Cell Cycle & Cell Death Lecture 8 PDF

CELL CYCLE & CELL DEATH Lecture 8 In Block PMS-102 Ass. Prof. Dr. Safaa Said Department of Histology &Cell Biology Assuit University  After this lecture, students should be able to: Define the cell cyc...

CELL CYCLE & CELL DEATH Lecture 8 In Block PMS-102 Ass. Prof. Dr. Safaa Said Department of Histology &Cell Biology Assuit University  After this lecture, students should be able to: Define the cell cycle, Illustrate the phases of cell Learning cycle & the events occurring in each phase.. objectives Recognize the relation between cell differentiation & cell cycle Differentiate between cell death by apoptosis & necrosis. Define apoptosis & know its characteristic features  The cell cycle Definition: It is a sequence of events that occur in the cell during its division (mitosis) and during its rest (interphase), that produce new cells. ▪ The cell cycle is formed of 2 periods: a. Interphase: Includes Gap 1 phase, Synthesis phase, and Gap 2 phase. b. Mitosis: Includes division of the nucleus (Karyokinesis) and division of the cytoplasm (Cytokinesis).  A.Interphase: A. Interphase: It is the interval between two consecutive cell divisions, and during which the cell performs its specialized functions. It is subdivided into three phases (Phases of cell cycle): 1- Gap 1 phase (G 1 phase): ▪ Is the interval between mitosis (M) and synthesis (S) phase. During this phase: ► The chromosomes are s-chromosomes (46 schromosomes). The nucleus does not show any increase in DNA content.  It is a period of active RNA and protein synthesis. ►Growth occurs and the cell almost attain the normal size of mother cell. ► The cells will be either: Dividable cells pass to the S-stage, e.g. GIT epithelium. Non dividable cells that either leave the cycle permanently (G0) (e.g.Nerve cells and Cardiac myocytes are terminally differentiated) or leave the cycle for an extended time to perform their specialized functions and return at the time of need to redivide again (e.g. Hepatocytes and fibroblasts).  2- Synthesis phase (S phase): ▪ It is the phase during which duplication of DNA molecules and centrioles takeplace. ▪ The cell contains 46 identical pairs of s- chromosomes and may be called double chromosomes.  3- Gap 2 phase (G 2 phase): ▪ It is the phase between the synthesis phase and mitosis (relatively short). This period in which cells prepare for mitotic division, the cells synthesize a great amount of cytoplasmic material such as proteins, and the organoids are increased in number. ▪ Chromosomes are d- chromosomes that contain double amount of DNA compared to those in the G1 phase. ▪ Most of cells pass from the G2 phase to the next mitosis  B. Mitosis (M):It the period when thechromosomes shorten, align in the middle of the cell and separate into two identical sets each of which forms the chromosomal content of the new daughter cell. The time required for a cell cycle to be completed is from 10 to 30 hours. Thus cell cycle begins again after the end of mitosis till the completion of the next one.  Controls at cell cycle checkpoints. Each phase of the cell cycle has one or more checkpoints where the quality of specific cell activities is checked. Progression to the next phase of the cycle does not occur until all activities of the preceding phase are completed satisfactorily. Three important checkpoints are shown here, including The start or restriction checkpoint just before the start of S The G2 /M checkpoint that ensures that DNA replication is complete The metaphase spindle checkpoint that ensures that all chromosomes will be segregated Overall progression in the cycle is regulated by proteins called cyclins and cyclin-dependent kinases (CDKs)  Cell Development The development of a single fertilized egg cell to form a complex multicellular organism involves Three major stages, A)Growth is defined as an increase in mass. This increase can result from the enlargement of cells and more often from the increase in the number of these cells through mitotic divisions. B) Differentiation : It is the process by which new features and new functions are achieved. C) Integration: Is the process by which growth and differentiation are kept in harmony result in tissues and organs which then form the organism with its specific features Cell division and differentiation are balanced by cell death.  Definition:The irreversible cessation of vital activities inside the cell". Two distinct processes of cell death can be recognized, necrosis and apoptosis. Necrosis Is cell death due to exogenous Cell factors such as injury or toxins. The cell swells, Death The chromatin is clumped, The cytoplasm becomes weakly stained and the organoids are destroyed. The cell bursts and its contents are scattered and then removed by phagocytes. Apoptosis: Is the normal cell death due to internal activities within the cell As a result of genetic information..So it is called programmed cell death It is a rapid, highly regulated cellularactivity shrinks and eliminates defective and unneeded cells.  During apoptosis there is Condensation of chromatin and destruction of DNA due to activation of the enzyme endonuclease The cell does not swell. Instead, the cell volume decreases and the organoids remain normal. The plasma membrane of the shrinking cell undergoes blebbing At last the cell fragments into a number of membranebound globules (apoptotic bodies) which are then phagocytozed by cells specialized for debris removal. AND LOVE YOU ALL MY BRILLIANT FUTURE DOCTORS REFERENCES: INTEGRATED SYSTEMS (PP. 55 ELSEVIER’S &56), THE NERVOUS SYSTEM INTEGRATED (PP. 32-36), HISTOLOGY (2007) BASIC HISTOLOGY (PP. 161-170). 1ST EDITION.