Exploring Cell Division and Chromosome Structure

1. Cell Division

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

3. Requirements • Distribution of identical DNA to daughter cells

4. Figure 12.0 Mitosis

5. 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

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

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

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

12. Genes • Organization of DNA informational units • Chromosomes contain hundreds to thousands of genes • Humans: 35,000 - 45,000 genes

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

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

17. 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

19. Cell Cycle • Chrom duplicate during INTERPHASE (90% of cell’s life) • G1 phase - cells grow and synthesize biological molecules • S phase - DNA replication • G2 phase - gap of time between S phase and mitosis (preparation for division)

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

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

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

27. 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

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

31. ASTER

32. Mitosis – Prophase (late) • The nuclear envelope disappears

35. 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

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

41. 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

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

49. 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