Cell cycle regulation

Basic Cell cycle regulation suitable for undergraduate students.
This presentation has been started from the basics to enable easy understanding. It covers all the details of cell cycle regulation in yeast as well as higher eukaryotes.

1. Cell cycle regulation WBSU Semester I Botany Core Course II (Group B) Dr. Riddhi Datta

2. The Eukaryotic Cell Cycle A typical eukaryotic cell cycle is illustrated by human cells in culture, which divide approximately every 24 hours . The cell cycle is divided into two basic parts: mitosis (or meiosis) interphase. Mitosis and cytokinesis last only about an hour , so approximately 95% of the cell cycle is spent in interphase-the period between mitoses. However , interphase is the time during which both cell growth and DNA replication occur in an orderly manner in preparation for cell division . Dr. Riddhi Datta

3. The timing of DNA synthesis thus divides the cycle of eukaryotic cells into four discrete phases: The M phase of the cycle corresponds to mitosis, which is usually followed by cytokinesis. This phase is followed by the G1 phase (gap 1), which corresponds to the interval (gap) between mitosis and initiation of DNA replication. During G1 the cell is metabolically active. G1 is followed by S phase (synthesis) dur­ing which DNA replication takes place. The comple­tion of DNA synthesis is followed by the G2 phase ( gap 2) during which cell growth continues and pro­teins are synthesized in preparation for mitosis. The Eukaryotic Cell Cycle Dr. Riddhi Datta

4. Cells at different stages of cell cycle can be differentiated by their DNA content. Animal cells in G1 are diploid (2n). During S phase, replication increase the DNA content from 2n to 4n. It remains in the 4n stage in G2 phase as well and is then decreased to 2n stage after M phase. Some cells in adult animals cease division altogether (e.g., nerve cells) and many other cells divide only occasionally (e.g. skin fibroblasts). T hese cells exit G1 to enter a quiescent stage of the cycle called G0 phase , where they remain metabolically active but no longer proliferate unless called on to do so by appropriate intracellular signals. The Eukaryotic Cell Cycle Dr. Riddhi Datta

5. Regulation of the Cell Cycle by extracel­lular signals The progression of cells through the division cycle is regulated by extracel­lular signals from the environment, as well as by internal signals that mon­itor and coordinate the various processes that take place during different cell cycle phases . This is accomplished by some control points that regulate progression through the cell cycle: START Restriction point Control of G2 to M transition Dr. Riddhi Datta

6. START A major cell cycle regulatory point in many types of cells occurs late in G1 and controls progression from G1 to S. This regulatory point was first defined by studies of budding yeast (Saccharomyces cerevisiae ), where it is known as START. Once cells have passed START, they are com­mitted to entering S phase and undergoing one cell division cycle. The importance of this regulation is particu­larly evident in budding yeasts in which cell division produces progeny cells of very different sizes: a large mother cell and a small daughter cell. Dr. Riddhi Datta

7. Some of the activities monitored at this check point are : Optimum cell size: In order for yeast cells to maintain a constant size, the small daughter cell, after one division, must grow more than the large mother cell does before they divide again. This regulation is accomplished by a control mecha­nism that requires each cell to reach a minimum size before it can pass START. START Nutrient status: If yeasts are faced with a shortage of nutrients, they arrest their cell cycle at START and enter a rest­ing state rather than proceeding to S phase. Thus START represents a deci­sion point at which the cell determines whether sufficient nutrients are available to support progression through the rest of the division cycle. Polypeptide factors that signal yeast mating also arrest the cell cycle at START, allowing haploid yeast cells to fuse with one another instead of pro­gressing to S phase. Dr. Riddhi Datta

8. Restriction point Such arrested cells then enter a quiescent stage , called G0 , in which they can remain for long periods of time without proliferating. G0 cells are metabolically active , although they cease growth and have reduced rates of protein synthesis . For example, skin fibroblasts are arrested in G0 until they are stimu­lated to divide as required to repair damage resulting from a wound. The proliferation of these cells is triggered by platelet-derived growth factor , which is released from blood platelets during clotting and signals the pro­liferation of fibroblasts in the vicinity of the injured tissue. In animals, a decision point in late G1 , called restriction point , functions analogously to START in yeasts. In contrast to yeasts, however, the passage of animal cells through the cell cycle is regulated primarily by the extracellular growth fac­tors that signal cell proliferation, rather than by the availability of nutrients . In the presence of the appropriate growth factors, cells pass the restriction point and enter S phase. Once it has passed through the restriction point, the cell is committed to proceed through S phase and the rest of the cell cycle. On the other hand, if appropriate growth factors are not available in G1 progression through the cell cycle stops at the restriction point. Dr. Riddhi Datta

