Cell division is essential but must be controlled Topic 1.6 IB Biology Miss Werba

ULTRASTRUCTURE OF CELLS TOPIC 1 – CELL BIOLOGY 1.1 INTRODUCTION TO CELLS 1.2 ULTRASTRUCTURE OF CELLS 1.3 MEMBRANE STRUCTURE 1.4 MEMBRANE TRANSPORT 1.5 THE ORIGIN OF CELLS 1.6 CELL DIVISION J WERBA – IB BIOLOGY 2

THINGS TO COVER U.1 U.2 U.3 U.4 U.5 U.6 A.1 S.1 S.2 IM.1 Statement Guidance U.1 Mitotic division produces 2 identical nuclei Know the sequence of events in the 4 phases of mitosis. U.2 Supercoiling of chromosomes Refer to two parts of a chromosome as sister chromatids during early phases of mitosis, and as chromosomes once they have separated during anaphase. U.3 Cytokinesis U.4 Interphase is an active period in the cell cycle U.5 Cyclins U.6 Tumour formation A.1 Correlation between smoking and cancer S.1 Identification of phases in mitosis in cells S.2 Determination of a mitotic index from a micrograph IM.1 Biologists worldwide are researching the causes and treatments of cancer NOS 1.4 Serendipity and scientific discoveries J WERBA – IB BIOLOGY 3

CELL DIVISION U.6 J WERBA – IB BIOLOGY Source: www.nature.com 4

CELL DIVISION All cells come from pre-existing cells U.6 All cells come from pre-existing cells Cells undergo division in order for an organism to grow, reproduce and repair tissue damage Cell division must be tightly controlled by the mechanisms of the cell cycle Tumours form from uncontrolled cell division J WERBA – IB BIOLOGY 5

MITOSIS Growth Asexual reproduction Tissue repair and Mitosis occurs whenever new cells are required. Remember: GATE Mitosis is used for: Growth Asexual reproduction Tissue repair and Embryonic development J WERBA – IB BIOLOGY 6

CELL CYCLE U.1 The cell cycle is a series of stages through which all cells must pass in order to divide. J WERBA – IB BIOLOGY 7

CELL CYCLE Two main stages: Interphase Mitosis (M) phase U.1 Two main stages: Interphase Includes G1, S and G2 stages Mitosis (M) phase Includes stages of mitosis J WERBA – IB BIOLOGY 8

CELL CYCLE U.1 U.3 M phase: The periods of nuclear division (mitosis) and cytoplasmic division (cytokinesis) The division of the nucleus creates two genetically identical daughter nuclei J WERBA – IB BIOLOGY 9

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CELL CYCLE U.1 The length of the cell cycle depends on the particular cell. For example: bacterial cells can divide every 20-30 minutes under suitable conditions skin cells divide every 12 hours on average liver cells every 2 years muscle cells never divide at all after maturing, so remain in the growth phase for decades J WERBA – IB BIOLOGY 11

CYCLINS U.5 The progression though the cell cycle depends on the presence of a family of proteins called cyclins. J WERBA – IB BIOLOGY 12

CYCLINS U.5 Cells can’t progress to the next phase of the cell cycle until a specific cyclin reaches a critical concentration These cyclins trigger a pathway of reactions that ultimately controls a specific task in the cell cycle J WERBA – IB BIOLOGY 13

SERENDIPITY CAN LEAD TO SCIENTIFIC DISCOVERY NOS 1.4 Serendipity and scientific discoveries The discovery of cyclin was accidental. Tim Hunt was researching protein synthesis and fertilisation in spawning sea urchin eggs when he discovered cyclin in 1982. J WERBA – IB BIOLOGY 14

SERENDIPITY & CYCLIN NOS 1.4 U.5 http://www.sciencedirect. com/science/article/pii/ S009286740800888X http://www.nobelprize. org/nobel_prizes/ medicine/laureates/2001/ presentation-speech.html J WERBA – IB BIOLOGY 15

INTERPHASE Before mitosis, interphase occurs. Interphase contains growth checkpoints to ensure division is viable: G1: the cell grows, duplicates organelles, synthesises proteins and produces ATP S: DNA is replicated G2: the copied DNA is checked for fidelity (mutations) and final metabolic reactions occur J WERBA – IB BIOLOGY 16

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INTERPHASE Protein synthesis (transcription/translation) U.4 Interphase is an active period in the life of a cell Remember: PAID Protein synthesis (transcription/translation) Aerobic respiration (ATP formation) Increase organelles (mitochondria/chloroplast) DNA replication (in S phase) J WERBA – IB BIOLOGY 18

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MITOSIS U.1 S.1 J WERBA – IB BIOLOGY 20

MITOSIS - Prophase DNA supercoils (chromatin → chromosome) Nuclear membrane dissolves Centrosomes move to opposite poles Centrioles start making spindle fibres centrosome centriole J WERBA – IB BIOLOGY 21

SUPERCOILING Chromosomes within the nucleus condense by supercoiling Human cells are ~10μm in diameter Human chromosomes are ~15,000-18,000μm in length Supercoiling allows the chromosomes to be separated efficiently during division centrosome centriole J WERBA – IB BIOLOGY 22

SUPERCOILING U.2 J WERBA – IB BIOLOGY 23 Source: http://www.genome.gov/dmd/img.cfm?node=Photos/Graphics&id=85282

MITOSIS - Metaphase U.1 Spindle fibres connect to centromere of each chromosome Chromosomes line up separately along equator J WERBA – IB BIOLOGY 24

