Cell junctions and cell division

Basic concept of Cell junctions

1. Cell Junctions and Cell Cycle

2. Intercellular space in closely packed tissue is about 20nm. The cells are bound together by the specific adhesive glycoprotein. Epithelial cells adhere to each other through glycoproteins called Cadherins Modified cell membranes contributing in cohesion and communication are called Cell junctions Cell Junctions

3. There are three types of Cell Junctions Occluding Junctions Adhering Junctions Communicating Junctions Types of Cell Junctions

4. Found in epithelial tissues Also known as “Tight Junctions” Do not allow passage of small molecules form impermiable membrane. Types: Zonula Occludens Fascia Occludens Occluding Junctions

5. Encircles the entire cell perimeter Occludes the intercellular space Series of focal fusions The adjacent cell membranes approach each other, outer leaflets fuse, diverge again then fuse again At fusions sites specific trans membranous proteins named ( Occludins , and Claudins ) perform the binding function Less in PCT and more in the intestinal mucosa Zonula Occludens

6. A strip like tight junction of limited extent Found between the endothelial cells of the blood vessels Fascia Occludens

7. Anchoring junctions Provide cell-cell or cell to basal lamina adherence Types: Zonula adherens Fascia adherencs Macula adherens (Desmosomes) Hemidesmosomes Adhering Junctions

8. A belt like junction No fusion of cell membranes Trans membranous glycoprotein “E-cadherin” occupies intercellular gap E-cadherin links to adherent proteins in cytoplasm which are: Catenin Vinculin Zonula Adherens

9. Structurally it is similar to Zonula adherence But its cell junction is strip-like and (not ring-like or belt-like ) i.e. Cardiac muscle cells. Fascia Adherens

10. Macula adherins are commonly known as desmosomes “Spot-weld” like junctions Randomly distributed along lateral plasma membranes of the cells in simple epithelium In stratified epithelium it is distributed throughout the plasma membrane It is also found in cardiac muscle cells Desmosomes

11. Cell membrane in the region of junctions are seen further apart (30mm) than the usual gap Electron dense attachment plaques are located opposite to each other on the cytoplasmic aspects Intermediate filaments of the cytoskeleton are anchored to the attachment plaques Two types of transmembranes glycoproteins named Desmocolins and Desmogleins provide adherence Desmosomes

12. These junctions serve to anchor the epithelial cells to the basal lamina A hemidesmosome is a spot like adhering junction which gives appearance of a half desmosome In hemidesmosome transmembrane linker proteins are integrins The cytoplasmic intermediate filaments of keratin are inserted in to the attachment plaque Hemidesmosomes

13. Characterized by presence of minute tubular passageways Provide direct cell to cell communication Tubular passages allow movement of ions and other small molecules between adjacent cells Communicating Junctions

14. Gap junction also called the “Nexus” which are communication junctions, occur frequently between the epithelial cells Also found in cardiac muscle cells, smooth muscles, neurons, astrocytes, and osteocytes Plasma membrane of the adjoining cells are closely opposed with a gap of only 2nm The gap junction contains closely packed numerous tubular intercommunicating channels Gap Junction

15. The lumens of the channels of gap junction have an average diameter of 1.5nm These channels permit free passage of ions, sugar and amino acids In cardiac and smooth muscles the gap junction provides electrical coupling of the adjacent cells Gap junctions are frequently found in embryonic cells Gap Junctions

16. Static cell population The cells incapable of mitotic division i.e. Neurons Cardiac Cells Stable Cell population Division occurs on stimulation as a result of an injury or a disease Renewing Cell population Undergoes regular mitotic division i.e. epidermis of skin and epithelial lining of GIT Cell Renewal

17. Cells belonging to renewing population undergoes a sequence of events which are repeated over and over again The cycle is divided in to two parts M PHASE: in which mitosis occurs (30 to 60 minutes) INTERPHASE: it is intervening period between two cell divisions consist of three sub phases 1- The G1 Phase (GAP-1) - During this phase synthesis of RNA and proteins occur s - Cell size is restored to normal - The duration of G1 is about 8 hours Cell Cycle

18. 2- The S-Phase: - During this synthesis of DNA takes place - It results in preparation of exact replica of genetic material and duplication of centrioles - Duration is 8 hours 3- G2 (GAP-2) Phase: - It is period between the end of S phase and beginning of mitosis - During this process production and accumulation of energy for mitosis takes place - Duration is 2 to 4 hours Cell Cycle

19. Cell Division

20. Mitosis The process of cell division which results in the production of two daughter cells from a single parent cell. The daughter cells are identical to one another and to the original parent cell .

21. Mitosis can be divided into stages Interphase Prophase Metaphase Anaphase Telophase

22. Interphase The cell prepares for division DNA replicated Organelles replicated Cell increases in size

23. Chromosomes become visible under LM Threads become shorter and thicker consist of two chromatids joined by centromere Nucleoli disappears Centrioles separates and migrate to each pole and starts giving out mitotic spindle Prophase The cell prepares for nuclear division

24. Chromosomes line up at the center of the cell Spindle fibers attach from daughter cells to chromosomes at the centromere Equatorial plate is formed Microtubules of mitotic spindle are attached at centromere Microtubules exert pull on chromosomes Metaphase The cell prepares chromosomes for division

25. Spindle fibers pull chromosomes apart ½ of each chromosome (called chromatid ) moves to each daughter cell Chromatids separate and move to respective poles as an independent chromosome In human cell two identical sets of 46 chromosomes move to the opposite poles Anaphase The chromosomes divide

26. A constriction called cleavage furrow appears in the middle of elongated cell Nuclear envelop is formed enclosing chromosomes 2 nuclei form Cell wall pinches in to form the 2 new daughter cells Telophase The cytoplasm divides

27. Mitosis -- Review Interphase                                                               Prophase                                                               Metaphase                                                               Anaphase                                                               Telophase                                                               Interphase                                                              

28. MEIOSIS

29. Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced. One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell Meiosis

30. During meiosis, DNA replicates once , but the nucleus divides twice . Four stages can be described for each division of the nucleus. Meiosis

31. Meiosis I First division of meiosis

32. Prophase is much longer consisting of five stages Leptotene : Chromosomes becomes visible in the nucleus Zygotene : Homologus chromosomes come together along their entire length and synapses are formed Pachytene : Chromosomes become thicker and shorter Each chromosome pair is called bivalent Diplotene : Chromosomes began to separate along their length. Each bivalent consists of four chromatids Diakinesis : S eparation of chromosomes continue. Nucleolus and the nuclear envelop disappears Prophase

33. A spindle of microtubules is produced by centrioles Equatorial plate is formed The bivalent chromosome pairs align in the centre of the spindle Metaphase

34. Chromosomes of homologous pairs completely separates and move to the opposite poles No division of centromere occurs and the whole chromosomes move to opposite poles Anaphase

35. Nuclei are reconstructed The parent cell is divided in to two daughter cells Each daughter cell contains haploid (23) chromosomes Each chromosome is double structured consisting of two sister chromatids Telophase

36. Meiosis II Second Division of Meiosis

37. Meiosis II

38. Differences in Mitosis & Meiosis Mitosis Asexual Cell divides once Two daughter cells Genetic information is identical Meiosis Sexual Cell divides twice Four haploid daughter cells Genetic information is different

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