9. G2 to M transition Although the proliferation of most cells is regulated primarily in G1 some cell cycles are instead controlled principally in G2. In fission yeast Schizosaccharomyces pombe , in con­trast to Saccharomyces cerevisiae , the cell cycle is regulated prima­rily by control of the transition from G2 to M , which is the principal point at which cell size and nutrient availability are monitored. Vertebrate oocytes can remain arrested in G 2 for long periods of time (several decades in humans) until their progression to M phase is triggered by hormonal stimulation . Extracellular signals can thus control cell proliferation by regu­lating progression from the G2 to M phase of the cell cycle. In addition, some haploid mosses use G2 control point. Dr. Riddhi Datta

10. Cell Cycle Checkpoints In addition to cell size and extracellular signals, the events that take place during different stages of the cell cycle must be coordinated with one another so that they occur in the appropriate order . T his coordination between different phases of the cell cycle is dependent on a series of cell cycle checkpoints that prevent entry into the next phase of the cell cycle until the events of the preceding phase have been completed. Stage of cell cycle progression arrest Response G1 DNA Damage S DNA Damage/incomplete DNA replication G2 DNA Damage/incomplete DNA replication M Chromosome misalignment Dr. Riddhi Datta

11. Check point to sense DNA damage Several cell cycle checkpoints function to ensure that incomplete or dam­aged chromosomes are not replicated and passed on to daughter cells. Cell cycle arrest at the G1, S , and G2 checkpoints is mediated by two related protein kinases, designated ATM and ATR, that recognize damaged or unreplicated DNA and are activated in response to DNA damage . ATM and ATR then activate a signaling pathway that leads not only to cell cycle arrest , but also to the activation of DNA repair and, in some cases, programmed cell death. Dr. Riddhi Datta

12. Another important cell cycle checkpoint that maintains the integrity of the genome occurs toward the end of mitosis . This check ­ point , called the spindle assembly checkpoint, monitors the alignment of chromosomes on the mitotic spindle, thus ensuring that a complete set of chromosomes is distributed accurately to the daughter cells. T he failure of one or more chromosomes to align properly on the spindle causes mitosis to arrest at metaphase, prior to the segregation of the newly replicated chromosomes to daughter nuclei. As a result of the spindle assembly checkpoint, the chromosomes do not separate until a complete complement of chromosomes has been organized for distribution to each daughter cell. Check point to sense chromosome misalignment Dr. Riddhi Datta

13. Restricting DNA Replication to Once per Cell Cycle DNA replication is restricted to once per cell cycle by the MCM helicase proteins that bind to origins of replication together with ORC (origin recognition complex) proteins and are required for the initiation of DNA replication . MCM proteins are only able to bind to DNA in G1 , allowing DNA replication to initiate in S phase. Once initiation has occurred, the MCM proteins are displaced so that replication cannot initiate again until after mitosis. Dr. Riddhi Datta

14. Regulators of Cell Cycle Progression Our current understanding of cell cycle regulation has emerged from a convergence of results obtained through experiments on organisms as diverse as yeasts, sea urchins, frogs, and mammals . Three initially distinct experimental approaches contributed to identifica­tion of the key molecules responsible for cell cycle regulation . Dr. Riddhi Datta

15. Discovery of MPF 1 st approach: In 1971, two independent teams of researchers ( Yoshio Masui and Clement Markert , as well as Dennis Smith and Robert Ecker ) found that oocytes arrested in G2 could be induced to enter M phase by microinjection of cytoplasm from oocytes that had been hormonally stimulated. Thus a cytoplasmic factor present in hormone ­ treated oocytes was sufficient to trigger transition from G2 to M . Because the entry of oocytes into meiosis is frequently referred to as oocyte maturation, this cytoplasmic factor was called maturation promoting factor (MPF). H owever , MPF also induces entry into M phase of the mitotic cycle. MPF thus appeared to act as a general regulator of the transition from G2 to M . Dr. Riddhi Datta

16. 2 nd approach: Studying the budding yeast Saccharomyces cerevisiae , Hartwell and his colleagues in the early 1970s identified temperature-sensitive mutants that were defective in cell cycle progression . cdc mutants in yeast The key characteristic of these mutants (called cdc for cell division cycle mutants ) was that they underwent growth arrest at specific points in the cell cycle. A particularly important mutant designated cdc28 caused the cell cycle to arrest at START , indicating that the Cdc28 protein is required for passage through this critical regulatory point in G1. Dr. Riddhi Datta

