cell structure mitosis and meiosis 2007 for upload

8/4/2019 Cell Structure Mitosis and Meiosis 2007 for UPLOAD

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Cell Structure/Mitosis &Meiosis

SE ShirleyDept of Pathology


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Lecture Objectives

At the end of the lecture, the student shouldbe able to:

Describe the structural features ofeukaryotic cells

Outline the specialized structural

adaptationsof epithelial cells Define mitosisand meiosis, and discuss the

roles of each in the cell cycle


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The Cell

Basic unit of structure and function in livingorganisms

Derived from Latin cella

little room

First used in a biologic sense by RobertHooke in 1665

German pathologist, Rudolf Virchow (18211905) credited with initiating the study of

disease at the cellular level


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The Cell

Eukaryotes: e.g. mammals and other higherorganisms - characterized by the presence of

membrane-bound nucleus

Prokaryotes: e.g. bacteria are non-nucleated

Approximately 100 billion cells in humans


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Cell Structure


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Cell Membrane

Boundary/outer limit of cell contents

Composed of protein (about 60%) and lipid(about 40%)

Primary lipid is phospholipid: molecules forma 'phospholipid bilayer'

Other lipids: cholesterol & glycolipids

Proteins are embedded throughout the bilayer


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Cell Membrane

Functions:

Selective barrier

TransportRecognition

Receptor function

Adhesion

Movement


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Cell Membrane

Selective barrier: Water, non-polar molecules and some small

polar molecules can cross; most polar

compounds e.g. amino acids, organic acids andinorganic salts have to be transported across.

Transfer Function:

bulk transfer(e.g. phagocytosisand pinocytosis), passive diffusion

active transferinvolving the expenditure of energy(e.g. the sodium pump)


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Cell Membrane

Cell Recognition: the cell membrane is associated with antigens

by which the body is able to recognize its own

cells and tolerate them; cells that are notrecognized are attacked by the immune system.

Receptor Function: most compounds that interact with the cell do

so at specific points called receptors. Receptorsare integral proteins of the membrane that canvary in number or change position under certaincircumstances.


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Cell Membrane

Cell Movement: The cell surface is in constant movement;

this can be especially appreciated inmacrophages, which display a characteristicundulating movement.

Projections from the cell surface such ascilia (respiratory epithelium) and flagella(spermatozoa) also assist in movement.


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Nucleus

Initiates and regulates most cellularactivities

Bound by inner and outer nuclear envelopes

Contains: DNA (genetic codes)

RNA (essential molecules for proteinsynthesis)


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Nucleus

DNA = deoxyribonucleic acid; double helix of 4deoxyribonucleotides

Complementary base pairing of adenine(A) withthymine (T) and cytosine (C) with guanine (G)

Held together by hydrogen bonds

Attached to sugar phosphate backbone

Chromatin = chromosomal material = DNA, histones,non-histone proteins, RNA


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Nucleus

RNA = ribonucleic acid

Genetic information in DNA is transcribed tomRNA (messenger RNA) and translated in thecytoplasm (protein synthesis), with help of

tRNA (transfer RNA)


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Nucleolus

Sites of ribosome synthesis

Ribosomal DNA is transcribed into rRNA(ribosomal RNA) precursor; further processedinto ribosomal subunits which function in

protein synthesis in the cytoplasm


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The Nucleus in Disease

Hyperchromasia: increased staining of the nucleususually due to chromatin (e.g. genetic

abnormalities

DNA)

Active protein synthesis = prominent nucleoli

Nucleoli assembled from specific chromosomal

regions (nucleolar organizer regions) which may bedisturbed in cancer cells multiple/odd shapednucleoli


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Cytoplasm

Composed largely of water

Approx. 8% of protein High concentrations of potassium, magnesium,

phosphate (osmotic pressure within cells is similarto that of the extracellular fluid)

Membrane-bound structures = organelles

Filaments and granules


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Organelles

Mitochondria

Rough endoplasmic reticulum

Smooth endoplasmic reticulum

Golgi apparatus

Lysosomes

Cytoskeletal system


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Mitochondria

Organelles of energy production

Products of carbohydrate, fat and proteinmetabolism are oxidized to produce energy

Final product = ATP/adenosine triphosphate


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Mitochondria

Have a double-membrane: outer membrane &

highly convoluted inner membrane

Inner membrane has folds called cristae thatcontain elementary particles; these particles

contain enzymes important in ATP production


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Endoplasmic reticulum &

Golgi apparatus

Biosynthesis and transport of proteins and

lipids

Flattened sheets or elongated tubules

Content depending on cellular metabolicactivity


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Rough endoplasmic

reticulum

Series of membranes studded withribosomes that are the site of proteinproduction.


