Cell Division Chapter 8

Cell Division Doubling organelles and proteins DNA replication Nuclear division Cytoplasmic division

Cell Increase and Decrease Maintain homeostasis Cell numbers kept in check by this mechanism  Through cell division of somatic cells and cell death Cell division – interphase, mitosis and cytokinesis Cell death – apoptosis

Cell Increase and Decrease Somatic cells  Asexual reproduction increase in number of somatic cells Increase in number unicellular organisms Germ cells  Sexual reproduction requires the production of eggs and sperm

Important terms: DNA Chromosomes Chromatin Chromatid

DNA Replication Duplicated chromosome  Composed of 2 sister chromatids  held together by a centromere Sister chromatids  Genetically identical  When separate, each daughter nucleus gets a chromosome DNA copied DNA divided

Mitosis verse Meiosis?? Mitosis  Division of somatic cell nuclei Meiosis  Division of gamete nuclei

Cell Cycle – Somatic cells Set of stages that involves cell growth and nuclear division Consists of:  Interphase G 1 S G 2  Meiotic stage Mitosis and Cytokinesis

Interphase When the cell carries on its usual functions Main stages:  G1 Gap before DNA synthesis begins  S Time when DNA duplicated***************  G2 Gap between time DNA duplication ends and mitosis begins

Interphase in Meiosis? Meiosis is a "one- way" process  Cannot be said to engage in a cell cycle as mitosis does Preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle

Cytokinesis in Animal Cells Cytoplasmic cleavage Accompanies mitosis  Separate process Cleavage furrow forms between daughter nuclei Contractile ring contracts deepening the furrow Continues until separation is complete

Human DNA in somatic cells pairs of homologous chromosomes  Difference?  Autosomes (1-22)  Sex chromosomes (23) Somatic cells have 46 chromosomes  Diploid  2n

Human DNA in gametes Due to reductional division Halves the diploid number (2n) to a haploid number (n) 23 total chromosomes

Division of the Nucleus Nucleus must be divided  Parent cell’s DNA into 2 nuclei 2 ways nucleus can divide:  Mitosis  Meiosis

Mitosis Maintaining the Chromosome Number

Mitosis  4 main stages: Prophase Metaphase Anaphase Telophase

Mitosis 1. Prophase – Mitosis begins!  Threadlike form  Spindle fibers appear  DNA start to condense  Aster formed  Nuclear envelope starts to break apart  Centrioles move to opposite sides of the cell

Mitosis 2. Metaphase  Duplicated chromosomes aligned midway between the poles  Associated with spindle fibers

Mitosis 3. Anaphase  Sister chromatids separate from each other and move to opposite poles Become daughter chromosomes

Mitosis 4. Telophase  Return to threadlike form as in prophase  New nuclear envelope separates each chromosome cluster  2 new nuclei!!!!!

Meiosis Reducing the Chromosome Number

Meiosis Mechanism for dividing the nucleus of germ cells Oogonia and spermatogonia 2n Meiosis must take place prior to formation of gametes Sperm and eggs n First stage in sexual reproduction

Meiosis Overview Occurs in the life cycle of sexually reproducing organisms Reduces the chromosome number 2 divisions, 4 daughter cells Cells are diploid at beginning of meiosis TWO consecutive divisions  Result is 4 haploid nuclei Divided into:  Meiosis I  Meiosis II

Reducing the Chromosome Number Genetic Recombination  Promotes genetic variability  Occurs in Two Ways Crossing Over Independent Assortment of paired chromosomes

Meiosis comparison Meiosis I Crossing over Homologous chromosomes line up Homologous chromosomes split Two haploid cells formed Cytokinesis occurs Meiosis II Essentially the same as Meiosis I  Starts with a haploid cell that has NOT undergone chromosome duplication

Comparisons between males and females Spermatogenesis  Begins at puberty and continues throughout life Oogenesis  Begins in the fetus  Primary oocytes are arrested in prophase I  At puberty, one primary oocyte continues the process of meiosis during each menstrual cycle

Comparisons Mitosis  DNA replication occurs only once  Requires only one division  Produces two daughter cells  Diploid daughter cells 2n  Genetically identical cells produced  Occurs all the time Meiosis  DNA replication occurs only once  Requires two divisions  Produces four daughter cells  Haploid daughter cells n  Genetically variable cells produced  Occurs only at certain times

Comparisons

Overview of the Life Cycle of Humans

Cell division in other organisms

How Plant Cells Divide Occurs in meristematic tissues Same phases as animal cells Plant cells do not have centrioles or asters

Plant Cells

Cytokinesis in Plant Cells Flattened, small disk appears between daughter cells Golgi apparatus produces vesicles which move to disk Release molecules which build new cell walls Vesicle membranes complete plasma membranes

Cell Division in Prokaryotes Binary Fission  Prokaryotes have a single chromosome  Chromosomal replication occurs before division  Cell elongates to twice its length  Cell membrane grows inward until division is complete

Mitosis and Meiosis Gone Awry Cancer Chromosomal Abnormalities

Cancer disease of the cell cycle Cancer cells  do not respond normally to the cell cycle control system  can form tumors if malignant, can spread to other parts of the body

Important Terms Tumor Benign Malignant Metastasis Cancer Carcinoma Sarcoma Lymphoma

Cancer Treatment Cancer treatment can involve  Radiation therapy which damages DNA and disrupts cell division  Chemotherapy which uses drugs that disrupt cell division

Chromosomal Abnormalities Nondisjunction  members of a chromosome pair fail to separate during anaphase  produces gametes with an incorrect number of chromosomes If normal sperm fertilizes an egg with an extra chromosome  result is a zygote with a total of 2n + 1 chromosomes

Chromosomal Abnormalities Down Syndrome:  Trisomy 21  condition in which an individual has an extra chromosome 21  Affects about one out of every 700 children

Chromosomal Abnormalities Nondisjunction can also affect the sex chromosomes  Klinefelter's Syndrome XXY males  Turner Syndrome XO female