Cell Structure/ Mitosis & Meiosis

1. Cell Structure/Mitosis & Meiosis SE Shirley Dept of Pathology

2. 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

3. 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

4. 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

5. Cell Structure

6. Cell Membrane • Acts as boundary to contain cell contents • Lipid bilayer; phopholipids and cholesterol • Proteins are embedded throughout the bilayer

7. Cell Membrane • Selective barrier • Transport • Communication • Recognition

8. Nucleus • Initiates and regulates most cellular activities • Bound by inner and outer nuclear envelopes • Contains: • DNA (genetic codes) • RNA (essential molecules for protein synthesis)

9. 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

10. 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)

11. 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

12. Normal cell Cancer cells

13. 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

14. 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

15. Organelles • Mitochondria • Rough endoplasmic reticulum • Smooth endoplasmic reticulum • Golgi apparatus • Lysosomes • Cytoskeletal system

16. Mitochondria • Organelles of energy production • Products of carbohydrate, fat and protein metabolism are oxidized to produce energy • Final product = ATP/adenosine triphosphate

17. Endoplasmic reticulum and Golgi apparatus • Biosynthesis and transport of proteins and lipids • Flattened sheets or elongated tubules • Content depending on cellular metabolic activity

18. Rough endoplasmic reticulum • Series of membranes studded with ribosomes that are the site of protein production.

19. 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)

20. 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

21. 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

22. 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 – actin • Intermediate filaments – 10 nm - 6 main proteins which vary between cells • Microtubules – 25 nm – tubulin

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

24. Epithelial cells • Cover body surfaces (skin) and line body cavities and tracts (e.g. respiratory, gastrointestinal) • Functional units of secretory glands

25. 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

26. 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)

27. Mitosis & Meiosis

28. 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)

29. 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)

30. Cell cycle • Begins at the completion of one cell division (mitosis) and ends at the completion of the next division • Dividing phase = Mitosis • Resting phase = Interphase

31. Interphase • Phases: • G1 (resting; variable length) • (G0 for quiescent cells) • S (DNA replication  tetraploid DNA content) • G2 (second gap; approx 4-5 hrs)

32. Mitosis • Thought to be initiated by triggering factors in cytoplasm or from other cells including various growth factors • 30-60 minutes • Division of nucleus* (karyokinesis) and cytoplasm (cytokinesis)

33. 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.

34. Mitosis • Mitosis facilitates: • Increased number of cells  increase in size (growth) of organ/organism • Replacement of dead cells

35. 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)

36. Info on the Web • The Biology Project at University of Arizona: Cell Biology • http://www.biology.arizona.edu/cell_bio/cell_bio.html