Cell Cycle & DNA Replication Lecture 5 PDF

BMS: 141 Lecture No: 5 Title: Cell Cycle & DNA Replication Instructor Name: Dr Amira Abdel Haleem Medicine and Surgery Program Fall 2024  Intended Learning Outcomes of lecture At the end of th...

BMS: 141 Lecture No: 5 Title: Cell Cycle & DNA Replication Instructor Name: Dr Amira Abdel Haleem Medicine and Surgery Program Fall 2024  Intended Learning Outcomes of lecture At the end of this lecture, the students will be able to: 1. Identify different phases of cell cycle 2. Recognize different check points of regulation of cell cycle 3. Enumerate different proteins involved in regulation of cell cycle 4. Explain role of Rb and P53 proteins in controlling cell cycle  Cell cycle and DNA replication Def.: The term cell cycle refers to sequence of events that occur during the lifetime of a cell Or Refers to the events occurring during the period between two mitotic divisions.  Cell cycle and DNA replication Phases of cell cycle: The cell cycle consists of four distinct phases in higher organisms: G1, S , G2 & M phases → G1(Gap 1) phase: -The period preceding replication. -During this phase the cell synthesizing all cellular components needed for replication →S (synthesis) phase: DNA replication occurs during this phase  Cell cycle and DNA replication → G2 (Gap2) phase: During this phase the cell synthesize more proteins sufficient for two cells (Cytoplasmic division). → The M (Mitotic) phase: -During this phase the cell splits into two daughter cells. -The daughter cells then either enter into Go (undividing or dormant) phase or re-enter the cell cycle when there is necessity for growth and repair.  Cell cycle and DNA replication N:B: -When the cell is not growing (stopped dividing), it exists in a dormant or undividing phase (Go) such as mature neurons. -The whole cycle lasts about 24 hours; out of which M phase is only 1-2 hours. -In a normal cell population, most of the cells are in Go phase. General metabolic events are taking place in Go phase. -Interphase is the period between the end of M phase and the beginning of the next mitosis (i.e. G1,S & G2 phases).  Regulation of cell cycle Def.: Is a way of informing the cell about the proper time to grow and stop the cell division when the time is not right. How? This can be done by a number of proteins which act mainly on checkpoint Checkpoints: 1- G1-S checkpoint 2- S phase checkpoint 3- G2-M checkpoint  Cell cycle regulatory proteins 1- Cyclins 2- The cyclin dependant kinases (CDK) 3- The CDK inhibitors 4- The tumor suppressor genes 5- Growth factors regulating cell cycle Cell cycle check points The cell cycle is controlled at a series of “checkpoints” that prevent entry into the next phase of the cycle until the preceding phase has been completed. There occurs a continuous monitoring of the cell cycle with respect to DNA replication, chromosome segregation and integrity.  Cell cycle check points lf any damage to DNA is detected either in G1o r G2 phase of the cycle, or if there is a formation of defective spindle ( i.e. Incomplete chromosomal segregation) the cell cycle will not progress until appropriately corrected. lf it is not possible to repair the damage done, the cells undergo apoptosis (programmed cell death).  Cell cycle check points -Cell cycle arrest mediated by a checkpoint may fail for a variety of reasons: 1st: like all cellular processes, checkpoints have an intrinsic error. 2nd: after a period of arrest, even though damage remains unrepaired, the cell may resume the cycle. - If damage is not repaired and the cell passes to the next stage, secondary lesions result. 3rd: check points may be mutated, leading to unchecked growth, leading to cancer.  Cell cycle regulatory proteins 1- Cyclins 2- The cyclin dependant kinases (CDK) 3- The CDK inhibitors 4- The tumor suppressor genes 5- Growth factors regulating cell cycle  Cyclins Cyclins are a group of proteins that are closely associated with the transition of one phase of cell cycle to another, hence they are so named. The most important cyclins are cyclin A, B,D and E.  