, gel-like fluid that fills the cell and is a where protein synthesis
and other fundamental chemical reactions occur.
– cytoskeleton, a protein structure that maintains cell shape and helps move organelles around the cell.
– organelles, membrane-bound components, each with a distinct structure and function
The nucleus– contains DNA, template to all protein in a cell
Inside the cell
Cytoskeleton
• Protein structure, organization and transport
• Shape change and movement, resist stress
• Connects cell to external environment
• Differ in size and stiffness
• Differ in assembly and disassemblydynamics
Microtubules, form tracks that
can span the length of the entire
cell, like ’highways’ for
intracellular traffic, can
reorganize rapidly.
Cytoskeleton
Actin filaments, support leading
edge of cell movement by steady
elongation. Support filopodial
extensions, and control shape.
Exo- and endocytosis.
Important role during compaction
and hatching.
Centrosome
• “Division centre” segregating chromosomes to each daughter cell
• Composed of centrioles and Pericentriolar material (PCM)
• Centrioles are of paternal origin, oocyte looses during oogenesis
• PCM is supplied by the oocyte
• Replicate at same time as DNA synthesis, prior to meiosis I and II in spermatogenesis and after fertilization
• Abnormalities result in
– Failure in chromosome segregation
– Aneuploidy
– Multinucleation
Nucleus
• Contains the template to all proteins in the cell – DNA
• Nucleolus – synthesizes rRNA and assembles ribosomes.
• Double membrane - outer membrane is in continuim with ER
• Nuclear pores are tightly controlled
The genome
• Chromatin consists of DNA, RNA and proteins
• Chromatin
– packages DNA into a smaller volume
– stabilizes DNA macromolecule during mitosis
– prevents DNA damage
– controls gene expression and DNA replication
• As cells prepares to divide (mitosis or meiosis) chromatin packs more tightly to facilitate segregation of chromosomes
Chromosome
Sperm DNA
• During spermiogenesis, histones are replaced by protamines, condensing the DNA into tightly packaged toroids
• High state of supercoiling inhibits transcription
Mol Hum Reprod (2010)16: p30-36
DNA structure
• Double stranded helix
• Nucleobases are purines (Adenine or Guanine) or pyrimidines (Cytosine or Thymine)
• Hydrogen bonds between purines and pyrimidines stabilize helix structure
DNA Replication Fork
Helicase: breaks hydrogen bonds to separate DNA strands
DNA polymerase: adds free nucleotides to the 3' end of the newly forming strand and proofreads
Topoisomerase: prevents overwinding, cuts and reseals phosphate backbone
DNA primase: creates a RNA primer for DNA polymerase to start replication
ehinger.nu
Cell cycle checkpoints
• The cell cycle is guarded at 3 checkpoints (boundaries)
– G1/S
– G2/M
– Metaphase/Anaphase (SAC)
• Respond to Cdks and cyclins complexes to progress through checkpoints
• Activity modulated by reduced availability of cyclins, or phosphorylation of complexes and positive and negative feedback loops
Spindle assembly checkpoint SAC
Mitosis
P - nuclear envelope breaks down, centrioles begin moving to opposite ends of the cell, microtubules extend from centrioles and begin to attach to the centromeres of chromosomes
M - spindle fibers align chromosomes at the metaphase plate at equal distance from centrioles, the mitotic spindle
A - each pair of sister chromatids splits at the centromere, separates and moves along the shortening spindle fibers to opposite sides of the cell, the number of centromeres and chromosomes within the cell is doubled
T - a new nuclear membrane forms around each of the two new daughter nuclei, spindle fibers begin to disperse, chromosomes decondense,
Cytokinesis - cytoplasm divides to form two identical daughter cells
Prophase 2n:4c
Diploid parent cell2n:2c
DNA replication
Metaphase 2n:4c
Anaphase 4n:4c
Telophase 4n:4c
Cytokinesis 2n:2c
SAC
Mitochondria
• POWER HOUSE: primary source of ATP
• Permeable outer membrane (porins)
• Impermeable inner membrane with folds (cristae)
• Own DNA and ribosomes
• Maternally inherited (highly conserved)
• Divide by binary fission in response to energy needs, or at same time as cell cycle
• During embryogeneisis, replication (increase in numbers/per cell) does not begin until after implantation
MitochondriaImbalance of ATP supply/demand associated with
– Chromosomal segregation disorders
– Maturation and fertilization failure
– Arrested cell division and abnormal cytokinesis
– Cell apoptosis
•A mtDNA threshold has been implicated as a new euploid embryo selection marker (Fragouli ESHRE 2014)
Sperm –Defects in structure, genome, membrane potential or O2 consumption associated to loss of sperm function
–Critical for fertilization
–Sperm mtDNA is then degraded inside the zygote Reproduction (2013) 146 163–174
Mt (green) found in midpiece
DNA (blue)
MII D3 D4
D5
Rough endoplasmic recticulum
Post translational modification and packaging of proteins, deliver to golgi and membranes.
brittocellproj.weebly.com
sciencephoto.com
Smooth endoplasmic recticulum
• Synthesis and metabolism of lipids, steroids and carbohydrates and detoxification of drugs and poisons (high number in cells of ovaries and testes)
• Regulation of Ca2+ ion concentrations
• SER+ oocytes have been reported to have lower fertilization, embryo development and pregnancy rates and compromised neonatal oucomes
• Presence of SER disturbs Ca2+ stores and oscillations
Golgi Apparatus
• Stacked membranes called Cisternae
• Central organelle mediating protein and lipid transport
• Processes proteins made by the endoplasmic reticulum (ER) before sending them out to the cell
• Golgi enzymes catalyze protein modifications
– addition or removal of sugars (glycosylation)
– addition of sulfate groups (sulfation)
– addition of phosphate groups (phosphorylation)
Xu, D. & Esko, J. D. Nature Chemical Biology 5, 612–613 (2009).
Lysosomes
• Primary lysosome from Golgi
• Contain digesting enzymes for nucleic acids, proteins, sugars and lipids
• Release digested contents outside of cell by exocytosis
Summary
• The eukaryotic cell is a highly functional unit
• Is able to take care of household functions with specialised organelles, to replicate, manufacture and dispose.