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REPRODUCTION AT THE CELLULAR LEVEL CHAPTER 6 THE GENOME Genome: all of the DNA in a given cell Prokaryotic Genome One double-stranded DNA molecule Forms a loop Eukaryotic Genome Multiple double-stranded DNA molecules Linear EUKARYOTIC CHROMOSOMES Chromosomes are a double-stranded DNA molecule and attached proteins Human chromosomes would be about 2 meters long Winds and coils up (condenses) to pack tightly into the nucleus DNA molecule winds twice around histones (protein “spools”) DNA + histone = nucleosome Looks like beads on a string under a microscope Coils like a telephone cord Have a characteristic X shape when completely supercoiled EUKARYOTIC CHROMOSOMES Chromosomes consist of one DNA molecule during most of a cell’s life When a cell divides the chromosomes are duplicated Each chromosome will consist of two DNA molecules Each is called a sister chromatid They are attached at the centromere Constricted region where sister chromatids are attached centromere EUKARYOTIC CHROMOSOMES Karyotyping shows all of a cell’s chromosomes Images of the chromosomes can be lined up by size, shape, length, and centromere location Shows Types of chromosomes How many chromosomes Missing or extra chromosomes or structural abnormalities Fig. 8-6a, p.129 EUKARYOTIC CHROMOSOMES Types of Chromosomes Each type of chromosome has a specific length, centromere location, shape, and genetic information EUKARYOTIC CHROMOSOMES Types of Chromosomes Autosomes Any chromosome except a sex chromosome Both females and males have the same autosomes Sex chromosomes Differ between females and males XX or XY Determine an individual’s sex EUKARYOTIC CHROMOSOMES Chromosome number Each species has a specific number of chromosomes Humans = 46 Others Gorillas = 48 Pea plants = 14 Mosquito = 8 EUKARYOTIC CHROMOSOMES Chromosome number Haploid (n) (“n” = a single set of chromosomes) Only one of each type of chromosome Diploid (2n) Two of each type of chromosome Pairs have the same length, shape, and information about heritable traits Called Homologous pairs of chromosomes or Homologs One of the pair came from the father and one came from the mother Polyploid Multiple copies of types of chromosomes QUESTIONS Chromosome Two of each type of chromosome Histone Any chromosome except the sex chromosomes Chromatid DNA molecule and attached proteins Centromere One of each type of chromosome Diploid Protein spool that DNA wraps around Haploid Image showing all of a cell’s chromosomes Karyotype Constricted region where sister chromatids attach Autosome One strand of a duplicated chromosome Questions 1 2 2 4 3 CELL CYCLE The cell cycle is a series of events including cell growth and cell division It starts when a new cell forms and ends when that cell divides In eukaryotes it consists of Interphase (G1, S, G2), Mitotic phase Replicated DNA and cytoplasm are separated The cell divides CELL CYCLE Interphase The longest interval of the cell cycle Has three stages G1 Interval or “gap” Cell growth and activity before the onset of DNA replication Most cells remain in G1 unless they need to divide S Time of “synthesis” or DNA replication This is when the DNA is duplicated resulting in sister chromatids G2 Second interval or gap The cell prepares for division MITOTIC PHASE Mitotic phase has two parts Mitosis which is division of the nucleus and chromosomes Cytokinesis which is division of the cytoplasm, membrane, and organelles MITOTIC PHASE Mitosis: mitotic spindle Sorts and divides chromosomes Also called “bipolar spindle” Composed of Centrosomes A pair of small barrel shaped centrioles that direct microtubule construction Divided during G2 of interphase just before prophase so there are two pair in each cell ready for mitosis Spindle fibers (microtubules) Centrosome Fibers MITOTIC PHASE Mitosis: mitotic spindle Spindle fibers (microtubules) Polar fibers: run from pole to pole Kinetochore fibers: run from a pole to one chromatid of a chromosome. Fiber from the other pole connects to its sister. The fibers attach to a specialized area at the centromere Polar fibers called the kinetochore. Aster fibers: In animal cells the aster fibers extend from the centrosome out towards the edge of the cell. The Aster gives the bipolar spindle support. Plant cells do not have asters. Kinetochore fibers Aster fibers MITOTIC PHASE Mitosis: mitotic spindle When a cell commits to divide by mitosis the centrosomes migrate toward opposite sides of the cell Once they reach opposite sides this creates “poles” Equidistant from the poles is the “equator” or metaphase plate pole microtubule Chromosomes at the equator pole MITOTIC PHASE Mitosis Prophase Chromosomes condense and become visible with a light microscope During Interphase chromosomes are loosely dispersed throughout the nucleus (chromatin) Condensing means that the chromosomes coil up into a compact form Helps to keep the chromosomes from tangling as they are moved and sorted out during nuclear division Mitotic spindle