17. A similar collection of cell cycle mutants was isolated in the fission yeast Schizosaccharomyces pombe by Paul Nurse and his collaborators . These mutants included cdc2 , which arrests the S. pombe cell cycle both in G1 and at the G2 to M transition (the major regulatory point in fission yeast). Further studies showed that S. cerevisiae Cdc28 and S. pombe Cdc2 are functionally homologous genes , which are required for passage through START as well as for entry into mitosis in both species of yeasts. cdc mutants in yeast Dr. Riddhi Datta

18. Further studies of cdc2 yielded two important insights: Molecular cloning and nucleotide sequencing revealed that Cdc2 encodes a protein kinase—the first indication of the prominent role of protein phosphorylation in regulating the cell cycle. A human gene related to Cdc2 was identified and shown to function in yeasts, providing a dramatic demonstration of the conserved activity of this cell cycle regulator. cdc mutants in yeast Dr. Riddhi Datta

19. Discovery of cyclins 3 rd approach: In 1983, Tim Hunt and his colleagues identified two proteins that display a periodic pattern of accumulation and degradation in sea urchin and clam embryos. These proteins accumulate throughout interphase and are then rapidly degraded toward the end of each mitosis . Hunt called these proteins cyclins (the two proteins were designated cyclin A and cyclin B ) and suggested that they might function to induce mitosis, with their periodic accumulation and destruction controlling entry and exit from M phase. Direct support for such a role was provided in 1986, when Joan Ruderman and her colleagues showed that microinjection of cyclin A into frog oocytes is sufficient to trigger the G2 to M transition . Dr. Riddhi Datta

20. Characterization of MPF These initially independent approaches converged dramatically in 1988 when MPF was purified from frog eggs in the laboratory of James Maller . MPF , a conserved regulator of the cell cycle is composed of two key subunits : Cdkl cyclin B Cyclin B is a regulatory subunit required for catalytic activity of the Cdkl protein kinase. MPF activity is controlled by periodic accumulation and degradation of cyclin B during cell cycle progression. Dr. Riddhi Datta

21. MPF regulation Cdkl forms complexes with cyclin B during G2. CdkI is then phosphorylated on threonine-161 by MO15 , which is required for Cdkl activity. It is then phosphorylated on tyrosine-15 (and threonine-14 in vertebrate cells ) by a protein kinase called Wee1 , which inhibits Cdkl activity and leads to the accumu­lation of inactive Cdkl / cyclin B complexes throughout G2 phase. Dephosphorylation of Thr14 and Tyr15 by a protein phosphatase called Cdc25C activates MPF at the G2 to M tran­sition . Activated Cdkl protein kinase phosphorylates a variety of tar­get proteins that initiate the M phase . MPF activity is terminated at the end of mitosis by degradation of cyclin B . Dr. Riddhi Datta

22. Regulation of CDK activity The activity of Cdk's during cell cycle progression is regulated by four molecular mechanisms: Activities of CdkI / cyclin B complexes can also be regulated by binding of inhibitory proteins called CKI’s ( Cdk inhibitors ). In mammals 2 families of CKI’s are present: Ink4 family: binds to Cdk4 & Cdk6; inhibits G1 to S progression Kip/ Cip family: binds to Cdk1 & Cdk2; inhibits various phases of cell cycle progression Dr. Riddhi Datta

23. Families of Cyclins and Cyclin -Dependent Kinases In yeast , passage through START is controlled by Cdkl in association with G1 cyclins ( Cln1 , Cln2 , and Cln3). Complexes of Cdkl with distinct B-type cyclins ( Clb’s ) then regu­late progression through S phase and entry into mitosis. In animal cells , progres­sion through the G1 restriction point is controlled by complexes of Cdk4 and Cdk6 with D-type cyclins . Cdk2/ cyclin E complexes function later in G1 and are required for the G1 to S transition. Cdk2 / cyclin A complexes are then required for progression through S phase. CdkI / cyclin A regulates progression to G2 Cdkl / cyclin B complexes drive the G2 to M transition. Dr. Riddhi Datta

