Biology of the cell

Aisling Ahlström

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.

Cells we work with..

Blastomere

• 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

labmonsters.com

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

Two types of cell division

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

Meiosis I & Meiosis II

Crossing over

4c

2c 1c

Spindle assembly checkpoint

GametogenesisSpermatogenesis Oogenesis

Age related meiotic errors in oocytes

Synthesis and Degradation

Metabolic pathways

Energy/ATP

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

Cell Respiration- Energy

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

Protein synthesis

classes.kumc.edu

Ribosomal

RNA

Transfer RNA

Messenger RNA

”Universal code”

en.wikipedia.org

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

Programmed Cell Death : Apoptosis

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.

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