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Chapter5: The Cell Cycle and Mitosis The cell cycle and mitosis happens in the nucleolus of the cell (the dense section of nucleus) The Cell Cycle A. The Role of Cell Division Why do cells divide? • • • • Growth Reproduction Replacement of dying cells – skin, RBC Reproduction in multi-cellular organisms – gamete formation (meiosis) In the case of growth, why divide, rather than simply get bigger? • Surface:volume ratio constraints The Cell Cycle --> Interphase 1. G1 Phase 1st growth phase 2. S Phase DNA duplicated 3. G2 Phase Final growth phase Purpose of the first three phases (Interphase) – to duplicate cell contents; 90% of the cell’s growth cycle 4. Mitosis Purpose of Mitosis – to divide the genetic material into exact two halves 5. Cytokinesis Purpose of Cytokinesis – to divide all other contents (except nucleus) into two cells Interactive Cell cycle • http://www.cellsalive.com/cell_cycle.htm Create a concept map • Use pages 153-155 to create a concept map explaining the cell cycle. – Start by drawing/tracing the drawing on page 153. – Label each step with information about the step. – Draw the arrows that separate each step and use colour to colour in your cell cycle drawing. – 4 marks for each step’s information (5 steps) – 5 marks for colour and clarity. – Due Monday /25 Important Notes DNA wraps some special proteins to form more stable structure called chromosomes Chromosome are found inside nucleus Human - 46 chromosomes, 23 pairs (1 set of 23 from egg, 1 set of 23 from sperm) Gene is a segment of DNA that is responsible for controlling a trait Each chromosome contains thousands of genes (these make up our traits) Types of cell division • Mitosis (this chapter): – Growth, development & repair – Asexual reproduction (creates identical cells) – Occurs in body cells • Meiosis (next chapter): – Sexual reproduction (yields different cells) – Occurs in specific reproductive cells Process of mitosis Aim: Two identical daughter cells exact copy of mother cell formed Requirements – the whole DNA must be copied exactly Phases of Mitosis 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase 1. Prophase - Chromatin condenses, this causes the chromosomes to begin to become visible - Centrosomes separate, moving to opposite ends of the nucleus - The centrosomes start to form a framework used to separate the two sister chromatids called the mitotic spindle, that is made of microtubules - Nucleolus disappears 2. Metaphase - Chromosomes align on an axis called the metaphase plate - Note: the spindle consists of microtubules, one attached to each chromosome 3. Anaphase - Each centromere splits making two chromatids free - Each chromatid moves toward a pole - Cell begins to elongate, caused by microtubules not associated with the kinetochore 4. Telophase • Formation of nuclear membrane and nucleolus • Short and thick chromosomes begin to elongate to form long and thin chromatin • Formation of the cleavage furrow - a shallow groove in the cell near the old metaphase plate • Cytokinesis = division of the cytoplasm Mitosis Mitosis in an onion root Asexual Reproduction • Only one parent is required to make offspring • Offspring are genetically identical to parents • Genetically identical organisms are called Clones • Mother cells create identical daughter cells Binary fission in bacteria Not the best method of chromosome division – Cells Chromosomes replicate then plasma membrane grows inwards Creates two identical daugther cells Happens in Bacteria have a single chromosome (versus the 46 human have). Budding • Some cells split resulting in mother and daughter cells • Offspring is smaller than mother • The buds grow into fully matured individuals which eventually break away from the parent organism. Fragmentation • http://www.teachersdo main.org/resource/tdc 02.sci.life.repro.asexu al/ • In this form, the body of the parent breaks into distinct pieces, each of which can produce an offspring. Regeneration • In this form, if a piece of a parent is detached, it can grow back! Grafting • Used to propagate a desired variety of tree or shrub. • Involves taking a twig (scion) from a mature plant and inserting it into a cut stump (the stock). • Check it out Vegetative Reproduction • Method used by plants by sending a runner (underground stems) to reproduce. • A new plant will grow at end of each runner • Some plants use other underground stems called rhizoids, bulbs, or tubers to reproduce asexually. Spore Production • Some fungi and plant species release cells called spores • Spores reproduce new organisms via mitosis • Spores can be carried by water, wind or animals. • Video Asexual Reproduction Advantages Disadvantages • Large number of offspring produced quickly • Large colonies can form • Large numbers or organisms may survive • No energy is required to find a mate • Offspring are clones, no differences and if mutation occurs.. Uh oh! • Too many individuals may fight with each other to survive • Entire colonies can be wiped out if not adapted to environment Asexual Reproduction Questions • Use notes and handout to answer questions • Work in partners Cloning Assignment • Read pages 176-77 • What is the difference between reproductive (DNA) and theraputic