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The Cell Cycle and Cellular Reproduction Chapter 9
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Outline DNA and Chromosomes Cell Division in Prokaryotes
Cell Division in Eukaryotes The Cell Cycle Interphase Mitosis Cytokinesis Cell Cycle Control Apoptosis The Cell Cycle and Cancer Characteristics of Cancer Causes of Cancer
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DNA and Chromosomes DNA serves as information storage molecules.
DNA is made up of nucleotides Each nucleotide has a sugar, a phosphate group and a nitrogenous (nitrogen-containing) base The bases are of two main types Purines – Large bases Adenine (A) and Guanine (G) Pyrimidines – Small bases Cytosine (C) and Thymine (T)
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The four nucleotide subunits that make up DNA.
Nitrogenous base 5-C sugar
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The two possible basepairs
DNA Double Helix The two possible basepairs
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Chromosomes Chromosomes are composed of DNA (40%) and protein (~ 60%).
Chromatin-tangled mass of threadlike DNA and protein in nondividing cell (seen primarily during interphase). Chromosomes-condensed rod-shaped DNA and protein molecules during cell division.
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Chromosomes A typical human chromosome contains about 140 million nucleotides in its DNA. In the cell, the DNA is coiled. The DNA helix is wrapped around positively-charged proteins, called histones. Eukaryotic chromosomes are linear. Prokaryotic chromosomes are circular.
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Cell Cycle Set of events that occur between one cell division and the next. In prokaryotes and unicellular eukaryotes, cell division is a form of asexual reproduction. In multicellular eukaryotes, cell division is important for growth, development, and repair. Plants retain the ability to divide throughout their life span In mammals, cell division is necessary: Fertilized egg becomes an embryo Embryo becomes a fetus Allows a cut to heal or a broken bone to mend
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Function of Mitosis Permits growth and repair.
In plants it retains the ability to divide throughout the life of the plant In mammals, mitosis is necessary: Fertilized egg becomes an embryo Embryo becomes a fetus Allows a cut to heal or a broken bone to mend 9
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Functions of Cell Division
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Prokaryotes Have a Simple Cell Cycle
The DNA of prokaryotes is a single circular chromosome. Cell division in prokaryotes takes place in two stages. The DNA is replicated. The cell elongates, then splits into two identical daughter cells. The process is called binary fission (asexual reproduction).
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Binary Fission of Prokaryotes
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Cell division in eukaryotes is more complex than in prokaryotes because
1. Eukaryotic DNA is packaged differently. It is in linear chromosomes compacted with proteins. 2. Eukaryotes contain far more DNA.
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Levels of Eukaryotic Chromosome Organization
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Chromosome Number in Eukaryotes
The number of chromosomes varies from species to species. Diploids have two chromosomes of each type. Humans have 23 different types of chromosomes Each type is represented twice in each body (somatic) cell- diploid. Only sperm and eggs have one of each type- haploid. The n number for humans is n=23 Two representatives of each type Makes a total of 2n=46 in each nucleus of a body cell. One set of 23 from individual’s father (paternal). Other set of 23 from individual’s mother (maternal).
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Chromosome Numbers of Some Eukaryotes
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Chromosomes Humans have 46 chromosomes.
The 23 pairs of chromosomes can be organized by size. This display is termed a karyotype.
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Eukaryotic Cell Cycle The major stages of the eukaryotic cell cycle:
Interphase Mitosis Cytokinesis Interphase and mitosis each have several substages.
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The Cell Cycle
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Interphase Longest phase of the cell cycle.
Normal cell functions occur as well as preparation for cell division. G1 phase: Cell doubles its organelles Accumulates materials needed for DNA synthesis S phase: DNA replication (synthesis) Chromosomes enter with 1 chromatid each Chromosomes leave with 2 identical chromatids each G2 phase: Between DNA replication and onset of mitosis (or meiosis) Cell synthesizes proteins necessary for division Such as microtubules for the mitotic spindle
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Mitosis (M) Stage Also called karyokinesis Nuclear division Occurs in somatic (non-reproductive) cells in animals Occurs in meristematic tissues in plants Daughter chromosomes divided between two daughter nuclei Four Phases (PMAT) Prophase Metaphase Anaphase Telophase
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Cytokinesis (C) Stage Cytoplasmic division
Results in two genetically identical diploid daughter cells. In animal, structure that forms to separate two cells is called a cleavage furrow. In plant cells, structure that forms to separate two cells is called a cell plate.