24. Growth factors & regulation of G1 to S transition Growth factors regulate cell cycle progression through the G1 restriction point by inducing synthesis of D-type cyclins via the Ras / Raf /MEK/ERK signaling pathway . Cyclin D1 continues to be synthesized as long as growth factors are present. However, cyclin D1 is also rapidly degraded. As long as growth factors are present through G1, complexes of Cdk4, 6/ cyclin D1 drive cells through the restric­tion point. If growth factors are removed prior to this key regulatory point, the levels of cyclin D1 rapidly fall and cells are unable to progress through G1 to S, instead becoming quiescent and entering G0. Dr. Riddhi Datta

25. While cyclin D and oncogene proteins like Ras drive cell proliferation, proteins encoded by many tumor suppressor genes (like Rb and Ink4 Cdk inhibitors ) act as brakes that slows down cell cycle progression. Mutations resulting in continual unregulated expression of cyclin D1 contribute to the development of a variety of human cancers, including lymphomas and breast cancers. Similarly, muta­tions that inactivate the Ink4 Cdk inhibitors that bind to Cdk4 and Cdk6 are commonly found in human cancer cells. Growth factors & regulation of G1 to S transition Dr. Riddhi Datta

26. C onnection between cyclin D and cancer is further fortified by the fact that a key substrate protein of Cdk4, 6/ cyclin D complexes, Rb , is itself frequently mutated in a wide array of human tumors. Rb is the prototype of a tumor suppressor gene whose inactivation leads to tumor development. In its under-phos­phorylated form, Rb binds to members of the E2F family (transcription factors that initiates transcription of genes required for initiation of S phase). This represses transcrip­tion of E2F-regulated genes. Phosphorylation of Rb by Cdk4,6/ cyclin D complexes results in its dissociation from E2F in late G1. E2F then stimulates expression of its target genes, which encode proteins required for cell cycle progression. Growth factors & regulation of G1 to S transition Dr. Riddhi Datta

27. Progression through the restriction point and entry into S phase is mediated by the activation of Cdk2/ cyclin E complexes. ln early G1, Cdk2/ cyclin E complexes are inhibited by the Cdk inhibitor p27 . Passage through the restriction point induces the synthesis of cyclin E via activation of E2F. In addition, growth factor signaling reduces the levels of p27 by inhibiting its tran­scription and translation. The resulting activation of Cdk2/ cyclin E leads to activation of the MCM helicase and initiation of DNA replication (i.e. S phase). Growth factors & regulation of G1 to S transition Dr. Riddhi Datta

28. DNA Damage Checkpoints DNA damage check ­ points arrests cell cycle progression in response to damaged or incompletely replicated DNA. Cell cycle arrest at the DNA damage checkpoints is initiated by the ATM or ATR protein kinases , which are components of protein complexes that recognize damaged or un-replicated DNA. ATM is activated principally by double-strand breaks , while ATR is activated by single ­ stranded or un-replicated DNA . ATM and ATR then phosphorylate and activate the CHK2 and CHKI protein kinases , respectively. CHKI and CHK2 phosphorylate and inhibit the Cdc25A and Cdc25C protein phosphatases . Cdc25A and Cdc25C are required to activate Cdk2 and Cdkl , respectively, so their inhibi­tion leads to arrest at the DNA damage checkpoints in G1, S, and G2. Dr. Riddhi Datta

29. In mammalian cells, arrest at the G1 checkpoint is also mediated by the action of an additional protein known as p53 , which is phosphorylated by both ATM and CHK2. Phosphoryl­ation by ATM and CHK2 stabilize p53 , resulting in rapid increases in p53 levels in response to DNA damage. The protein p53 then activates transcription of the gene encoding the Cdk inhibitor p21 , leading to inhibition of Cdk2/ cyclin E complexes and cell cycle arrest. DNA Damage Checkpoints Loss of p53 function as a result of mutations prevents G1 arrest in response to DNA damage, so the damaged DNA is replicated and passed on to daughter cells instead of being repaired. This inheritance of damaged DNA results in an increased frequency of mutations and general instability of the cellular genome, which contributes to cancer development. Dr. Riddhi Datta

30. M phase During prophase, chromosomes condense and centrosomes move to opposite sides of the nucleus , initiating formation of the mitotic spindle. Breakdown of the nuclear envelope then allows spindle microtubules to attach to the kinetochores of chromosomes. During pro-metaphase the chromosomes shuffle back and forth between the centrosomes and the center of the cell, eventually aligning in the center of the spindle ( metaphase). At anaphase, the sister chromatids separate and move to opposite poles of the spindle . Mitosis then ends with re-formation of nuclear envelopes and chromosome de-condensation during telophase, and cytokinesis yields two interphase daughter cells. Note that each daughter cell receives one centrosome, which duplicates prior to the next mitosis. Dr. Riddhi Datta