MITOSIS - Anaphase Spindle fibres contract U.2 Spindle fibres contract Causes sister chromatid chromosomes to split into genetically identical single chromatid chromosomes Chromosomes move to opposite poles J WERBA – IB BIOLOGY 25

MITOSIS - Telophase + Cytokinesis U.1 U.3 DNA decondenses (chromosome → chromatin) Spindle fibres dissolve Nuclear membranes reform around chromatin The division of the cell into two daughter cells (cytokinesis) occurs concurrently with telophase J WERBA – IB BIOLOGY 26

CYTOKINESIS U.3 Cytokinesis is the division of the cytoplasm, and therefore the cell itself. Cytokinesis is very different in plant and animal cells. This is due to the presence of the cell wall. J WERBA – IB BIOLOGY 27

CYTOKINESIS U.3 J WERBA – IB BIOLOGY 28

CYTOKINESIS Animal cells: U.3 Animal cells: Contractile proteins (microfilaments) inside the plasma membrane pull the membrane in at the equator This produces a cleavage furrow. When this furrow reaches the centre of the cells, they are able to separate. J WERBA – IB BIOLOGY 29

CYTOKINESIS Plant cells: U.3 Plant cells: Cell wall components are transported in vesicles to the centre of the cell during telophase. This forms the cell plate, which eventually connects with the existing cell’s membrane. The components of the vesicles are released into the space between the cells, sticking them together. Both daughter cells secrete cellulose to build their walls. J WERBA – IB BIOLOGY 30

MITOSIS - SKILLS S.1 S.2 You are required to identify the phases of mitosis in cells viewed: with a microscope in a micrograph You are also required to determine the mitotic index from a micrograph: J WERBA – IB BIOLOGY 31

MITOSIS - SKILLS S.1 S.2 Online activity: http://www.biology.arizona.edu/cell_bio/activities/cell_ cycle/cell_cycle.html Follow the instructions and complete the table provided. Practical activity: Calculate the mitotic index of fixed onion root tips viewed at 1000x magnification under a light microscope. J WERBA – IB BIOLOGY 32

TUMOURS U.6 Tumours (cancers) are the result of uncontrolled cell division. These can occur in any organ or tissue. The cell cycle is controlled by a chemical control system proteins from tumour suppressor genes inhibit cell division proteins from proto-oncogenes promote cell division Mutations to these genes result in uncontrolled cell division, resulting in the formation of a tumour J WERBA – IB BIOLOGY 33

TUMOURS Mutagens: agents that cause gene mutations not all mutations cause cancer, but they all have the potential to cause cancer There are different types of mutagens: chemical mutagens (carcinogens) – eg. tobacco linked to lung cancer incidence radiation – eg. UV radiation linked to skin cancer incidence viruses – eg. HPV linked to cervical cancer incidence J WERBA – IB BIOLOGY 34

TUMOURS Oncogenes: genes that control the cell cycle and cell division mutations in these genes result in uncontrolled cell division this leads to tumour formation eg. Activated oncogenes could cause cells destined for programmed cell death (apoptosis) to survive and proliferate J WERBA – IB BIOLOGY 35

TUMOURS Primary tumours: result from several mutations in the same cell (very low probability) they are named after the site where the abnormal growth first occurred Secondary tumours: result from cancerous cells invading other tissues Metastasis: the name given to the movement of cells from the primary tumour J WERBA – IB BIOLOGY 36

SCIENTIFIC COLLABORATION IM.1 Biologists in laboratories throughout the world are researching into the causes and treatments of cancer. For example, the correlation between smoking and the incidence of lung cancer. J WERBA – IB BIOLOGY 37

SMOKING & CANCER A.1 Smoking is linked to a high incidence of lung cancer, as well as higher than average incidences of mouth and throat cancers. Other cancers are also more common in smokers: eg. breast cancer colon cancer pancreatic cancer J WERBA – IB BIOLOGY 38

Source: http://intro.bio.umb.edu/111F98Lect/consTiming.gif SMOKING & CANCER A.1 Source: http://intro.bio.umb.edu/111F98Lect/consTiming.gif J WERBA – IB BIOLOGY 39

SMOKING & CANCER Discuss the relationship shown here. What type of correlation is shown? Is it a strong correlation? J WERBA – IB BIOLOGY 40

SMOKING & CANCER Remember that correlation is not causation! Just because there is a lot of evidence, one can never be 100% certain that smoking was the sole cause of the cancer observed. There is a strong positive correlation seen in first few decades. As one increases, so does the other. However, the correlation is lagged. There is an ~20yr time gap between the factors here. J WERBA – IB BIOLOGY 41

SMOKING & CANCER Lung cancer rates in males are actually decreasing. They are increasing in females. Why??? A high percentage of the women developing lung cancer are life-long NON-SMOKERS! Links to chemical exposure in the home and oestrogen treatments J WERBA – IB BIOLOGY 42

CELL DIVISION Q1. The diagram below shows a cell during mitosis. What are the structures (I and II) and stage of mitosis? J WERBA – IB BIOLOGY 43

CELL DIVISION Q2. Cyclins were discovered by Timothy R Hunt in 1982 while studying sea urchins. What is a function of cyclins? Circulation of seawater for gas exchange Rotation of tentacles Control of the cell cycle Recycling of nutrients J WERBA – IB BIOLOGY 44

CELL DIVISION Q3. Explain how mitosis produces two genetically identical nuclei. [8] J WERBA – IB BIOLOGY 45