Cell Theory 1 Cellular Reproduction, Part 1: Mitosis Lecture 10 Fall 2008 Cell theory: All organisms are made of cells All cells arise from preexisting cells How do new cells arise? Cell division the reproduction of cells Cell Division 2 Cell Division 3 What is cell division used for? Growth Wound repair Reproduction Both sexual and asexual Fig. 8.1 Multicellular organisms start out life as a single cell A series of cell divisions occur that give rise to all the cells in an organisms body
Cell Division 4 Cell Division 5 Two components Cytokinesis ( cell movement ) Division of the cytoplasm into two daughter cells Division of the nucleus (and of the DNA) Two different types of nuclear division possible Mitosis Form of cell division that leads to identical daughter cells with the full complement of DNA Occurs in somatic cells Cells of body that are not reproductive cells Meiosis Form of cell division that leads to non-identical daughter cells with one-half the complement of DNA Forms gametes Reproductive cells (sperm cells & egg cells) Chromosomes 6 Chromosomes 7 Genome the complete complement of an organisms genetic material, DNA Gene a discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA Prokaryotes have one long DNA molecule Eukaryotes have many DNA molecules = chromosomes Chromosome: a thread-like, gene-carrying molecule located in the nucleus Chromosomes in the nucleus contain most of an organisms genes Some DNA in mitochondria and chloroplasts Number of chromosomes vary depending on species Somatic cells of humans = 46 chromosomes Two sets of 23 chromosomes 23 from egg and 23 from sperm Gametes of humans = 23 chromosomes
Chromosomes 8 DNA Packing 9 Chromosomes made of chromatin Complex of DNA and protein molecules Role of protein molecules Organize chromatin Help control activity of gene Mass of long fibers spread out in the nucleus Chromatin fibers form compact chromosomes only during cell division Fig. 12.3 Each chromosome contains one very long, linear DNA molecule that carries several hundred to a few thousand genes Chromosomes condense to structures visible with a light microscope 1. Histones attach to DNA Histones small proteins that assist with DNA packing Creates an area called a nucleosome DNA wrapped around histones beads on a string 2. Tight helical fiber 3. Supercoil 4. More looping & folding Fig. 16 21 Replication of Chromosomes 10 The Cell Cycle 11 DNA needs to be duplicated prior to cell division Duplicated chromosome = sister chromatids Held together by adhesive proteins (cohesins) Centromere Specialized region where sister chromatids most closely attached During mitosis, sister chromatids separate, one each to the two daughter cells Once separated, they are now individual chromosomes Fig. 12.4 Cell cycle Life cycle of the cell Starts with origination of a cell and ends with cell division Two main phases Fig. 12.5 Interphase ~90% of time Mitotic phase ~10% of time
The Cell Cycle 12 The Cell Cycle 13 Interphase Normally activity of cell Cell grows as it produces more contents of cell cytoplasm Chromosome duplication occurs Fig. 12.5 Interphase Cell growth occurs in all three sub-phases G 1 (first gap) S phase (synthesis) Chromosome replication Sister chromatids G 2 (second gap) Cell preparing to divide Fig. 12.5 Mitosis 14 Mitotic phases Prophase Prometaphase Metaphase Anaphase Telophase Activity Cytokinesis occurs during telophase
G2 of Interphase 15 Mitosis: Prophase 16 Chromosomes (chromatin) duplicated Chromosomes still uncondensed Not clearly distinguishable as separate chromosomes Centrosome Non-membrane bound organelle Organizes the cell s microtubules Centrosome replicates into 2 Contains centrioles Structures made of microtubules Found only in animals Microtubules will form even if centrioles destroyed Chromosomes condense Each chromosome appears as two identical sister chromatids joined together Formation of mitotic spindle Centrosomes move towards poles of cell Microtubules extend from centrosomes Asters short microtubules extending from centrosome Will guide the separation of the daughter chromosomes Mitosis: Prometaphase 17 Mitosis: Metaphase 18 Breakdown of nuclear envelope Microtubules of mitotic spindle start to attach to chromosomes Centromere contains special attachment proteins kinetochore Movement of chromosomes towards center of cell Non-kinetochore microtubules interact with others from opposite pole Centrosomes at opposite poles Mitotic spindle fully formed Chromosomes lined up at equator of cell (metaphase plate) Each sister chromatid is attached to a kinetochore microtubule Other non-kinetochore microtubules meet with each other from opposite ends of the cell
Mitosis: Anaphase Sister chromatids of each chromosome separate Cohesion proteins cleaved Now a daughter chromosome Chromosomes move away from center, towards poles Kinetochore microtubules shorten & bring chromosomes with them towards each pole Read Fig. 12.8 Inquiry Cell elongates Non-kinetochore microtubules lengthen Pushes poles of cell further apart End of Anaphase Both poles have equivalent and complete set of chromosomes 19 Mitosis: Telophase Chromosomes at poles of cell Two daughter nuclei begin to form in the cell Nuclear envelope forms around each one Chromosomes become less condensed Mitotic spindle deconstructed 20 Cytokinesis 21 Cytokinesis 22 Cytokinesis in animals Cleavage Cleavage furrow Shallow groove at equator of cell Parent cell pinched in two by contraction of ring of microfilaments Cytokinesis in plants Cell wall material bound in vesicles Vesicles line up, fuse to form cell plate Grows until it fuses with plasma membrane Cell wall contents bind with cell wall Fig. 12.9 Fig. 12.9
Asexual Reproduction 23 Asexual Reproduction 24 Mitosis also used to reproduce entire organisms Asexual reproduction Reproduction involving only one parent that produces genetically identical (clone) offspring Does not involve sperm and egg (no meiosis) Japanese knotweed broken stem sprouts shoots and roots Fig. 8.2 Aspen root suckers produce new shoots and stems Asexual Reproduction 25 Evolution of Mitosis 26 Binary fission Prokaryotes Not mitosis Single large circular chromosome Chromosome duplicated Starts at origin of replication Copy of origin site moves to other pole of cell Cell elongates Plasma membrane grows inward to form two daughter cells Hypothesis: mitosis evolved from binary fission in prokaryotes Some similar proteins Possible intermediate stages still seen other organisms Fig. 12.11 Fig. 12.12