cloning (stem cells)? • Do you have a problem with either? • Make notes for a discussion…. • Do you find them morally wrong? Reproductive Cloning Duplicate existing organism by removing nucleus of egg cell and fusing with mammary gland cells Theraputic Cloning Embryonic Stem cells grown into desired organs What is Sexual Reproduction? • Genetic information from two parents passed on to offspring • Both parents make special cells called gametes • Gametes contain only one set of chromosomes • (sperm and egg) • When two gametes come together (ask your parents) this is called fertilization and form a new cell called a Zygote • A zygote will mature and undergo mitosis to become an embryo. Diploid vs. Haploid • Diploid (Di = two) • Every cell in body except gametes (sperm/egg) • Contain 2n or two sets of chromosomes • 2 sets of 23 = 46 chromosomes • Haploid • Gametes (sperm and egg) • Contain n or one set of chromosome • 23 chromosomes How do cells become Haploid? • Meiosis…. Meiosis – key differences from mitosis • Meiosis reduces the number of chromosomes by half. • Daughter cells differ from parent, and each other. • Meiosis involves two divisions, Mitosis only one • End product is 4 haploid cells (1n) instead of 2 diploid cells (2n) • Occurs only in sex cells to produce gametes (sperm and egg) Methods of Genetic Variation Crossing Over: During Meiosis 1 nonsister chromatids will exchange genetic information. Independent Assortment Different combinations of chromatids can end up in each gamete (egg and sperm cells) Animation Meiosis 1 First division of meiosis • Prophase 1: Each chromosome duplicates and remains closely associated. These are called sister chromatids. Crossing-over can occur during the latter part of this stage. • Metaphase 1: Homologous chromosomes align at the equatorial plate. (Tetrads or two full chromosomes pair up) • Anaphase 1: Homologous pairs separate with sister chromatids remaining together. • Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair . Meiosis II Second division of meiosis: Gamete formation • Prophase 2: DNA does not replicate. • Metaphase 2: Chromosomes align at the equatorial plate. • Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole. • Telophase 2: Cell division is complete. Four haploid daughter cells are obtained. Mitosis vs. meiosis Meiosis vs Mitosis Mitosis Meoisis mitosis produces daughter cells identical to parent cell. No genetic variation Meiosis creates: genetic Variation 1 cycle 2 cycles 2 diploid cells created 4 haploid cells (gametes) result Both diploid cells are identical All four haploid cells are different from each other and different from parent cells All cells (except sex cells) Truly unique offspring will be created when male and female gametes meet (Sperm and egg) Only in sex cells (sperm and egg) Review….What is Meiosis? • Type of cell division that halves number of chromosomes (e.g., 2N to 1N) • Process of 2 successive divisions • Product is gamete, essential for sexual reproduction A comparison of mitosis and meiosis: summary Evolutionary advantage • asexual reproduction (mitosis) – easy, rapid, effective way to reproduce – useful in stable environment – lack of genetic diversity among offspring • sexual reproduction (meiosis) – promotes genetic variability – useful in changing environment Chromosome Mutations • Gene mutations may have no effect on organism because they are small differences in genetic information • When big chunks of DNA are lost, duplicated or moved within a chromosome or to an chromosome during Meiosis the result can be drastic • Chromosome mutations also occur from mutagens. • Fruit flies can grow legs where their attenae’s should be, eyes growing out of their wings etc… Where mutations occur? • Whole mutations can occur during Meiosis I when homologous chromosomes (tetrads) fail to separate or in Meiosis II when sister chromatids fail to separate. • One gamete will have two copies of chromosome while the other will have none. • Do you think these chromosomes are genetic? Think about the result in people when chromosomal mutations occur? Chromosome Mutations • Down Syndrome – Chromosome 21 does not separate correctly. – They have 47 chromosomes in stead of 46. – Children with Down Syndrome develop slower, may have heart and stomach illnesses and vary greatly in their degree of inteligence. Chromosome Mutations • Cri-du-chat – Deletion of material on 5th chromosome – Characterized by the cat-like cry made by cri-du-chat babies – Varied levels of metal handicaps Sex Chromosome Abnormalities • Klinefelter’s Syndrome – – – – – XXY, XXYY, XXXY Male Sterility Small testicles Breast enlargement Sex Chromosome Mutations • Turner’s Syndrome – X – Female – sex organs don't mature at adolescence – sterility – short stature Sex Chromosome Mutations • XXX – – – – – – Trisomy X Female Little or no visible differences tall stature learning disabilities limited fertility Karyotypes • Geneticists can prepare a picture of one’s chromosomes called a Karyotype • Take chromosomes during mitosis, cut and paste them and paired by size. • When analyzing, geneticists can determine mutations and treatment for genetic disorders or syndromes • Syndrome is a disease with a specific group of symptoms that occur together (down’s syndrome)