cell cycle,cell division_lec 6.pdfrdthdrhdrhdr
- 2. CONTENTS 1.Cell division: mitosis. 2.Cell cycle and its regulation
- 3. CELL CYCLE Two principal phases 1. Interphase ( represents continuous growth of the cell) G1(gap) phase, S(synthesis phase), G2 (gap2) phase. 2. M phase ( mitosis)
- 4. G1 PHASE 1. longest and the most variable phase of the cell cycle 2. It begins at the end of M phase. 3. Cell's progress is monitored by 2 checkpoints: (1) the restriction point- sensitive to the size of the cell, the state of the cell’s physiologic processes, and its interactions with extracellular matrix; 2) the G1 DNA-damage checkpoint- which monitors the integrity of newly replicated DNA
- 5. S PHASE 1. Initiation of DNA synthesis 2. DNA of the cell is doubled 3. New chromatids are formed 4. Chromosome replication is initiated at sites called replicons along the chromosomal DNA.
- 6. G2 PHASE 1. the cell prepares for cell division. 2. It is short- 1 hour 3. 2 check point- prevents the progression of cell into M phase before DNA synthesis is complete
- 7. G0-phase -G0 phase or quiescent stage. -cells fail to divide further and undergo differentiation.
- 8. M PHASE 1. Mitosis occurs in the M phase. 2. Separation of two identical daughter cells concludes the M phase. 3. Two checkpoints: spindle-assembly checkpoint- prevents premature entry into anaphase chromosome-segregation checkpoint- prevents process of cytokinesis
- 10. Regulation of the Cell Cycle 1. Passage through the cell cycle is driven by proteins that are cyclically synthesized and degraded during each cycle. 2. Two-protein complex -cyclin and a cyclin- dependent kinase (Cdk) They help power the cells through the checkpoints of cell-cycle division.
- 11. Controls at Cell Cycle checkpoints
- 12. CELL DIVISION- MITOSIS 1. It is a process of chromosome segregation and nu-clear division followed by cell division that produces two daughter cells with the same chromosome number and DNA content as the parent cell. 2. A parent cell divides and each of the two daughter cells receives a chromosomal set identical to that of the parent cell. 3. INTERPHASE- long period between mitosis( G1,S,G2) 4. Four phases: PROPHASE METAPHASE ANAPHASE TELOPHASE
- 13. PROPHASE 1. The nucleolus disappears and the replicated chromatin condenses into discrete threadlike chromosomes, each consisting of duplicate sister chromatids joined at the centromere. 2. The two centrosomes with their now-duplicated centrioles separate and migrate to opposite poles of the cell and organize the microtubules of the mitotic spindle. 3. Late in prophase, lamins and inner nuclear membrane are phosphorylated, causing the nuclear lamina and nuclear pore complexes to disassemble and disperse in cytoplasmic membrane vesicles.
- 15. METAPHASE 1. Chromosomes condense further and large protein complexes called kinetochores 2. They are located at the centromere DNA region of each chromosome and attach to the mitotic spindle. 3. Microtubules move the chromosomes into alignment at the equatorial plate.
- 16. ANAPHASE
- 17. TELOPHASE
- 19. SIGNIFICANCE OF MITOSIS Mitosis helps the cell in maintaining proper size and equilibrium in the amount of DNA and RNA in the cell. ii. The growth of multicellular organism is due to mitosis. After particular size has been reached, the cell divides to restore the nucleoplasmic index. Thus, growth takes place by an increase in the number of cells, rather than by increase in the size of the cells. iii. The old, decaying and dead cells of the body are replaced by the process of mitosis. For example, dead cells of upper layer of epidermis, cells lining the gut and red blood cells, are constantly being replaced by mitosis. iv. Maintenance of cell size v. Genetic stability
- 20. MEIOSIS 1. Meiosis involves two sequential nuclear divisions followed by cell divisions that produce gametes containing half the number of chromosomes and half the DNA found in somatic cells 2. consists of two successive divisions of the diploid mother nucleus: A. (i) Meiosis division I in which the diploid chromosome number (2n) is reduced to haploid chromosome number n, and (ii) Meiosis division II which is a mitotic division.
- 22. PACHYTENE 1. chromosomes become shorter, thicker . 2. Split into chromatids linked at the centromeres
- 23. DIPLOTENE 1. Centromeres of paired chromosomes move away from each other 2. homologous chromosomes remain connected at one or more points called chiasmata. 3. Exchange of genetic material takes place at each chiasma under the process called crossing over.
- 24. DIAKINESIS 1. chromosomes are shortest and thickest. 2. nuclear membrane starts disinte- grating 3. nucleolus also disintegrates and disappears 4. chromosome bivalents move towards the periphery of the nucleus 5. chromosomes are finally released into the cytoplasm.
- 25. METAPHASE I 1. complete disintegration of nuclear membrane and the formation of spindle 2. All the chromosomes, each along with their two chromatids, move to the equatorial region of the newly formed spindle.
- 26. ANAPHASE I
- 27. TELOPHASE I 1. nuclear membrane develops around each group of homologous chromosomes 2. The nucleolus reappears. 3. Daughter nuclei formed contain haploid number (n) of chromosomes 4.
- 28. MEIOSIS DIVISION II Four different phases which constitute meiosis division II are prophase II, metaphase II, anaphase II and telophase II
- 29. TELOPHASE II
- 31. Significance of meiosis 1. Formation of gametes 2. Genetic information 3. Meiosis facilitates stable sexual reproduction 4. Crossing over 5. Mutations
- 34. REFERENCES Junqueira’s Basic histology Page 58- 67