31. Phases of mitosis Dr. Riddhi Datta

32. Cdkl / Cyclin 8 and Progression to Metaphase Mitosis involves dramatic changes in multiple cellular components, leading to a major reorganization of the entire structure of the cell. These events are initiated by activation of the Cdkl / cyclin B protein kinase (MPF). The Cdkl / cydin B complex induces multiple nuclear and cytoplasmic changes at the onset of M phase both by activating other protein kinases and by phosphorylating proteins such as condensins , components of the nuclear envelope , Golgi matrix proteins , and proteins associated with centrosomes and microtubules . Dr. Riddhi Datta

33. Chromatin condensation: Phosphorylation of cohesins & condensins Cohesins bind to DNA during S phase and maintain the linkage between sister chromatids following DNA replication in S and G2. As the cell enters M phase the cohesins are replaced by condensins along most of the chromosome, remaining only at the centromere. Phosphorylation by Cdkl activates the condensins , which drive chromatin condensation . Dr. Riddhi Datta

34. Breakdown of the nuclear envelope Cdkl / cyclin B phosphorylates the nuclear lamins as well as proteins of the nuclear pore complex and inner nuclear membrane. Phosphorylation of the lamins causes the filaments that form the nuclear lamina to dissociate into free lamin dimers . Dr. Riddhi Datta

35. Fragmentation of Golgi apparatus The Golgi apparatus fragments into small vesicles at mitosis, which may either be absorbed into the endoplasmic reticulum or distributed directly to daughter cells at cytokinesis. The breakdown of these membranes is mediated by Cdkl phosphorylation of Golgi matrix proteins ( such as GM130 and GRASP-65), which are required for the docking of COPI-coated vesicles to the Golgi membrane. Phosphorylation of these pro­teins by Cdkl inhibits vesicle docking and fusion, leading to fragmentation of the Golgi apparatus . Dr. Riddhi Datta

36. Spindle formation At the beginning of prophase, activation of Cdkl leads to separation of the centrosomes, which were duplicated during S phase . Centrosome maturation and spindle assembly involves Aurora and Polo-like protein kinases, which are located at the centrosome . Like Cdkl , Aurora and Polo-like kinases are activated in mitotic cells, and they play important roles in spindle formation and kinetochore function, as well as in cytokinesis . Microtubules from opposite poles of the spindle eventually attach to the two kinetochores of sister chromatids (which are located on opposite sides of the chromosome ), and the balance of forces acting on the chromosomes leads to their alignment on the metaphase plate in the center of the spindle . The spindle consists of four kinds of microtubules: Kinetochore microtubules and chromosomal microtubules are attached to chromosomes ; polar microtubules overlap in the center of the cell; and astral microtubules radiate from the centrosome to the cell periphery. Dr. Riddhi Datta

37. The spindle assembly checkpoint The spindle assembly checkpoint monitors the alignment of chromosomes on the metaphase spindle . The progression from metaphase to anaphase results from ubiquitin-mediated proteolysis of key regulatory proteins, called the anaphase-pro­moting complex (APC). Dr. Riddhi Datta

38. The spindle assembly checkpoint Activation of APC is induced at the beginning of mitosis, so the activation of Cdkl / cyclin B ultimately triggers its own destruction . The APC remains inhibited until the cell passes the spindle assembly checkpoint , after which activation of ubiquitin degradation system brings about the transition from metaphase to anaphase and progression through the rest of mitosis. Dr. Riddhi Datta

39. The spindle assembly checkpoint Unattached kinetochores lead to assembly of Mad/Bub protein complex that bind to Cdc20 -a required component of the APC. Mad proteins are activated in this complex, and then released in active form that inhibits Cdc20, maintaining the APC in inactive state. Once all chromosomes are aligned on the spindle, the Mad/Bub complex dissociates, relieving inhibition of Cdc20 and leading to APC activation . Dr. Riddhi Datta

40. The spindle assembly checkpoint Activation of APC results in ubiquitination and degradation of two key target proteins. APC ubiquitinates cyclin B , leading to its degradation and inactivation of Cdkl . APC also ubiquitinates securin , leading to activa­tion of separase . Separase degrades a subunit of cohesin , breaking the link between sister chromatids and initiating anaphase. Dr. Riddhi Datta