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At end of S phase of interphase:
Each chromosome is duplicated. Consists of two identical DNA double helix chains Called sister chromatids Attached together at a single point called a centromere During mitosis: Centromeres holding sister chromatids together split Sister chromatids separate Each becomes a daughter chromosome Sisters of each type distributed to opposite daughter nuclei
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Duplicated Chromosome
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Mitosis in Animal Cells
Outside nucleus is the centrosome The centrosome was replicated in S-phase. Now two centrosomes This is the microtubule organizing center. Organizes the mitotic spindle Each composed of a bundle of microtubules In animals, contains two barrel-shaped centrioles. Oriented at right angles to each other within centrosome
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Prophase Nuclear envelope breaks down Nucleolus disappears
Chromatin condenses Chromosomes distinguishable with microscope Visible double (two sister chromatids attached at centromere) Spindle apparatus forms Two centrosomes move away from each other Form microtubules in star-like arrays called asters
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Prometaphase Centromere of each chromosome develops two kinetochores
On opposite sides of the centromeres One over each sister chromatid Spindle fibers attach at the kinetochores Physically hook sister chromatids up with spindle apparatus. Connect sister chromatids to opposite poles of mother cell.
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Metaphase Chromosomes align along the equatorial plane of cell
Metaphase plate Represents plane through which mother cell will be divided Chromosomes are pulled around by kinetochore fibers
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Anaphase Centromere splits Sister chromatids separate
Now called daughter chromosomes Pulled to opposite poles by shortening of the microtubules attached to them Cytokinesis begins in anaphase.
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Telophase Spindle apparatus disappears
Now two clusters of daughter chromosomes Still two of each type with all types represented Nuclear envelopes form around the two separate cluster of daughter chromosomes Chromosomes uncoil and become chromatin again Nucleolus reappears in each daughter nucleus
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Mitosis in Animal Cells
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Overview of Mitosis Nuclear division in which chromosome number stays constant. DNA replication produces duplicated chromosomes. Each duplicated chromosome is composed of 2 sister chromatids held together by a centromere. Sister chromatids are genetically identical. During mitosis, the centromere divides and each chromatid becomes a daughter chromosome.
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Mitotic Phases Summarized
Prophase-nuclear membrane disappears, centrosomes migrate, spindle fibers appear Prometaphase- the kinetochore of each chromatid is attached to a spindle fiber. Metaphase-chromosomes line up at equator, associated with spindle fibers Anaphase-centromeres divide, sister chromatids migrate to opposite poles, cytokinesis begins Telophase-nuclear membranes form, spindle disappears, cytokinesis occurs
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Cytokinesis: Animal Cells
Division of cytoplasm Divides mother cell’s cytoplasm equally to daughter nuclei Encloses each in it’s own plasma membrane Often begins in anaphase Animal cytokinesis: A cleavage furrow appears between daughter nuclei Formed by a contractile ring of actin filaments Eventually pinches mother cell in two
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Cytokinesis in Animal Cells
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Cytokinesis: Plant Cells
Rigid cell walls outside plasma membrane do not permit furrowing Forms a cell plate Many small membrane-bounded vesicles Eventually fuse into one thin vesicle extending across the mother cell The membranes of the cell plate become the plasma membrane between the daughter cells
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Cytokinesis in Plant Cells
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Special Characteristics of Mitosis in Plants
Same phases as animal cells Plant cells do not have centrioles or asters Cytokinesis via cell plate
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Mitosis in Plant Cells
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Cell cycle controlled by internal and external signals
Growth factors Received at the plasma membrane Cause completion of cell cycle Internal signals Family of proteins called cyclins Must be present for stages to progress. Increase and decrease as cell cycle continues Without them cycle stops at G1, M or G2 Allows time for any damage to be repaired
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The Cell Cycle Some cells do not go through the cell cycle. These cells are in the G0 stage-extended rest period; cell are still metabolically active; for example, some muscle cells and neurons.