begins to form Nuclear envelope starts to break up Nucleolus disappears MITOTIC PHASE Mitosis Prometaphase Chromosomes continue to condense Kinetochores appear at the centromeres Mitotic spindle microtubules attach to kinetochores Centrosomes move toward opposite poles MITOTIC PHASE Mitosis Metaphase Mitotic spindle is fully developed Centrosomes are at opposite poles Chromosomes are lined up at the metaphase plate/equator Each sister chromatid is attached to a spindle fiber originating from opposite poles MITOTIC PHASE Mitosis Anaphase Sister chromatids are separated and pulled toward opposite poles Microtubules drag the chromatids Polar fibers lengthen, elongating the cell MITOTIC PHASE Mitosis Telophase Chromosomes arrive at opposite pole and begin to decondense Nuclear envelope forms around each set of chromosomes Mitotic spindle breaks down QUESTIONS What are the components of the bipolar spindle? What organelles are located at the poles? What are spindle fibers made of? What do spindle fibers attach to? What is the role of the aster? What are the phases of Mitosis? MATCHING QUESTION 1. Interphase A. Kinetochores appear 2. Prophase B. Chromosomes line up at the equator 3. Prometaphase C. Chromosomes condense 4. Metaphase D. Chromosomes relax and new nuclear membranes form 5. Anaphase E. The cell grows and prepares to divide 6. Telophase F. Chromatids are pulled toward the poles MITOTIC PHASE Cytokinesis Divides the cytoplasm and its contents between the two new daughter cells Usually occurs between late anaphase and end of telophase Different mechanisms are used by different organisms Animals: Contractile Ring Formation Plants: Cell Plate Formation MITOTIC PHASE Cytokinesis Animal: Contractile Ring Formation At the spindle equator a ring of actin filaments contracts Cleavage furrow is formed The contractile ring continues to shrink pulling the cell surface inward until the ring pinches the cytoplasm in two MITOTIC PHASE Cytokinesis Plant: Cell Plate Formation Vesicles containing cell wall materials form From the Golgi apparatus Vesicles collect at the metaphase plate and fuse Deposit cell wall components Vesicle membranes become the plasma membrane on either side of the new cell wall Starts in the center and grows toward the cell walls MITOTIC PHASE The end result of mitosis plus cytokinesis is two identical daughter cells G1 Genetically identical to each other and to the original parent cell Sister chromatids are separated during anaphase ensuring that each daughter cell receives one copy of each chromosome Used for growth, cell replacement, tissue repair, asexual reproduction S G2 Mitosis and Cytokinesis REGULATION OF THE CELL CYCLE G0 Phase Cells that are not actively preparing to divide The cell has exited the cell cycle An external signal triggers some to re-enter the cell cycle Some cells remain in G0 permanently REGULATION OF THE CELL CYCLE Internal Checkpoints To prevent cells with mistakes in the chromosomes from continuing to divide Check for Integrity of the DNA Adequate reserves and cell size CANCER AND THE CELL CYCLE Sometimes the checkpoints fail and cell division becomes uncontrollable Tumor formation and cancer are the outcome Basal cell carcinoma Squamous cell carcinoma Melanoma Slow growing, raised lump Grows under the surface of the skin Spreads fastest Dark encrusted lumps CANCER AND THE CELL CYCLE Proto-oncogenes Normal genes that code for positive cell-cycle regulators When mutated become oncogenes Less- or non-functional in regulating the cell cycle causing cells to become cancerous CANCER AND THE CELL CYCLE Tumor suppressor genes Code for negative regulator proteins that can prevent the cell from undergoing uncontrolled division Mutated tumor suppressor genes Cell can no longer halt the cell cycle if there is a problem PROKARYOTIC CELL DIVISION Binary fission (prokaryotic fission) Less complicated and much quicker than mitosis Populations can grow very rapidly Single circular chromosome located in the “nucleoid” region Origin: the starting point of replication Close to a site on the chromosome which binds to the plasma membrane Replication occurs in both directions from the origin PROKARYOTIC CELL DIVISION Binary fission (prokaryotic fission) Bacterial DNA is attached to the plasma membrane PROKARYOTIC CELL DIVISION Binary fission (prokaryotic fission) DNA is replicated (both directions from the origin of replication) PROKARYOTIC CELL DIVISION Binary fission (prokaryotic fission) Both strands are attached PROKARYOTIC CELL DIVISION Binary fission (prokaryotic fission) The cell elongates and as the membrane grows the two chromosome copies move apart PROKARYOTIC CELL DIVISION Binary fission (prokaryotic fission) New membrane and cell walls start (septum) PROKARYOTIC CELL DIVISION Binary fission (prokaryotic fission) When the cell walls are finished two new cells separate SUMMARY The genome Eukaryotic cell cycle Interphase The mitotic phase Mitosis Cytokinesis Control Cancer and the cell cycle Prokaryotic cell division
