1 Chapter 8 Cell Growth and DivisionPhoto Credit: © CAMR/A.B. Dowsett/Science Photo Library/Photo Researchers, Inc.

2 Limits to Cell Growth What problems does growth cause for cells?The larger a cell becomes, the more demands a cell places on its DNA and the harder it is for the cell to move enough nutrients and wastes across the cell membrane. Think of a library. If the amount of books doesn’t change and the town continues to grow, will the library be able to meet the needs of the community?

3 Limits to Cell Growth The rate at which food, oxygen, water, and wastes are moved in and out of the cell is dependent on the surface area of the cell. The rate at which food, oxygen, and water are used and waste is produced depends on the cell’s volume.

4 Limits to Cell Growth Ratio of Surface Area to Volume- As the length of a cell increases, its volume increases faster than the surface area. - The decrease in the cell’s ratio of surface area to volume makes it more difficult for the cell to move needed materials in and waste products out quickly enough for the cell to survive.

5 Limits to Cell Growth As the length of a cell increases, its volume increases faster than its surface area. The resulting decrease in the cell’s ratio of surface area to volume makes it more difficult for the cell to move needed materials in and waste products out.

6 Division of the Cell Before it becomes too large, a growing cell divides forming two “daughter” cells. The process by which a cell divides into two new daughter cells is called cell division.

7 8–1 Review As a cell increases in size, which of the following increases most rapidly? surface area volume

8 8–1 Review If an imaginary cube-shaped cell has a length of 6 cm, its ratio of surface area to volume is 1 : 1. 6 : 1. 36 : 1. 1 : 6.

9 8–1 Review The process by which a cell divides into two new daughter cells is called cell growth. cell division. DNA replication. cell multiplication.

10 8–1 Review When one cell divides in two, what happens to the surface area to volume ratios in the new cells? There is no change in the amount of material exchanged. Each new cell can exchange more material than the original cell. Each new cell can exchange less material than the original cell. The two new cells cannot be compared to the original cell.

11 8–1 Review When a growing cell undergoes cell division, each new cell gets half the DNA from the original cell. twice as much DNA as the original cell. a random sample of the DNA in the original cell. a full copy of all the DNA in the original cell.

12 Chromosomes Eukaryotic cells: during cell division, DNA is coiled tightly into compact structures called chromosomes. - Rod-shaped structures made of DNA and protein.

13 Chromosomes (cont’d) Between cell divisions, DNA is not so tightly coiled into chromosomes and is called chromatin.

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15 Chromosomes (cont’d) Each chromosome consists of two identical “sister” chromatids. Each pair of chromatids is attached at an area called the centromere. When the cell divides, the chromatids separate. Each new cell gets one chromatid. Sister chromatids Centromere Photo Credit: © Gunther F. Bahr/AFIP/Stone

16 Chromosome Number Each species has a characteristic chromosome numberHuman beings have 46 chromosomes. Dogs have 78 chromosomes. The number of chromosomes does NOT determine the complexity of the organism.

17 Human Karyotype

18 Chromosome Number (cont’d)Chromosomes are characterized as 2 types: - Sex chromosomes – determines the sex of an organism; may carry genes for other characteristics. - Autosomes – represent all of the other chromosomes.

19 Sex Chromosomes Humans have 2 sex chromosomes: X or Y.Normal females have XX and males have XY.

20 Autosomes All other chromosomesHumans have 2 sex chromosomes and 44 autosomes.

21 Homologous ChromosomesEvery cell produced by sexual reproduction has 2 copies of each autosome. - One from each parent. The 2 copies are homologous chromosomes or homologues.

22 Homologous Chromosomes (cont’d)Same size and shape and carry genes for the same traits. Humans have 22 homologous pairs and 2 sex chromosomes.

23 Diploid Represented by 2n.Cells with 2 sets of chromosomes, with both chromosomes for each homologous pair.

24 Diploid (cont’d) Cells also have 2 sex chromosomes.All normal human cells, except reproductive cells (sperm cells and egg cells), are diploid. Humans have a diploid number of 46.

25 Haploid Represented by 1n or n.Human sperm and egg cells have only one set of chromosomes, which is half the number in a diploid.

