University of Balamand BioL 205 - Principles of Human Biology - Cell Division PDF

University of Balamand Faculty of Health Sciences BIOL 205– Principles of Human Biology CHAPTER 4 – CELL DIVISION Dr. Espérance Debs  Cells 2  Cells are the basic units of life  Pre-existing cells divide to form new cells  This proc...

University of Balamand Faculty of Health Sciences BIOL 205– Principles of Human Biology CHAPTER 4 – CELL DIVISION Dr. Espérance Debs  Cells 2  Cells are the basic units of life  Pre-existing cells divide to form new cells  This process enables an organism to grow, repair damaged parts and reproduce  Cells are the essential link between generations  The simplest cell contains a massive amount of genetic information called the organism’s genome  Genomes are organized in informational units called the genes  Genes control the activities of the cell and are passed on to its descendants Cell division  Eukaryotic chromosomes 3  Chromosomes means “colored body” but chromosomes are virtually colorless  Chromosomes are made of chromatin, a material consisting of DNA and associated proteins  When a cell is not dividing the chromosome is present but in an extended partially unraveled form  During cell division, chromatin fibers condense and the chromosomes become visible as distinct structure Cell division  DNA organization 4  An organism’s genome may contain hundreds or thousands of genes  DNA is packaged in a highly organized way  The total length of DNA is 1000 times longer than the cell itself  The DNA molecule with the help of proteins is twisted and folded compactly to fit inside the cell Cell division  DNA organization 5  Histones are positively charged whereas DNA is negatively charged  They will associate together to create the nucleosome  Each nucleosome consists of 146 bp of DNA wrapped around a disc- shaped core of eight histone molecules  Nucleosomes function as tiny spools preventing DNA from becoming tangled  Scaffolding proteins are non histone proteins that help maintaining chromosome structure Cell division  DNA organization 6 Mouse metaphase chromosome depleted of histones Cell division  DNA organization 7 Condensed chromatin Extended chromatin Packed nucleosomes Nucleosomes DNA double helix Cell division  DNA organization 8 Cell division  Chromosome number 9  Chromosome number differs among species  It is not the number of chromosomes that makes each species unique but what they contain and the information specified by the genes  Normal human body cells have 46 chromosomes, studying the chromosome constitution is called karyotyping  Most animals and plants have between 8 to 50 chromosomes per body cell Cell division  The cell cycle 10  When cells reach a certain size they either stop growing or divide  The stages through which a cell passes from one cell division to the next are referred to as cell cycle  The cycle consist of two main phases:  Interphase  M phase  M phase involves two main processes:  Mitosis  Cytokinesis Cell division  Interphase 11  Chromosomes duplicate during Interphase  Interphase is the time where no cell division occurs  The cell will synthesize most of its biological material in this phase  The gap phase is an interval where no DNA synthesis occurs  Growth and normal metabolism take place during G1  G1 is typically the longest phase  Towards the end of G1 the enzymes required for DNA synthesis become more active Cell division  Interphase 12  During the Synthesis phase -S phase-, DNA replicates and histones are synthesized  After it completes the S phase, the cell enters a second gap phase, G2 phase  At G2, increased protein synthesis occurs  Generally G2 is short relatively to G1 & S  At the end of Interphase, cell goes into Mitosis Cell division  Mitosis 13  Mitosis is the cell (nuclear) division that produces two cells (nuclei) identical to the parental cell  Mitosis is divided into four stages:  Prophase  Metaphase  Anaphase  Telophase Cell division  Structure of a chromosome 14  Structure of duplicated chromosome  Pair of sister chromatids ◼ Contain identical DNA sequences ◼ Tightly associated in regions of centromeres  Each chromatid ◼ Includes region called centromere ◼ Kinetochore, a microtubule attachment, attached to each centromere Cell division  Structure of a chromosome 15 Centromere region Microtubules Kinetochore Sister 1.0µm chromatids Cell division  Stages of mitosis – Prophase 16  Chromosome condensation  Animal cells differ from the plant cells in the details of mitotic spindle formation Cell division  Stages of mitosis – Metaphase 17 Metaphase plate (midplane) Kinetochore microtubule (chromosomal spindle fibers) Centrioles Astral microtubules Pericentriolar material Polar microtubule 10µm Cell division  Stages of mitosis – Anaphase 18  The newly separated chromosomes move to opposite poles using the spindle microtubules as tracks Cell division  Stages of mitosis – Telophase 19 Cell division  Stages of mitosis – Cytokinesis 20  Cytokinesis is the division of the cytoplasm to yield 2 daughter cells  In animals, cytokinesis starts with a ring of contractile actin filaments Cleavage furrow Ring of contractile microfilaments (actin and 10µm myosin filaments) Cell division  Stages of mitosis – Cytokinesis 21  In plants, cytokinesis occurs by forming a cell plate  The cell plate forms as a line of vesicles originated in the Golgi complex Vesicles gather on Small vesicles fuse, forming Eventually one large New cell walls (from vesicle cell’s midplane larger vesicles vesicle exists contents) Plasma Cell Cell plate New plasma membranes (from membrane wall forming vesicle membranes) Cell division  Mitosis 22  Mitosis produces 2 cells genetically identical to the parental cell  With few exceptions, every cell of a multicellular organism has the same genetic makeup  Mitosis provides the distribution of chromosomes, but what about the various cytoplasmic organelles?  The nonmitotic division process is similar to prokaryotic cell division  Many copies of each organelle are present in each cell, so organelles are apportioned with the cytoplasm that each daughter will receive Cell division  Sexual reproduction and meiosis 23  Asexual reproduction: a single parent splits, fragments to produce two or more individuals  Offspring have hereditary traits identical to single parents, clones  Mitosis is the basis  Sexual reproduction: involves the union of two sex cells or gametes  Gametes fuse to form a zygote  Offspring are not genetically identical to their parents  Meiosis is the basis Cell division  Sexual reproduction and meiosis 24  Diploid cells (somatic cells 23X2 ch.) contain two sets of chromosomes (2n)  Characteristic number of chromosome pairs per cell ◼ Homologous chromosomes ◼ Similar in length, shape, other features, and carry similar attributes  Haploid cells (gametes 23 ch.) contain one member of each homologous pair (n)  Polyploid cells contain three or more sets of chromosomes Cell division  Sexual reproduction and meiosis 25 o One set of chromosome o Two sets of chromosome o In humans, number (n)=23 o In humans, 2n=46 o Gametes (sperm and ova) o All body cells (other than are haploid gametes) are diploids Cell division  Stages of meiosis 26  The term meiosis means to make smaller  A diploid cell (one cell with 2n) undergoing meiosis yields four haploid cells (4 cells with n)  The events of meiosis are similar to mitosis with 4 important differences:  Meiosis involves two successive nuclear and cytoplasmic divisions, producing 4 cells  Despite two successive nuclear divisions, the DNA and other chromosomal components duplicate only once during the Interphase preceding the first meiotic division  Each of the four cells produced by meiosis contains the haploid chromosome number  During meiosis the genetic information of both parents is shuffled so each resulting haploid cell has a virtually unique combination of genes Cell division  Stages of meiosis 27 2 sets of chromosomes, 2 chromatids (2n, 2c) Reductional division 1 set of chromosome, 2 chromatids (1n, 2c) Equational division 1 set of chromosome, 1 chromatid (1n, 1c) Cell division  Stages of meiosis – Meiosis I 28 INTERPHASE Interphase preceding meiosis; DNA MEIOSIS I replicates Homologous PROPHASE I chromosomes Homologous chromosomes synapse, forming Sister tetrads; nuclear envelope breaks down chromatids METAPHASE I Tetrads line up on cell's midplane. Tetrads held together at chiasmata (sites of prior crossing-over) ANAPHASE I Homologous chromosomes separate and move to opposite poles. Note that sister chromatids remain attached at their centromeres TELOPHASE I One of each pair of homologous chromosomes is at each pole. Cytokinesis occurs Cell division  Meiotic tetrad with two chiasmata 29 Cell division  Stages of meiosis – Meiosis II 30 PROPHASE II MEIOSIS II Chromosomes condense again following a brief period of interkinesis. DNA does not replicate again METAPHASE II Chromosomes line up along cell's midplane ANAPHASE II Sister chromatids separate, and chromosomes move to opposite poles TELOPHASE II Nuclei formed at opposite poles of each cell. Cytokinesis occurs HAPLOID CELLS Four gametes (animal) or four spores (plant) are produced Cell division  Meiosis in the trumpet lily 31 Cell division  Major differences between mitosis and meiosis 32  Mitosis involves a single nuclear division in which the two daughter cells formed are genetically identical to each other and to the original cell. Synapsis of homologous chromosomes does not occur during mitosis  Meiosis involves two successive nuclear divisions and forms four haploid cells. Synapsis of homologous chromosomes occurs during prophase I of meiosis Cell division  Major differences between mitosis and meiosis 33 Cell division  Major differences between mitosis and meiosis 34 Cell division