1 / 68

Cell Cycle

Cell Cycle. Why Cells Divide. Why do cells need to divide?. Cell Division - Purpose. Provides a means of reproduction for organisms. Provides a means of growth in multicellular organisms. Provides a means of repair in multicellular organisms. Reproduction.

awen
Télécharger la présentation

Cell Cycle

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Cell Cycle

  2. Why Cells Divide • Why do cells need to divide?

  3. Cell Division - Purpose • Provides a means of reproduction for organisms. • Provides a means of growth in multicellular organisms. • Provides a means of repair in multicellular organisms.

  4. Reproduction • Unicellular organisms divide in order to form new organisms. No Elizabeth, don’t go! Apologies to Gary Larson and the FAR SIDE

  5. Reproduction • Multicellular organisms divide to reproduce special cells (gametes) that will carry out the formation of a new organism.

  6. Growth • Multicellular organisms are made of millions to trillions of microscopic cells rather than a few large cells. Growth mainly occurs by increasing the number of cells.

  7. Repair • Many microscopic cells as opposed to only a few large ones is also advantageous in the case of injury. How so?

  8. Is there any advantage to possessing many small cells as opposed to a few large cells? Examine the two cells below. The blue color represents the area of diffusion of glucose within the cell over equal periods of time. Do you see a potential problem that might suggest an answer to the question above? nucleus

  9. Surface Area to Volume Ratio • Relative size of the surface area of the plasma membrane and the volume of the cell reach a critical point. Analyze the following cubic cell:

  10. Nuclear Limitations • Limited capability of the nucleus -- there is a finite amount of genetic material because the genome size remains constant even as the cell grows.

  11. Why Cells Divide • What actually triggers or cues the cell about the need to divide? Most of it comes down to chemicals.

  12. Why Cells Divide • Two irreversible points in cell cycle • replication of genetic material • separation of sister chromatids   REPLICATION (S phase) SEPARATION (anaphase)

  13. Why Cells Divide • Checkpoints • process is assessed & possibly halted • 3 major checkpoints: • G1/S • can DNA synthesis begin? • G2/M • has DNA synthesis been completed correctly? • commitment to mitosis • spindle checkpoint • are all chromosomes attached to spindle? • can sister chromatids separate correctly?

  14. Checkpoint control system • Checkpoints • cell cycle controlled by STOP & GOchemical signals at critical points • signals indicate if key cellular processes have been completed correctly

  15. Why Cells Divide The “decision” to divide has both external and internal chemical influences.

  16. Why Cells Divide • EXTERNAL • Cells can have direct contact with each other through cell junctions or surfaces. • Certain chemicals can easily come in contact with adjacent cells in this way.

  17. Why Cells Divide • Cells can communicate with each other by secreting chemical messengers into the extracellular fluid. • Paracrine signaling– target is near the signaling source (ex. Adjacent cells) • Hormonal signaling – signal travels through bloodstream from source to target (pituitary – ovaries) • Synaptic signaling – chemicals travel across small “gap” or synapse (neurotransmitters from neuron to neuron)

  18. Why Cells Divide • The signaling process consists of 3 stages: • EXTERNAL • Reception - Chemical messengers interact with receptors, often those of the plasma membrane.

  19. Why Cells Divide INTERNAL • Transduction- This begins a chain of events in a chemical pathway within the cell, such as a “phosphorylation cascade” • Phosphorylation cascading refers to the transfer of phosphate groups from one protein molecule to the next, via a type of kinase enzyme, subsequently activating the molecules in the pathway. (Think Domino effect)

  20. Why Cells Divide • Response - cellular activity • Could be: • Rearrangement of cytoskeleton • Opening/closing ion channels • Initiation of metabolic activity, such as cell division.

  21. Chemical Regulation of the Cell Cycle • Once signaled, Kinase* proteins give the go ahead signals at G1 and G2 checkpoints. • *Kinase proteins are a family of related proteins that activate proteins which in turn activate certain cell processes such as cell division.

  22. These kinases (Cdks) themselves are not activated until they are attached to cyclin (becoming MPFs) • Cyclin concentrations fluctuate within a cell, slowly building up until cell division begins.

  23. The MPF’s (Cdk-cyclin) cause nuclear membrane destruction and stimulate other kinases, setting the chain of events in motion known as mitosis.

  24. The Cell Cycle(Internet click here)(click on the words above to go to website and ACQUIRE more information on the cell cycle.) The Cell Cycle(Internet click here)(click on the words above to go to website and ACQUIRE more information on the cell cycle.) • Consists of phases characterized by important events in the life of a cell

  25. After Mitosis • MPFs (partially made of cyclin) levels must drop to allow the cell to enter interphase again. • Proteins such as ubiquitin*, regulate the cycle by causing the degradation of cyclin and kinases. • This brings about the end of mitosis and the reforming of the nuclear membrane. • Thus the new cells continue to interphase. *Ubiquitin – common across most eukaryotic species; hence ubiquitous.

  26. Other Division Halting Processes Density-dependent inhibition – when the cell density reaches a certain maximum, many cells stop dividing. Anchorage dependence – contact with a substratum may influence if a cell stops dividing.