26 Haploid (cont’d) Haploid cells contain only one chromosome of each homologous pair and only one sex chromosome. When a sperm (1n) and an egg (1n) combine, they form a cell which is diploid (2n).

27 Cell Division: ProkaryotesDivide by binary fission division into 2 offspring.

28 Cell Division: Prokaryotes3 stages: Chromosome copies itself. Cell grows to twice the original size. Cell wall forms between the 2 chromosomes and cell splits into 2 new cells, identical to the original.

29 Cell Division: EukaryotesTwo types of cell division - Mitosis: new cells form with genetic material identical to that of the original cell. Reproduction in unicellular organisms Growth and repair of tissue and organs in multicellular organisms

30 Cell Division: Eukaryotes (cont’d)Meiosis: reduces the chromosome number by half to produce sex cells.

31 Cell Division In eukaryotes, cell division occurs in two major stages.The first stage, division of the cell nucleus, is called mitosis. The second stage, division of the cell cytoplasm, is called cytokinesis. Reproduction by mitosis is considered asexual, since the cells produced by mitosis are genetically identical to the parent cell.

32 Chromosomes Genetic information is passed from one generation to the next on chromosomes. Before cell division, each chromosome is duplicated, or copied.

33 The Cell Cycle The cell cycle is the series of events that cells go through as they grow and divide. During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells, each of which then begins to cycle again. Interphase is the period of growth that occurs between cell divisions.

34 The Cell Cycle The cell cycle consists of four phases:G1 (First Gap Phase) S Phase G2 (Second Gap Phase) M Phase

35 Events of the Cell CycleDuring the normal cell cycle, interphase can be quite long, but the process of cell division takes place quickly. Interphase is divided into three phases: G1, S, and G2. During G1, the cell, increases in size (this is where cells do most of their growing) synthesizes new proteins and organelles

36 Events of the Cell CycleDuring the S phase, chromosomes are replicated DNA synthesis takes place Once a cell enters the S phase, it usually completes the rest of the cell cycle.

37 Events of the Cell CycleThe G2 Phase (Second Gap Phase) When the DNA replication is completed, the cell enters the G2 phase – the shortest of the three phases of interphase! Organelles and molecules required for cell division are produced during this phase. Once G2 is complete, the cell is ready to start the M phase—Mitosis.

38 Events of the Cell CycleDuring the cell cycle, the cell grows, replicates its DNA, and divides into two daughter cells. DNA synthesis takes place during the S phase. Cell division takes place during the M phase. G1 and G2 are gap phases.

39 Mitosis Biologists divide the events of mitosis into four phases:Prophase Metaphase Anaphase Telophase

40 Chromosomes (paired chromatids)Mitosis Section 10-2 Prophase Prophase Spindle forming Chromosomes (paired chromatids) Centromere Most eukaryotic cells go through a regular cycle of interphase, mitosis and cytokinesis. Mitosis has four phases: Prophase, Metaphase, Anaphase and Telophase. The events shown here are typical of animal cells.

41 Mitosis Prophase Prophase is the first and longest phase of mitosis.Spindle forming Centromere Chromosomes (paired chromatids) Centriole Prophase Prophase is the first and longest phase of mitosis. The centrioles separate and take up positions on opposite sides of the nucleus.

42 Mitosis The centrioles lie in a region called the centrosome.Spindle forming Centromere Chromosomes (paired chromatids) Centriole The centrioles lie in a region called the centrosome. The centrosome helps to organize the spindle, a fanlike microtubule structure that helps separate the chromosomes.

43 Mitosis Chromatin condenses into chromosomes.Spindle forming Centromere Chromosomes (paired chromatids) Centriole Chromatin condenses into chromosomes. The centrioles separate and a spindle begins to form. The nuclear envelope breaks down.

44 Mitosis Metaphase Centriole Spindle Centriole Metaphase

45 Mitosis Metaphase The second phase of mitosis is metaphase.Centriole Spindle Metaphase The second phase of mitosis is metaphase. The chromosomes line up across the center of the cell. Microtubules connect the centromere of each chromosome to the poles of the spindle.

46 Individual chromosomesMitosis Anaphase Individual chromosomes Anaphase

47 Mitosis Anaphase Anaphase is the third phase of mitosis.Individual chromosomes Anaphase Anaphase is the third phase of mitosis. The sister chromatids separate into individual chromosomes. The chromosomes continue to move until they have separated into two groups.