Cyclins The concentration of cyclins increase or decreased using the course of cell cycle. These cyclins act on cyclin-dependent kinases (CDKs) that phosphorylate certain substances essential for the transition of one cycle to another.  Cyclin dependent kinases Def.: -These are a groups of protein when activated (by binding to cyclin) they undergo a cascade of phosphorylation reaction for other proteins regulating cell cycle -These proteins control cell cycle at different checkpoint of cell cycle -5 different cyclin dependent kinases (CDK 1, 2, 4, 5 and 6)  Cyclin dependent kinases -Mechanism of action: During G1-S checkpoint, cyclin D binds to and activates CDK 4 & CDK 6 →phosphorylate retinoblastoma (Rb) protein → Inhibition of binding of Rb protein to a family of transcription protein (E2-F) → E2-F become free → ↑ transcription of protein required for replication (e.g. Replication enzymes) Tumor suppressor genes Rb (retinoblastoma) gene -It encode a protein known as Rb protein (Rb protein is the product of an oncosuppressor gene) -It is so named, because it was isolated from patients of retinoblastoma (cancer arising from retina). Tumor suppressor genes Mechanism of action: -Rb protein bind to and inhibits a family of transcription factors (e.g. E2-F) → inhibits transcription of many proteins required for cell division (e.g. enzymes of replication). Tumor suppressor genes Certain tumor antigens derived from viruses such as SV40, HSV, HPV may combine with Rb. Then, Rb cannot inhibit cell cycle, leading to continuous cell division and cancer Tumor suppressor genes In the hypo-phosphorylated state, Rb is active and carries out its role as tumor suppressor by inhibiting cell cycle progression. Phosphorylation inactivates Rb, during the M-to-G1 transition.  Tumor suppressor genes P53 gene It encode a protein known as P53 protein It is known as policeman of the cell It is so named because it is a protein with 53 kD in size. It is a DNA binding protein -Mechanism of action: -When P53 binds to DNA → stimulate transcription of genes producing the following proteins (p21/WAF1) → Inhibits CDKs → Inhibits phosphorylation and activation of Rb protein occurs → thus Rb bind E2F in inactive form→ Stop cell cycle Tumor suppressor genes P53 inactivation: Once the cell begun to cycle and has passed the restriction point, CDK2 activity increased at the end of G1 phase → phosphorylation and inactivation of p53 and get it out of the nucleus Tumor suppressor genes N:B: P53 inhibits cell division, allowing them to repair. If damage is extensive and repair is not possible, the p53 directs the cell to apoptosis. It is phosphorylated in a cell cycle dependent manner by CDK4. Maximum level of p53 phosphorylation is reached during mitosis.  Cyclin dependent kinase inhibitors (CDKs inhibitors) Types: 2 families: a- INK4 (inhibitors kinase 4): This family work in stress or when a cell should not undergo cell cycle. b- WAF/KIP ( Wild p53 associated factors/Kinase inhibitor protein): Example of this family is (p21/WAF1) previously discussed  Growth factors Mechanism of action: - They bind to growth factor receptors → activation of diverse signaling pathways → phosphorylation and activation of specific transcription factors → binding of these factors to DNA → stimulate transcription of certain genes essential for passing restriction point e.g. cyclin D gene  Cancer and cell cycle Cancer represents an excessive division of cells. ln cancer, a large quantity of cells are in mitosis and most of them in S-phase. Majority of the drugs used for cancer therapy are designed to block DNA replication or inhibit the enzymes that participate in replication (directly or indirectly).  Cancer and cell cycle Methotrexate (inhibits dihydrofolate reductase) and 5- fluorouracil (inhibits thymidylate synthase) block nucleotide synthesis. Topoisomerase inhibitors are being used. They allow the tangling of parental DNA strands and prevent replication.  References for further readings Lippincott Illustrated Review Integrated system Lippincott Illustrated Review 6th edition Oxford Hand book of Medical Science 2nd edition Clinical Key Student THANK YOU