2. Chapter 12~ The Cell Cycle

3. 2007-2008 Biology is the only subject in which multiplication is the same thing as division…

4. Why do cells divide?  There are functional limits to cell size, determined by: –1.) SA/V Ratio –The volume of a cell increases faster than the surface area when a cell grows. WHY? –As the cell grows, the SA/V ratio decreases. When it gets too small, the cell stops growing or begins cell division.  Key: Large SA/V ratio = good; Small SA/V ratio = bad –2.) Genome to Volume Ratio –If cell grows too large (G1), the DNA will not be able to control the cell adequately. –The cell divides before the G/V ratio gets too small.

6.  For reproduction –asexual reproduction  unicellular organisms  For growth –from fertilized egg to multi-celled organism  For repair & renewal –replace cells that die from normal wear & tear or from injury Why do cells divide? amoeba

7.  Unicellular organisms reproduce by cell division. 100 µm (a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM). Figure 12.2 A

8.  Multicellular organisms depend on cell division for: –Development from a fertilized cell. –Growth. –Repair. 20 µm200 µm (b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM). (c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM). Figure 12.2 B, C

9. Importance of Cell Division 1. Growth and Development 2. Asexual Reproduction 3. Tissue Renewal Zygote EmbryoFetus Adult 1 Cell 100 cellsmillions cells100 trillion cells

10. DNA organization in Prokaryotes  Nucleoid region  Bacterial Chromosome –Single (1) circular DNA –Small  (e.g. E. coli is 4.6X10 6 bp, ~1/100 th human chromosome)  Plasmids – extra chromosomal DNA

11. Bacterial Fission

12.  The cell division process is an integral part of the cell cycle.

13. The Cell Cycle  Interphase (90% of cycle) G1 phase~ growth G1 phase~ growth S phase~ synthesis of DNA S phase~ synthesis of DNA G2 phase~ preparation for cell division G2 phase~ preparation for cell division  Mitotic phase (M-phase) Mitosis~ nuclear division Mitosis~ nuclear division Cytokinesis~ cytoplasm division Cytokinesis~ cytoplasm division

14. Some Vocabulary before we get into it all…  Genome: cell’s genetic information  Somatic (body cells) cells  Gametes (reproductive cells): sperm and egg cells  Chromosomes: condensed DNA molecules  Diploid (2n): 2 sets of chromosomes  Haploid (1n): 1 set of chromosomes  Chromatin: DNA-protein complex  Chromatids: replicated strands of a chromosome  Centromere: narrowing “waist” of sister chromatids  Mitosis: nuclear division  Cytokinesis: cytoplasm division  Meiosis: gamete cell division

15. 14 Chromosome Organization  When cells divide, daughter cells must each receive complete copy of DNA  Each cell has about 2 meters of DNA in the nucleus; thin threads called chromatin  Before division, condenses to form chromosomes  DNA also replicates before cell division to produce paired chromatids

16. 15 Normal Karyotype (Fig 18.1)

17. Facts About Cell Division  Cell division results in genetically identical daughter cells.  Cells duplicate their genetic material before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA.

18. Cellular Organization of the Genetic Material  A cell’s endowment of DNA, its genetic information is called its genome.

19.  The DNA molecules in a cell are packaged into chromosomes. 50 µm Figure 12.3

20.  Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division.  In animals: –Somatic cells have two sets of chromosomes –Gametes have one set of chromosomes

21. Distribution of Chromosomes During Cell Division  In preparation for cell division DNA is replicated and the chromosomes condense.

22.  Each duplicated chromosome has two sister chromatids, which separate during cell division. 0.5 µm Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Sister chromatids Centromeres Sister chromatids A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule. Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. Figure 12.4

23.  Eukaryotic cell division consists of –Mitosis, the division of the nucleus. –Cytokinesis, the division of the cytoplasm.  In meiosis –Sex cells are produced after a reduction in chromosome number. This reduction is accomplished by a second division.

24.  The mitotic phase alternates with interphase in the cell cycle.  A labeled probe can reveal patterns of gene expression in different kinds of cells.

25. Phases of the Cell Cycle  The cell cycle consists of –The mitotic phase –Interphase INTERPHASE G1G1 S (DNA synthesis) G2G2 Cytokinesis Mitosis MITOTIC (M) PHASE Figure 12.5

26.  Interphase can be divided into subphases 1.G 1 phase 2.S phase 3.G 2 phase

27.  The mitotic phase is made up of mitosis and cytokinesis.

28. 5 Phases of Mitosis  5 Phases: 1.Prophase 2.Prometaphase 3.Metaphase 4.Anaphase 5.Telophase

29. CELL DIVISION (In Humans)  Prophase : 1. The nuclear membrane disappears. 2. The centrioles (only in animals) move to the opposite ends/poles of the cell. 3. Spindle Fibers begin to form. The genetic material recoils into the chromosome state (92 chromatids).