41. Check points Dr. Riddhi Datta

42. Cdk / Cyclins Dr. Riddhi Datta

43. Growth factors Ras / Raf / ERK/MEK Synthesis of Cyclin D Cdk4,6/ CycD Rb E2F Rb E2F P Transcriptional activation Cyclin E Cyclin E Cdk2 p27 Growth factors p27 Cyclin E Cdk2 MCM helicase ATM ATR Double strand break Single strand break CHK1 Cdc25C Cdc25A Cdk2 Cdk1 P P G1 phase G1 to S transition M phase G1, S, G2 phases p53 p21 CHK2 Unattached kinetochore Mad Bub Mad Cdc20 APC All kinetochores attached Bub Bub Mad Cdc20 APC Cyclin B degraded Cdk1 inactive Securin degraded Separase active Anaphase Dr. Riddhi Datta

44. Regulation of Meiosis Vertebrate oocytes (developing eggs) have been particularly useful models for research on the cell cycle. T he discovery and subsequent purification of MPF ( Cdkl / cyclin B) was made from frog oocytes . The first regulatory point in oocyte meiosis is in the diplotene stage of the first meiotic division. Meiosis is arrested at the diplotene stage , during which oocytes grow to a large size . Oocytes then resume meiosis in response to hormonal stimulation and complete the first meiotic divi­sion , with asymmetric cytokinesis giving rise to a small polar body. Most vertebrate oocytes are then arrested again at metaphase II . Dr. Riddhi Datta

45. Regulation of Meiosis Like the M phase of somatic cells, the meiosis of oocytes is controlled by the activity of Cdkl / cyclinB complexes . The regulation of Cdkl during oocyte meiosis, however, displays unique features that are responsible for progression from meiosis l to meiosis II and for metaphase II arrest. Hormonal stimulation of diplotene -arrested oocytes initially triggers the resumption of meiosis by activating Cdkl , as at the G2 to M transition of somatic cells . Dr. Riddhi Datta

46. Regulation of Meiosis Cdkl then induces chromosome condensation , nuclear envelope breakdown, and formation of the spindle. Activation of the anaphase-promoting complex then leads to the metaphase to anaphase transition of meiosis I, accompanied by a decrease in the activity of Cdkl . However, in contrast to mitosis, Cdkl activity is only partially decreased, so the oocyte remains in M phase , chromatin remains condensed, and nuclear envelopes do not re-form. Following cytokinesis, Cdkl activity again rises and remains high while the egg is arrested at metaphase II. Dr. Riddhi Datta

47. Regulation of Meiosis Cytoplasm from a metaphase II egg is microinjected into one cell of a two-cell embryo. The injected embryo cell arrests at metaphase, while the uninjected cell continues to divide. A factor in metaphase II in egg cytoplasm ( cytostatic factor ) therefore has induced metaphase arrest of the injected embryo cell. Dr. Riddhi Datta

48. More recent experiments have identified a protein-serine/threonine kinase known as Mos as an essential component of CSF . Mos is specifically synthesized in oocytes around the time of completion of meiosis I . The Mos protein kinase maintains Cdkl I cyclin B activity both by stimulating the synthesis of cyclin B and by inhibiting the degradation of cyclin B by the anaphase-promoting complex . The action of Mos is mediated by MEK , ERK , and Rsk protein kinases, and inhi­bition of the anaphase-promoting com­plex is mediated by Mad/Bub proteins. Mos thus maintains Cdkl / cyclin B activity during oocyte meiosis, leading to the arrest of oocytes at metaphase II . Oocytes can remain arrested at this point in the meiotic cell cycle for several days, awaiting fertilization. Regulation of Meiosis Dr. Riddhi Datta

49. Fertilization At fertilization, the sperm binds to a receptor on the surface of the egg and fuses with the egg plasma membrane , initiating the development of a new diploid organism containing genetic information derived from both parents. A sperm binds to a plasma membrane receptor of the egg and this induces an increase in Ca2+ level in egg cytoplasm , via hydrolysis of PIP2 (phosphatidylinositol4,5-isphosphate). The Ca2+ induces exocytosis of secretory vesicles that are present in large numbers beneath the egg plasma membrane . This, in turn, induces surface alterations that prevent additional sperm from entering the egg . The increase in cytosolic Ca2+ following fertilization also signals the com­pletion of meiosis . Following completion of oocyte meiosis, the fertilized egg (zygote ) contains two haploid nuclei (called pronuclei ), one derived from each parent . T he two pronuclei then enter S phase and repli­cate their DNA as they migrate toward each other . As they meet, the zygote enters M phase of its first mitotic division. Dr. Riddhi Datta

50. Thank you Dr. Riddhi Datta