48 Nuclear envelope reformingMitosis Telophase Nuclear envelope reforming Telophase

49 Mitosis Telophase Telophase is the fourth and final phase of mitosis.Chromosomes gather at opposite ends of the cell and lose their distinct shape. A new nuclear envelope forms around each cluster of chromosomes.

50 Cytokinesis Cytokinesis Cytokinesis

51 Cytokinesis During cytokinesis, the cytoplasm pinches in half.The area of the cell membrane that pinches in half is the cleavage furrow. Each daughter cell has an identical set of duplicate chromosomes.

52 Cytokinesis: Animals

53 Cytokinesis In plants, a structure known as the cell plate forms midway between the divided nuclei. Cell wall Cell plate

54 Cytokinesis The cell plate gradually develops into a separating membrane. A cell wall then begins to appear in the cell plate.

55 8-2 The series of events that cells go through as they grow and divide is called the cell cycle. mitosis. interphase. cytokinesis.

56 8-2 The phase of mitosis during which the chromosomes line up across the center of the cell is prophase. metaphase. anaphase. telophase.

57 8-2 Cytokinesis usually occurs at the same time as telophase.after telophase. during interphase. during anaphase.

58 8-2 DNA replication takes place during the S phase of the cell cycle.G1 phase of the cell cycle. G2 phase of the cell cycle. M phase of the cell cycle.

59 8-2 During mitosis, “sister” chromatids separate from one another during telophase. interphase. anaphase. metaphase.

60 Regulating the Cell Cycle

61 Controls on Cell DivisionExperiments show that normal cells will reproduce until they come into contact with other cells. When cells come into contact with other cells, they respond by not growing. This demonstrates that controls on cell growth and division can be turned on and off. What happens when you sustain an injury like a cut or a break?

62 Controls on Cell DivisionContact Inhibition Cells in a petri dish will continue to grow until they come into contact with other cells.

63 Cell Cycle Regulators Cell Cycle RegulatorsThe cell cycle is regulated by a specific protein. The amount of this protein in the cell rises and falls in time with the cell cycle. Scientists called this protein cyclin because it seemed to regulate the cell cycle. Cyclins regulate the timing of the cell cycle in eukaryotic cells. More recently, dozens of other proteins have been discovered that help to regulate the cell cycle.

64 Cell Cycle Regulators Internal Regulators Proteins that respond to events inside the cell are called internal regulators. Internal regulators allow the cell cycle to proceed only when certain processes have happened inside the cell. Ex) several regulatory proteins make sure a cell does not enter mitosis until all its chromosomes have been replicated.

67 Uncontrolled Cell GrowthCancer cells do not respond to the signals that regulate the growth of most cells. Cancer cells divide uncontrollably and form masses of cells called tumors that can damage the surrounding tissues. Cancer cells may break loose from tumors and spread throughout the body, disrupting normal activities and causing serious medical problems or even death. Caused by smoking tobacco, radiation exposure, and even viral infection. Many cancer cells have a defect in gene p53, which normally halts the cell cycle until all chromosomes have been replicated.

69 Tumors Types of tumors: (1) Benign Non-cancerous Usually don’t spreadUsually no threat Usually removed by surgery (if needed) Ex. Cysts, warts

70 Tumors (2) Malignant CancerousIf a person develops a malignant tumor, that person is said to have cancer (diagnosed w/ cancer) Invades and destroys healthy tissues of the body Metastasize (cells spread beyond their original site)

71 Biopsy A procedure done to check for cancerous cells

72 Stem Cells A cell whose job is not yet determinedAll other cells in the body “stem” from these cells They wait in the body until they get a signal to DIFFERENTIATE (gradually change into the desired cell type) They can become skin cells, RBC’s, nerve cells, skeletal muscle cells

73 What are stem cells & why are they important?Undifferentiated cells that renew themselves over long periods through cell division Under certain conditions, these cells will differentiate and become functioning body cells such as muscle or neurons A 3-5 day embryo (blastocyst) has a group of 30 or more cells that will become all the cells of the body (all cells stem from these) Scientists hope to use these cells to treat diseases