The Cell Cycle Chapter 9 BIOL 1000 Dr

The Cell Cycle Chapter 9 BIOL 1000 Dr. Mohamad H. Termos

Cell Structure Review • Eukaryotic cells: • Membrane bound nucleus containing DNA • Organelles 2

Cell Structure • Nucleus: contains the genetic information: DNA Chromatin- long and tangled threads of DNA Chromosomes – condensed chromatin; rod shaped Chromosome Number: • Diploid: Have two chromosomes of each type (2 n) The number for humans is n=23 Homologous chromosomes- each set • One set of 23 from individual’s father (paternal) • Other set of 23 from individual’s mother (maternal) 2 n=46 in each nucleus • Haploid: Have one chromosomes of each type (n) 3

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Diploid Chromosome Numbers of Some Eukaryotes 5

The Cell Cycle • Stages in a cell- from formation to division • Two major stages of the cell cycle: Interphase Most of the cell cycle Normal cell function and Prepare for division Mitotic stage Division of cell Why? • A cell divides for: Growth, repair, and development 6

The Cell Cycle • Interphase • Most of the cell cycle is spent in interphase • Cell performs its usual functions • Time spent in interphase varies by cell type • Nerve and muscle cells do not complete the cell cycle (remain in the G 0 stage) 7

The Cell Cycle • Interphase consists of: G 1, S, and G 2 phases –G 1 Phase: • Recovery from previous division • Cell doubles its organelles • Cell grows in size • Cell accumulates raw materials for DNA synthesis • G 0 Phase: some cells do not complete cell cycle and stay here 8

The Cell Cycle • S Phase: • DNA replication • Chromosomes enter with 1 chromatid each • Chromosomes leave with 2 identical chromatids (sister chromatids) each. 9

The Cell Cycle • G 2 Phase: • Between DNA replication and onset of mitosis • Cell synthesizes proteins necessary for division 10

Interphase DNA- genetic code Chromatin- tangled threads Chromosome- condensed Sister Chromatids- duplicated chromosomes Centromere- holds chromatids together Kinetochore- where spindle fibers attach to chromatids • Homologous chromosomes- sets of chromosomes (2 n) • • •

The Cell Cycle • M (Mitotic) Stage: Includes: • Mitosis • Nuclear division • Daughter chromosomes are distributed to two daughter nuclei • Cytokinesis • Division of the cytoplasm • Results in two genetically identical daughter cells 12

Mitosis and Cytokinesis • DNA is in very long threads. Stretched out and intertwined between divisions. DNA is associated with histones (proteins). DNA and histone proteins are collectively called chromatin • Before mitosis begins: –Chromatin condenses (coils) into distinctly visible chromosomes • At the end of S phase: • Each chromosome duplicated • Consists of two identical DNA chains • Sister chromatids (two strands of genetically identical chromosomes) • Attached together at a single point (called centromere) 13

Mitosis and Cytokinesis • During mitosis: • Centromeres holding sister chromatids together separate • Sister chromatids separate • Each becomes a daughter chromosome • Sisters of each type are distributed to opposite daughter nuclei 14

Mitosis and Cytokinesis • Just outside the nucleus is the centrosome. This is the microtubule organizing center, which organizes the mitotic spindle • Centrosome was also replicated in S-phase, so there are two centrosomes before mitosis begins 15

Phases of Mitosis • Prophase • Chromatin has condensed • Nuclear envelope disintegrates • Spindle begins to assemble • The two centrosomes move away from each other 16

Phases of Mitosis • Prometaphase • The centromere of each chromosome develops two kinetochores • Connect sister chromatids to spindle fibers on opposite poles of the mother cell 17

Phases of Mitosis • Metaphase • Chromosomes are pulled around by kinetochore fibers • Align across the equatorial plane of the cell 18

Phases of Mitosis • Anaphase • Centromere dissolves, releasing sister chromatids • Sister chromatids separate • Now called daughter chromosomes • Pulled to opposite poles 19

Phases of Mitosis • Telophase • Spindle disappears • Now two clusters of daughter chromosomes • Still two of each type with all types represented • Nuclear envelopes form around the two daughter nuclei • Each daughter nucleus receives one chromosome of each type 20

Cytokinesis • Cytokinesis = division of cytoplasm • Mother cell’s cytoplasm divided equally to daughter nucleus • Encloses each daughter cell in its own plasma membrane 21

