Cell Structure/Mitosis & Meiosis

SE ShirleyDept of Pathology

Lecture Objectives

At the end of the lecture, the student should be able to: Describe the structural features of

eukaryotic cells

Outline the specialized structural adaptations of epithelial cells

Define mitosis and meiosis, and discuss the roles of each in relation to the cell cycle

The Cell

Basic unit of structure and function in living organisms

Derived from Latin cella –little room First used in a biologic sense by Robert

Hooke in 1665 German pathologist, Rudolf Virchow

(1821–1905) credited with initiating the study of disease at the cellular level

The Cell

Eukaryotes: e.g. mammals and other higher organisms - characterized by the presence of membrane-bound nucleus

Prokaryotes: e.g. bacteria are non-nucleated

Approximately 100 billion cells in humans

Cell Structure

Cell Membrane

Acts as boundary to contain cell contents

Lipid bilayer; phopholipids and cholesterol

Proteins are embedded throughout the bilayer

Cell Membrane

Selective barrierTransportCommunicationRecognition

Nucleus Initiates and regulates

most cellular activities Bound by inner and

outer nuclear envelopes

Contains: DNA (genetic codes) RNA (essential

molecules for protein synthesis)

Nucleus

DNA = deoxyribonucleic acid; double helix of 4 deoxyribonucleotides Complementary base pairing of adenine(A)

with thymine (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

Nucleus

RNA = ribonucleic acid Genetic information in DNA is transcribed

to mRNA (messenger RNA) and translated in the cytoplasm (protein synthesis), with help of tRNA (transfer RNA)

Nucleolus

Sites of ribosome synthesis Ribosomal DNA is transcribed into rRNA

(ribosomal RNA) precursor; further processed into ribosomal subunits which function in protein synthesis in the cytoplasm

Normal cell Cancer cells

The Nucleus in Disease

Hyperchromasia: increased staining of the nucleus usually due to chromatin (e.g. genetic abnormalities DNA)

Active protein synthesis = prominent nucleoli

Nucleoli assembled from specific chromosomal regions (nucleolar organizer regions) which may be disturbed in cancer cells multiple/odd shaped nucleoli

Cytoplasm

Composed largely of water Approx. 8% of protein High concentrations of potassium,

magnesium, phosphate (osmotic pressure within cells is similar to that of the extracellular fluid)

Membrane-bound structues = organelles Filaments and granules

Organelles

MitochondriaRough endoplasmic reticulumSmooth endoplasmic reticulumGolgi apparatusLysosomesCytoskeletal system

Mitochondria

Organelles of energy productionProducts of carbohydrate, fat and

protein metabolism are oxidized to produce energy

Final product = ATP/adenosine triphosphate

Endoplasmic reticulum and Golgi apparatus

Biosynthesis and transport of proteins and lipids

Flattened sheets or elongated tubulesContent depending on cellular

metabolic activity

Rough endoplasmic reticulum

Series of membranes studded with ribosomes that are the site of protein production.

Smooth endoplasmic reticulum

Series of membranes without attached ribosomes that function in synthesis of lipids and processing of proteins (including steroid, carbohydrate and drug metabolism)

Golgi apparatus

Series of flattened sacs and vesicles that functions in the modification and packaging of material synthesized in the endoplasmic reticulum

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

Lysosomes

Organelles containing a range of lytic enzymes that are involved in the digestion of unwanted extrinsic as well intrinsic material

Enzymes include nucleases, proteases, lipases, phosphatases

Cytoskeletal system

Internal scaffolding: system of filaments and microtubules provides rigidity, as well as allows for movement within the cell (e.g. excretion of material) and locomotion

Microfilaments – 5 nm – actinIntermediate filaments – 10 nm - 6 main

proteins which vary between cellsMicrotubules – 25 nm – tubulin

Intermediate filaments

Cytokeratin- epithelial cells Desmin – smooth/skeletal muscles Glial fibrillary acidic protein – astrocytes Neurofilament protein – neurone Nuclear lamin - nucleus Vimentin – mesenchymal cells

Epithelial cells

Cover body surfaces (skin) and line body cavities and tracts (e.g. respiratory, gastrointestinal)

Functional units of secretory glands

Epithelial Cell Specialization

Cell surface projections Cilia: facilitate transport along cell surface Microvilli: increase surface area for

absorption

Secretory adaptations Well developed ER and Golgi apparatus

Cell junctions

Cell junctions

Specialized junctional areas between epithelial cells allow for: Adherence to each other Communication channels

Three types of junction: Occludens type/tight junction (barrier) Nexus/gap junction (2nm; communication) Adherens type (20nm; adhesion)

Mitosis & Meiosis

MITOSIS

Mechanism of cell division leading to the production of two daughter cells with exactly the same number of chromosomes and DNA content as parent cell

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

Meiosis

Specific type of cell division leading to the production of gametes (ova and spermatazoa)

Each gamete contains a haploid number of chromosomes (23; 22 autosomes and one sex chromosome; X in ova and either X or Y in spermatozoa)

Cell cycle

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

Dividing phase = MitosisResting phase = Interphase

Interphase

Phases:G1 (resting; variable length)(G0 for quiescent cells)S (DNA replication tetraploid DNA

content)G2 (second gap; approx 4-5 hrs)

Mitosis

Thought to be initiated by triggering factors in cytoplasm or from other cells including various growth factors

30-60 minutesDivision of nucleus* (karyokinesis)

and cytoplasm (cytokinesis)

Mitosis

Prophase: condensation and shortening of chromosomes/formation of mitotic spindle cell apparatus: centrosomes with intervening microtubules

Metaphase: centromeres attach to centre of apparatus

Anaphase: centromeres split and each half of chromosome (chromatid) move to opposite pole

Telophase: nuclear membrane reforms.

Mitosis

Mitosis facilitates: Increased number of cells increase

in size (growth) of organ/organism Replacement of dead cells

Meiosis

Crossover events are possible between maternally and paternally derived chromosomal material

Chiasmata = points of junction of the exchanged segments

Each gamete contains a haploid number of chromosomes (pairing at fertilization will result in restoration of diploid number)

Info on the Web

The Biology Project at University of Arizona: Cell Biologyhttp://www.biology.arizona.edu/

cell_bio/cell_bio.html