  27. External signals • Growth factors • coordination between cells • protein signals released by body cells that stimulate other cells to divide • density-dependent inhibition • crowded cells stop dividing • each cell binds a bit of growth factor • not enough activator left to trigger division in any one cell • Degradation of protein growth factors also possible • anchorage dependence • to divide cells must be attached to a substrate • “touch sensor” receptors

  28. Example of a Growth Factor • Platelet Derived Growth Factor (PDGF) • made by platelets in blood clots • binding of PDGF to cell receptors stimulates cell division in connective tissue • heal wounds

  29. Other Division Halting Processes Necrosis and Apoptosis Necrosis – death due to insult/injury Apoptosis – programmed cell death What benefits would there be for an organism to destroy its own cells? http://virtuallaboratory.colorado.edu/Biofundamentals/lectureNotes/Topic5-4_CellDeath.htm

  30. Cancer • Transformation – Alterations in genes implicated with cell cycle begin the conversion of a normal cell to cancerous cell. • Oncogenes • Tumor Supressor genes • Tumor – mass of abnormal cells • Benign – tumor remains at site • Malignant – Becomes invasive enough to interfere with organ function • Metastasis – cancer cells spread to other sites

  31. Development of Cancer • Cancer develops only after a cell experiences ~6 key mutations (“hits”) • unlimited growth • turn on growth promoter genes • ignore checkpoints • turn off tumor suppressor genes (p53) • escape apoptosis • turn off suicide genes • immortality = unlimited divisions • turn on chromosome maintenance genes • promotes blood vessel growth • turn on blood vessel growth genes • overcome anchor & density dependence • turn off touch-sensor gene

  32. p53 — master regulator gene NORMAL p53 p53 allows cells with repaired DNA to divide. p53 protein DNA repair enzyme p53 protein Step 2 Step 1 Step 3 DNA damage is caused by heat, radiation, or chemicals. Cell division stops, and p53 triggers enzymes to repair damaged region. p53 triggers the destruction of cells damaged beyond repair. ABNORMAL p53 abnormal p53 protein cancer cell Step 2 Step 1 Step 3 The p53 protein fails to stop cell division and repair DNA. Cell divides without repair to damaged DNA. DNA damage is caused by heat, radiation, or chemicals. Damaged cells continue to divide. If other damage accumulates, the cell can turn cancerous.

  33. Explain the purpose of meioisis. Compare and contrast the processes of mitosis and meiosis. Distinguish between male and female gametogenesis in humans. Define tetrad, homologous chromosome, synapsis. Describe the process of crossing-over. Identify the composition of a eukaryotic chromosome. Explain the results of a duplication, deletion, inversion and translocation of chromosomes. Define nondisjunction and provide examples of several genetic disorders resulting from nondisjunction.       

  34. Genomes and Chromosomes A Closer Look at Reproduction

  35. Genomes and Chromosomes • An organism is determined by its organism’s genetic material or genome. • In order to maintain life, any new cells created must possess the same exact genome.

  36. Genome and Chromosomes • The mitotic portion of the cell cycle ensures that the genome is transferred correctly to the new cells created.

  37. Genes and Chromosomes Chromosomes are the condensed version of the DNA-protein complex called chromatin. http://images.sciencedaily.com/2009/10/091005110401-large.jpg http://www.damours.iric.ca/Site/Projects_files/Chromatin_Nucleosomes.png

  38. Genes and Chromosomes • Once the chromatin is replicated during the S phase of the cell cycle a cell is ready to divide. Kinetochore

  39. Mitosis vs. Meiosis Mitosis Meiosis Both

  40. Mitosis vs. Meiosis During S phase Once -two Twice –four Same as original: Diploid Half of the original- haploid

  41. Meiosis For what purposes would “HALF” the chromosome material be appropriate? For the union of two cells (gametes) – sexual reproduction.

  42. Meiosis I • Homologous Chromosomes pair up forming a “tetrad” in a process called synapsis. • At certain points the chromatids of the homologs may crisscross, forming a chiasmata.

  43. Meiosis I • As the cell transists from metaphase to anaphase it is the homologs which are separated rather than the chromatids.

  44. Meiosis I • Cytokinesis occurs, resulting in two haploid cells. • Depending on the type of gamete, meiosis II may proceed directly or be carried out at a later date.

  45. Meiosis II • Proceeds much like the process of mitosis, however, the end results differ due to Meiosis I. 4 haploid cells are created.

  46. Meiosis • Humans – • Spermatogenesis – 4 viable haploid sperm produced. • Oogenesis – 1 viable haploid egg and 3 non-functional polar bodies produced.

  47. What is the advantage of producing one LARGE egg as opposed to 4 smaller ones? http://legacy.owensboro.kctcs.edu/gcaplan/anat2/notes/gametogenesis.jpg

  48. Meiosis • Plants – produce spores which then mitotically divide to form gametophyte. This produces gametes which then fuse to form sporophyte.

  49. Genetic Variation • Does not exist in cells produced by mitosis, unless some mutation arises. • Sexual reproduction provides a recombination of genetic material in 3 ways.

More Related