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Smooth endoplasmic reticulum

Series of membranes without attached

ribosomes that function in synthesis oflipids and processing of proteins(including steroid, carbohydrate and drug

metabolism)


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Golgi apparatus

Series of flattened sacs and vesicles that

functions in the modification and packaging ofmaterial synthesized in the endoplasmicreticulum

Examples: addition of sugars, proteolysis ofproteins , sorting of macromoleciles


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Lysosomes

Organelles containing a range of lyticenzymes that are involved in the digestion ofunwanted extrinsic as well intrinsic material

Enzymes include nucleases, proteases,

lipases, phosphatases


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Cytoskeletal system

Internal scaffolding: system of filaments andmicrotubules that provides rigidity, as well as allows

for movement within the cell (e.g. excretion ofmaterial) and locomotion

Microfilaments 5 nm actin

Intermediate filaments

10 nm - 6 main proteinswhich vary between cells

Microtubules 25 nm tubulin


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Intermediate filaments

Cytokeratin- epithelial cells

Desmin smooth/skeletal muscles

Glial fibrillary acidic protein astrocytes

Neurofilament protein neurones

Nuclear lamin

nuclei

Vimentin mesenchymal cells


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Specialized StructuralAdaptations Of

Epithelial Cells


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Epithelial cells

Cover body surfaces (skin) and line body

cavities and tracts (e.g. respiratory,gastrointestinal)

Functional units of secretory glands


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Epithelial Cell Specialization

Cell surface projections

Cilia: facilitate transport along cell surface

Microvilli: increase surface area forabsorption

Secretory adaptations

Well developed ER and Golgi apparatus

Cell junctions


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Cell Junctions

Adjacent cells exhibit specialized junctionalareas that allow:

1) Adhesion of cells to each other, in some casesforming a seal to prevent substances passingbetween them

2) Provision of areas of close contact throughwhich cells can communicate with each other; forcell growth, regeneration and repaircommunication is critical


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Cell Adhesion

The adherence of cells of a similar type is vital forthe maintenance of the architecture of tissuesand organs.

Adhesion molecules are glycoproteins that aredistributed in the cell membrane and which allowfor adhesion of cells to each other or theextracellular matrix.

The 5 major families are: the cadherins, theimmunoglobulins, the integrins, the selectinsandCD44.


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Cell Junctions

Occludens type (tight junction):

the membranes of two adjacent cells

come into intimate contact; a completeband encircling cells at their free bordersis called a zonula occludens; an incompleteband is called a macula occludens.


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Cell Junctions

Nexus or gap junction:

the two membranes are separated by anelectron-lucent gap of 2 nm through whichmolecules up to 150 nm in diametercanpass. Gap junctions are kept in close

apposition by cell adhesion moleculessuch as cadherins and integrins.


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Cell Junctions

Adherens type:

the cell membranes are separated by a gap of

about 20nm that is filled by electron-lucentmaterial that glues the cells together.

The zonula adherens forms a band around cellsand together with the zonula occludens and

desmosome constitutes thejunctional complex. The macula adherensor desmosomeis formed by

tonofilaments that do not cross from one cell tothe other.


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Mitosis &

Meiosis


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Mitosis

Mechanism of cell division leading to theproduction of two daughter cells with exactlythe same number of chromosomes and DNAcontent as parent cell

Diploid number of chromosomes = 46; 22pairs of autosomes and 1 pair of sexchromosomes; XX in females; XY in males)


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Meiosis

Specific type of cell division leading to the

production of gametes (ova and spermatazoa)

Each gamete contains a haploid number ofchromosomes (23; 22 autosomes and one sex

chromosome; X in ova and Y in spermatozoa)


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Cell cycle

Begins at the completion of one cell division(mitosis) and ends at the completion of thenext division

Dividing phase = Mitosis

Resting phase = Interphase


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Interphase

Phases:

G1 (resting; variable length)

(G0 for quiescent cells)

S (DNA replication tetraploid DNAcontent)

G2 (second gap; approx 4-5 hrs)


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Mitosis

Thought to be initiated by triggering factors incytoplasm or from other cells including variousgrowth factors

30-60 minutes

Division of nucleus* (karyokinesis) andcytoplasm (cytokinesis)


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Mitosis

Prophase:condensation and shortening ofchromosomes/formation of mitotic spindle cellapparatus: centrosomes with intervening microtubules

Metaphase:centromeres attach to centre ofapparatus

Anaphase:centromeres split and each half of

chromosome (chromatid) move to opposite pole

Telophase:nuclear membrane refroms.


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Mitosis

Mitosis facilitates:

Increased number of cells increase insize (growth) of organ/organism

Replacement of dead cells


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Meiosis

Crossover eventsare possible between maternallyand paternally derived chromosomal material

Chiasmata= points of junction of the exchangedsegments

Each gamete contains a haploid number of

chromosomes(pairing at fertilization will result inrestoration of diploid number)


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Info on the Web

The Biology Project at University of Arizona:Cell Biology

http://www.biology.arizona.edu/cell_bio/cell_bio.html

cell structure mitosis and meiosis 2007 for upload

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