Cell Cycle - 3rd Week PDF

CELL CYCLE AND CELL DIVISION Division Protein Phase Synthesis M G1 Phase G2 S Protein Chromose...

CELL CYCLE AND CELL DIVISION Division Protein Phase Synthesis M G1 Phase G2 S Protein Chromose Synthesis Duplicate Prof. Dr.Mekky Abouzied Professor pf Biochemistry and molecular biology Outline By the end of this session, the students will be 1. Introduction able to: 2. Cell division  Describe the stages of the cell cycle, including G1, S, G2, and M phases.  Explain the role and regulation of 3. Cell cycle checkpoints within the cell cycle.  Identify the key events and processes that 4. Regulation of cell occur during each phase of the cell cycle cycle  Outline the stages of mitosis: prophase, metaphase, anaphase, and telophase 5. Cell death  Define Apoptosis and Necrosis Question to think about?  Why do we need new cells?  Where do new cells come from?  How are new cells formed?  Is the production of new cells similar in all organisms and for all cell types?  why are new cells identical to their ancestors? 1. Introduction The different organisms need new cells either to grow in size or in number or to replace for damaged or lost ones New cells are derived from ancestor cells, i.e. new skin cells are derived from older skin cells …. These new cells are produced by two different types of reproduction: Asexual reproduction: where one cell is divided into two identical cells. For example: mitosis (in eukaryotes) and binary fission (in bacteria) Sexual reproduction: where two cells (an egg and a sperm) are joined to produce a new cell (zygote) that is not identical to the parent cells, For example meiosis 2. Cell division A. Cell division in prokaryotes (Binary fission) 1. Prokaryotes have a single circular DNA molecules. 2. First this DNA molecule replicates and each copy is attached to cell membrane at different positions. 3. The cell starts then to pull apart so DNA molecule (replicate and original) are separated. 4. The new cells are finally separated forming two identical daughter cells.  2. Cell division B. Cell division in Eukaryotic When does a cell decide to divide?? When there is a need for new cells (growth, damage, ….) When the cell gets too large, nutrients and wastes exchange becomes difficult How and when does the division process starts? It starts when the cell reaches the appropriate size and has enough protein and energy (ATP) stores. To completely answer the question we need to learn about the CELL CYCLE 3. Cell cycle The cell cycle represents the different stages starting from the moment when a cell is divided into two distinct (daughter) cells until these daughter cell are dividing themselves into new cells It can be divided into three phases  G1: primary growth phase Phase 1  S : synthesis; DNA replicated  G2: secondary growth phase collectively these 3 stages are called interphase Phase 2  M - mitosis Phase 3  C - cytokinesis 3. Cell cycle  3. Cell cycle A. Interphase includes: G1 phase:  Cell grows to acquire the appropriate size for division  It increases its supply of different proteins needed for division and number of organelles like mitochondria and ribosomes. S phase: DNA amount is doubled Chromosomes are replicated in the form of two sister chromatids. G2 phase:  2nd growth phase after DNA duplication  All cell structures needed for division are made (e.g. By the end of the centrioles) interphase the cell is  proteins and organelles needed for division are synthesized ready to start.....MITOSIS 3. Cell cycle B. Mitosis Mitosis or karyokinesis means division of the nucleus It happens only in eukaryotes Some cells like nerve and brain cells DO NOT undergo mitosis It consists of four phases: (PMAT) a. Prophase b. Metaphase c. Anaphase d. Telophase Mitosis stages: a c b d c. Anaphase a. Prophase b. Metaphase d. Telophase Spindle fibers pull Chromatin condenses Chromosomes Chromosomes reappear as sister chromatids apart into chromosomes attached to chromatin (uncondensed) at there centromeres Nucleolus and Nuclear kinetochore fibers Nuclear envelope starts to form forming two poles membrane disappear are lined up along Nucleolus is visible Each pole has a whole Mitotic spindle forms cell equator Spindle breaks apart set of original genes from centrioles 3. Cell cycle C. Cytokinesis Cleavage furrow animal cell Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells. After mitosis has come to an end, the cytoplasm of the newly formed cells is separated by the aid of actin and myosin contractile ring which forms a cleavage furrow Can you name the following phases of cell cycle?? Prophase Metaphase Anaphase Telophase Cytokinesis Tissue Repair Cell division helps in growth, repair, and replacement of damaged cells  4. Regulation of cell cycle It aims to control cell cycle to prevent uncontrolled division of cells The regulatory molecules that are involved are called cyclins and cyclin-dependent kinases (CDK) Cyclin (regulatory subunit) and CDK (catalytic subunit) form a heterodimer that phosphorylates and so activates or deactivates target proteins  4. Regulation of cell cycle Check points of cell cycle They are used to monitor and regulate the progress of the cell cycle. They prevent cell cycle progression at specific points, allowing verification of necessary phase processes and repair of DNA damage.  G1 check point: ensures that every thing is ready for DNA replication  G2 check point: ensures that cell is ready for mitosis  Metaphase check point: ensures that the cell is ready to complete cell division  What happens if a cell loses control on its cell cycle ? Uncontrolled cell cycle = uncontrolled cell division = TUMOR FORMATION How can the cell lose control? A damaged/defective p53 gene causes it to lose information needed to respond to control signals This means that a cancer cell with defective p53 does not respond to cell cycle controlling signals = uncontrolled division  Normal cell versus cancer cell Normal Cell Cancer Cells 1. DNA is replicated properly. 1. Mutations occur in the DNA when it is replicated. 2. Chemical signals start and stop the cell 2. Chemical signals that start and stop cycle. the cell cycle are ignored. 3. Cells communicate with each other so 3. Cells do not communicate with each they don’t become overcrowded. other and tumors form. Due to DNA mutations, cancer cells cannot communicate with neighboring cells. Cells continue to grow and form tumors. Skin cancer Liver cancer  Cancer is essentially a failure of cell division control causing unrestrained, uncontrolled tissue growth What control is lost? lose checkpoint stops P53 gene plays a key role in G1/S check point p53 protein halts cell division if it detects damaged DNA ALL cancers have to shut down p53 activity 5. Cell death Necrosis Apoptosis (Unregulated cell death) (Regulated cell death) the death of most or all of the cells in An energy-dependent programmed an organ or tissue due to disease, cell death for removal of unwanted injury, or failure of the blood supply. individual cells 5. Cell death 5. Cell death Necrosis VS Apoptosis  Apoptosis in physiologic situations In the human body about 100,000 cells are produced every second by mitosis and a similar number die by apoptosis APOPTOSIS: important in embryogenesis Morphogenesis (eliminates excess cells): Removal of unnecessary cells between fingers and toes Apoptosis during the metamorphosis of a tadpole into a frog  APOPTOSIS: important in adults Tissue remodeling (eliminates cells no longer needed): Apoptosis Virgin mammary gland Late pregnancy, lactation Involution (non-pregnant, non-lactating) - Testosterone Apoptosis Prostate gland  APOPTOSIS: Role in Disease TOO MUCH: Tissue atrophy Neurodegeneration Thin skin…etc Cancer TOO LITTLE: Hyperplasia Athersclerosis…..etc Question Why do cells divide rather than simply increase in size, and what are the advantages of cell division over cell enlargement for maintaining cellular function?