30. Prometaphase –spindle fibers attach to centromeres  creating kinetochores –microtubules attach at kinetochores  connect centromeres to centrioles –chromosomes begin moving

31. CELL DIVISION (In Humans)  Metaphase: 1. The chromosomes have lined up at the “equator” of the nucleus 2. Spindle fibers coming from the centrioles have attached to the chromosomes at the centromeres. The genetic material is in the chromosome state (92) chromatids).

32. CELL DIVISION (In Humans) - 4 Phases:  Anaphase: 1. The chromatids are pulled apart by the spindle fibers. 46 chromatids going toward each end of the cell (two ends = 92 chromatids). 2. Cytokinesis begins.

33. CELL DIVISION (In Humans) - 4 Phases:  Telophase: “Opposite of prophase.” 1. Two nuclear membranes reappear 2. The centrioles get out of the way 3. Cytokinesis finishes. There are now two identical daughter cells entering G1 Phase, each with 46 chromatids which will quickly turn into a chromatin state and double during the S phase of interphase to give each body cell 23 pairs or 46 individual chromosomes.

34. G 2 OF INTERPHASE PROPHASE PROMETAPHASE Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Kinetochore Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore microtubule Figure 12.6 Nonkinetochore microtubules 5 Phases

35. Centrosome at one spindle pole Daughter chromosomes METAPHASEANAPHASETELOPHASE AND CYTOKINESIS Spindle Metaphase plate Nucleolus forming Cleavage furrow Nuclear envelope forming Figure 12.6 5 Phases

36. The Mitotic Spindle: A Closer Look  The mitotic spindle Is an apparatus of microtubules that controls chromosome movement during mitosis.

37.  The spindle arises from the centrosomes and includes spindle microtubules and asters (made of centrioles).

38.  Some spindle microtubules –Attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate CentrosomeAster Sister chromatids Metaphase Plate Kinetochores Overlapping nonkinetochore microtubules Kinetochores microtubules Centrosome Chromosomes Microtubules 0.5 µm 1 µm Figure 12.7

39.  In anaphase, sister chromatids separate a nd move along the kinetochore microtubules toward opposite ends of the cell. EXPERIMENT 1 The microtubules of a cell in early anaphase were labeled with a fluorescent dye that glows in the microscope (yellow). Spindle pole Kinetochore Figure 12.8

40.  Non-kinetechore microtubules from opposite poles overlap and push against each other, elongating the cell.  In telophase, genetically identical daughter nuclei form at opposite ends of the cell.

41. Cytokinesis: A Closer Look  In animal cells –Cytokinesis occurs by a process known as cleavage, forming a cleavage furrow. Cleavage furrow Contractile ring of microfilaments Daughter cells 100 µm (a) Cleavage of an animal cell (SEM) Figure 12.9 A

42.  In plant cells, during cytokinesis –A cell plate forms. Daughter cells 1 µm Vesicles forming cell plate Wall of patent cell Cell plate New cell wall (b) Cell plate formation in a plant cell (SEM) Figure 12.9 B

43.  Mitosis in a plant cell 1 Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is staring to from. Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelop will fragment. Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of cell as their kinetochore microtubles shorten. Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell. 23 4 5 Nucleus Nucleolus Chromosome Chromatin condensing Figure 12.10

44. Binary Fission  Prokaryotes (bacteria) reproduce by a type of cell division called binary fission.

45. Binary Fission  The bacterial chromosome replicates.  The two daughter chromosomes actively move apart. Origin of replication E. coli cell Bacterial Chromosome Cell wall Plasma Membrane Two copies of origin Origin Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. 1 Replication continues. One copy of the origin is now at each end of the cell. 2 Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. 3 Two daughter cells result.4 Figure 12.11

46. The Evolution of Mitosis  Since prokaryotes preceded eukaryotes by billions of years it is likely that mitosis evolved from bacterial cell division.  Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis carried out by most eukaryotic cells.

47.  A hypothetical sequence for the evolution of mitosis Most eukaryotes. In most other eukaryotes, including plants and animals, the spindle forms outside the nucleus, and the nuclear envelope breaks down during mitosis. Microtubules separate the chromosomes, and the nuclear envelope then re-forms. Dinoflagellates. In unicellular protists called dinoflagellates, the nuclear envelope remains intact during cell division, and the chromosomes attach to the nuclear envelope. Microtubules pass through the nucleus inside cytoplasmic tunnels, reinforcing the spatial orientation of the nucleus, which then divides in a fission process reminiscent of bacterial division. Diatoms. In another group of unicellular protists, the diatoms, the nuclear envelope also remains intact during cell division. But in these organisms, the microtubules form a spindle within the nucleus. Microtubules separate the chromosomes, and the nucleus splits into two daughter nuclei. Prokaryotes. During binary fission, the origins of the daughter chromosomes move to opposite ends of the cell. The mechanism is not fully understood, but proteins may anchor the daughter chromosomes to specific sites on the plasma membrane. (a) (b) (c) (d) Bacterial chromosome Microtubules Intact nuclear envelope Chromosomes Kinetochore microtubules Intact nuclear envelope Kinetochore microtubules Fragments of nuclear envelope Centrosome Figure 12.12 A-D