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G1 checkpoint Cell growth is assessed at G1 checkpoint Protein p53 stops cycle if DNA damaged G2 checkpoint DNA replication is assessed at G2 checkpoint Stops cycle if DNA is not done replicating or is damaged M checkpoint Spindle assembly checkpoint Stops if chromosomes not aligned properly
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Cell Increase and Decrease
Increase and decrease of cell numbers Cell division increases number of somatic cells Mitosis-division of nucleus of cell Cytokinesis-division of cytoplasm Occurs throughout life; important for growth, development, and repair in multicellular organisms Apoptosis- programmed cell death decreases cell number Occurs throughout life also Prevents abnormal cells from proliferating Signal protein P53 Stops cycle at G1 when DNA damaged Initiates DNA attempt at repair If successful, cycle continues to mitosis If not, apoptosis is initiated
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Apoptosis Progressive series of events resulting in cell destruction.
Cells rounds up, and lose contact with surrounding cells. Nucleus breaks up and DNA undergoes fragmentation. Mediated by enzymes called caspases.
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Apoptosis
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Fingers and toes form from these paddlelike hands and feet
During fetal development, many cells are programmed to die Programmed cell death Human cells appear to be programmed to undergo only so many cell divisions About 50 in cell cultures Only cancer cells can divide endlessly
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The Cell Cycle and Cancer
Cancer is unrestrained cell growth and division The result is a cluster of cells termed a tumor. Benign tumors Encapsulated and noninvasive Malignant tumors Not encapsulated and invasive Can undergo metastasis Leave the tumor and spread throughout the body Invade other tissues and form new tumors
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Progression of Cancer Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. New mutations arise, and one cell (brown) has the ability to start a tumor. primary tumor lymphatic vessel blood vessel Cancer in situ. The tumor is at its place of origin. One cell (purple) mutates further. lymphatic vessel blood vessel Cancer cells now have the ability to invade lymphatic and blood vessels and travel throughout the body. New metastatic tumors are found some distance from the primary tumor. 48
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Characteristics of Cancer Cells
Lack differentiation Have abnormal nuclei Form tumors Mitosis controlled by contact with neighboring cells – contact inhibition 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
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Carcinogenesis – development of cancer
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Causes of Cancer Most cancers result from mutations in growth-regulating genes. There are two general classes of these genes. 1. Proto-oncogenes Encode proteins that stimulate cell division If mutated, they become oncogenes. 2. Tumor-suppressor genes Encode proteins that inhibit cell division Retinoblastoma (Rb) protein acts during G1 checkpoint to inhibit cell division p53 protein
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Tumor-suppressor Genes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Proto-oncogenes Growth factor receptor: more per cell in many breast cancers. Ras protein Src kinase Cytoplasm Ras protein: activated by mutations in 20–30% of all cancers. Rb protein p53 protein Tumor-suppressor Genes Cell cycle checkpoints Rb protein: mutation in 40% of all cancers. p53 protein: mutated in 50% of all cancers. Mammalian cell
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Growth Factor Receptor Her2 and Cancer
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) In a normal cell there are few Her2 membrane receptors so cell division is controlled. Growth factor Her2 membrane receptor Growth Factor Receptor Her2 and Cancer Her2 gene on DNA (b) Some cancer cells have excess Her2 receptors. These cells divide at an increased rate when growth factors are present.
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Herceptin: Breast Cancer Treatment
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Retinoblastoma Protein
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p53 and Control of the Cell Cycle
The p53 gene plays a key role in the G1 checkpoint of cell division p53 is also called the guardian angel gene of the cell The p53 protein (the gene’s product), monitors the integrity of DNA If DNA is damaged, the protein halts cell division and stimulates repair enzymes. If the p53 gene is mutated Cancerous cells repeatedly divide even if the DNA is damaged. No stopping at the G1 checkpoint.
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Telomerase Chromosomes have caps on their ends called telomeres.
Every time the cell divides part of the cap breaks off. Eventually the cap becomes so small that the chromosome cannot be copied; limits the life span of the cell (~50 cell divisions). Telomerase is the enzyme that can repair the ends of chromosomes. In normal cells telomerase is inhibited so the cell will eventually stop dividing. In cancer cells telomerase is not inhibited, so the telomeres don’t shorten; no limit on the life span of the cell.
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Causes of Cancer Chemicals Radiation Viruses
Mutations in specific genes on chromosomes; for example a mutant p53 gene Chromosomal mutations- extra chromosomes or missing parts of chromosomes Telomerase
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