Cell Division

Why divide? Characteristic of life Continuity Growth (zygote → multicellular org) Repair, renewal, replacement

Requirements Distribution of identical DNA to daughter cells

Figure 12.0 Mitosis

DNA – a closer look Genome – cell’s entire genetic info Prok – often a single long DNA molecule Euk – several DNA molecules Human cells must copy ~ 3 m of DNA before division

Packaging DNA Chromosomes Contain DNA and Protein Called chromosomes because they can be stained with certain dyes

Eukaryotic Chromosomes Composed of CHROMATIN protein DNA Chromosomes become visible as distinct structures when the cell divides When not dividing, the chromosomes decondense

Chromosome Structure Duplicated chromosome = 2 sister chromatids Chromatids → identical copies of the DNA As they condense, the area where strands connect shrinks → centromere

Genes Organization of DNA informational units Chromosomes contain hundreds to thousands of genes Humans: 35, ,000 genes

Number of Chromosomes Differ by species Humans - 46 chrom (somatic cells) The number is not indicative of complexity What is this called? KARYOTYPE

Gametes Gametes contain half the # of chromosomes present in somatic cells Human gametes – 23 chromosomes WHY?

Cell Cycle A sequence of cell growth and division Numerous factors control when cells divide Cell Division Mitosis- division of chromosomes Cytokinesis- division of cytoplasm

Cell Cycle Chrom duplicate during INTERPHASE (90% of cell’s life) G 1 phase - cells grow and synthesize biological molecules S phase - DNA replication G 2 phase - gap of time between S phase and mitosis (preparation for division)

Mitosis Purpose is to ensure the orderly distribution of chromosomes Four Stages: Prophase Metaphase Anaphase Telophase

Late Interphase Chrom duplicated, still loosely packed Centrosomes duplicated, organization of microtubules into an “aster”

Mitosis – Prophase (early) Duplicated chromosomes visible Chromatin condenses Sister chromatids are bound at the centromere Centromeres have kinetochores (proteins) to which microtubules will bind

Mitosis – Prophase (early) The mitotic spindle, composed of microtubules, forms between the poles The MTOC (microtubule organizing center) surrounds a pair of centrioles in animal cells and some plant cells Centrioles are surrounded by pericentriolar material

Mitosis – Prophase (middle) Asters extend from the MTOCs at the poles (in cells that have centrioles) The nucleolus disappears

ASTER

Mitosis – Prophase (late) The nuclear envelope disappears

Mitosis - Metaphase Duplicated chromosomes line up at midplane Chromatids are highly condensed Polar microtubules extend from the pole to the equator, typically overlap Kinetochore microtubules extend from the pole to the kinetochores

Mitosis – Anaphase (early) Chromosomes move toward the poles Chromatids separate at the centromeres and are now referred to as chromosomes

Mitosis – Anaphase (late) The chromosomes are pulled by the kinetochore microtubules to the poles and form a “V” shape The movement mechanism by which the microtubules and other mitotic spindle components move the chromosomes is largely unknown

Mitosis- Telophase Two separate nuclei form Cell returns to conditions similar to interphase Nuclear envelope reforms; nucleoli reappear Cytokinesis occurs

Cytokinesis Formation of two separate daughter cells Begins during telophase In animals cells, a furrow develops caused by contractile actin filaments that encircle the equatorial region In plant cells, a cell plate forms originating from the Golgi complex

How do chromosomes move? A chromosome’s kinetochore is “captured” by microtubules, & it moves toward the pole connected to microtubules Microtubules attach to the other pole, tug- of-war ensues Other microtubules from opposite poles interact as well, elongating the cell

Chromosome movement Hypothesis for chromosome movement is that motor proteins at kinetochore “walk” attached chromosome along microtubule toward opposite pole Excess microtubule sections depolymerize

Experiments support hypothesis that spindle fibers shorten from end attached to chromosome, not centrosome

Mitosis - General Notes Mitosis (generally) produces two daughter cells genetically identical to the parent cell Most cytoplasmic organelles are distributed randomly to the daughter cells Mitochondria and chloroplasts divide on their own during interphase

Mitosis- General Notes Eukaryotic cells typically divide less frequently than prokaryotes

The Cell Cycle Frequency of cell division varies with cell type Driven by specific chemical signals in the cytoplasm

Experiment Fusion of S phase cell and G 1 phase cell induces G 1 nucleus to start S phase Fusion of cell in mitosis with one in interphase induces 2 nd cell to enter mitosis

Control of the Cell Cycle Protein kinases are active when complexed with cyclins (regulatory proteins) When Cdk complexes with a certain cyclin, it activates specific enzymes and can inactivate others Colchicine (one of a number of drugs that block cell division) can block cell division in eukaryotes by interfering with spindle formation

More about CdKs Protein kinases activate/deactivate other proteins by phosphorylating them Levels of kinases are constant, but they require cyclin, to become activated Levels of cyclin fluctuate (cyclically) Form cyclin-dependent kinases (Cdks)

Cyclin levels rise sharply in interphase, fall abruptly during mitosis Peaks in the activity of cyclin-Cdk complex, MPF, correspond to M phase

MPF “Maturation-promoting factor”; cyclin-Cdk complex Phosporylates kinases Fragmentation of nuclear envelope Breakdown of cyclin

External Control Growth factors (mammalian cells), proteins released by one group of cells - stimulate other cells to divide platelet-derived growth factors (PDGF)

PDGF Fibroblasts in culture only divide in a medium that contains PDGF

Density-dependent inhibition Cultured cells divide until they form single layer on inner surface of dish If gap is created, cells will grow to fill gap At high densities, amt of growth factors & nutrients insufficient to allow continued cell growth

Anchorage Dependence To divide, cells must be anchored to something, typically extracellular matrix of a tissue Control mediated by connections between extracellular matrix, plasma membrane proteins, & parts of cytoskeleton Cancer cells – HOW DO THEY WORK?