Phases of Mitosis in Animal and Plant Cells Copyright © The Mc. Graw Hill Companies, Inc. Permission required for reproduction or display. centrosome has centrioles Animal Cell at Interphase aster duplicated chromosome 20 µm MITOSIS nuclear envelope fragments chromosomes at metaphase plate 9 µm centromere 20µm daughter chromosome 20µm kinetochore cleavage furrow 16µm nucleolus chromatin condenses nucleolus disappears centrosome spindle pole kinetochore spindle fibers forming Early Prophase Centrosomes have duplicated. Chromatin is condensing into chromosomes, and the nuclear envelope is fragmenting. Prophase Nucleolus has disappeared, and duplicated chromosomes are visible. Centrosomes begin moving apart, and spindle is in process of forming. polar spindle fiber Prophase Nucleolus has disappeared, and duplicated chromosomes are visible. Centrosomes begin moving apart, and spindle is in process of forming. kinetochore spindle fiber Metaphase Anaphase Centromeres of duplicated chromosomes Sister chromatids part and become daughter are aligned at the metaphase plate (center chromosomes that move toward the spindle of fully formed spindle). Kinetochore spindle poles. In this way, each pole receives the same fibers attached to the sister chromatids number and kinds of chromosomes as the parent cell. come from opposite spindle poles. Telophase Daughter cells are forming as nuclear envelopes and nucleoli reappear. Chromosomes will become indistinct chromatin. lacks centrioles Plant Cell at Interphase 25µm cell wall chromosomes 6. 2µm spindle pole lacks centrioles and aster 20µm spindle fibers 6. 2µm cell plate 6. 6µm Animal cell(Early prophase, Prophase, Metaphase, Anaphase, Telophase): © Ed Reschke; Animal cell(Prometaphase): © Michael Abbey/Photo Researchers, Inc. ; Plant cell(Early prophase, Prometaphse): © Ed Reschke; Plant cell(Prophase, Metaphase, Anaphase): © R. Calentine/Visuals Unlimited; Plant cell(Telophase): © Jack M. Bostrack/Visuals Unlimited; 22

Functions of Mitosis Growth and repair In mammals, mitosis is necessary when: • • • A fertilized egg becomes an embryo An embryo becomes a fetus A cut heals or a broken bone mends 23

The Cell Cycle • The cell cycle is controlled by signals • A signal is a molecule that either stimulates or inhibits a metabolic event. • Internal signals • Family of proteins called cyclins that increase and decrease as the cell cycle continues • Without cyclins, the cell cycle stops at G 1, M or G 2 (checkpoints) • Allows time for any damage to be repaired 24

The Cell Cycle • Apoptosis is programmed cell death • It involves a sequence of cellular events • Apoptosis is caused by enzymes called caspases • Mitosis and apoptosis are opposing forces. • Cells harbor caspases that are kept in check by inhibitors (Can be unleashed by internal or external signals). • Signal protein • Stops the cell cycle at G 1 when DNA is damaged • Initiates an attempt at DNA repair (If successful, the cycle continues to mitosis, if not, apoptosis is initiated). 25

Phases of Mitosis in Animal and Plant Cells Stem Cells • Many mammalian organs contain stem cells • • • Retain the ability to divide Red bone marrow stem cells divide to produce various types of blood cells Therapeutic cloning to produce human tissues can begin with either adult stem cells or embryonic stem cells Embryonic stem cells can be used for reproductive cloning, the production of a new individual 26

Reproductive and Therapeutic Cloning Copyright © The Mc. Graw Hill Companies, Inc. Permission required for reproduction or display. remove and discard egg nucleus egg remove Go nucleus fuse egg with Go nucleus Go cells from animal to be cloned a. Reproductive cloning culture remove and discard egg nucleus egg embryonic stem cells Implant embryo into surrogate mother Clone is born nervous remove Go nucleus Go somatic cells fuse egg with Go nucleus blood culture embryonic stem cells muscle b. Therapeutic cloning 27

The Cell Cycle and Cancer • Abnormal growth of cells is called a tumor • Benign tumors are not cancerous • Encapsulated • Do not invade neighboring tissue or spread • Malignant tumors are cancerous • • Not encapsulated Readily invade neighboring tissues May also detach and lodge in distant places (metastasis) Results from mutation of genes regulating the cell cycle • Development of cancer • Tends to be gradual • May take years before a cell is obviously cancerous 28

The Cell Cycle and Cancer • Characteristics of Cancer Cells • • Lack differentiation Have abnormal nuclei Do not undergo apoptosis Form tumors • Cancer cells have lost contact inhibition • Undergo metastasis • Original tumor easily fragments • New tumors appear in other organs • Undergo angiogenesis • Formation of new blood vessels 29

Cancer Cells vs. Normal Cells 30

The Cell Cycle and Cancer • Origin of Cancer • Oncogenes • Proto-oncogenes promote the cell cycle in various ways • If a proto-oncogene is mutated, it may become an oncogene • Tumor suppressor genes inhibit the cell cycle in various ways • If a tumor suppressor gene becomes inactive, it may promote cancer development 31

Prokaryotic Cell Division • The prokaryotic chromosome is a ring of DNA • Folded up in an area called the nucleoid • Binary fission • Splitting in two • Two replicate chromosomes are distributed to two daughter cells • Produces two daughter cells identical to original cell – asexual reproduction 32

Binary Fission Copyright © The Mc. Graw Hill Companies, Inc. Permission required for reproduction or display. chromosome 1. Attachment of chromosome to a special plasma membrane site indicates that this bacterium is about to divide. cell wall plasma membrane cytoplasm 2. The cell is preparing for binary fission by enlarging its cell wall, plasma membrane, and overall volume. 3. DNA replication has produced two identical chromosomes. Cell wall and plasma membrane begin to grow inward. 4. As the cell elongates, the chromosomes are pulled apart. Cytoplasm is being distributed evenly. 5. New cell wall and plasma membrane has divided the daughter cells. © Dennis Kunkel Microscopy, Inc. /Visuals Unlimited SEM 2, 345 X 33

Functions of Cell Division 34