1. Cell Division  The process by which cells reproduce themselves  Two types of cell division:  Somatic cell division  Reproductive cell division

2. Somatic Cell Division  Somatic cell - any body cell other than a germ cell (gamete)  Two stages of somatic cell division:  Nuclear division - mitosis  Cytoplasmic division – cytokinesis  Produces two identical diploid cells  Replaces dead or injured cells  Adds new cells during tissue growth

3. Reproductive Cell Division  Germ cell - a gamete (sperm or oocyte)  Two stages of reproductive cell division:  Nuclear division – meiosis  Cytoplasmic division – cytokinesis  Produces unique haploid cells  Supplies gametes

4. Maternal Paternal

5. Somatic Cell Chromosomes  23 pairs of chromosomes in human body cells  One member of each pair is inherited from each parent.  22 pairs of homologous chromosomes.  1pair of sex chromosomes.  In females – XX  In males – X Y  Because somatic cells contain two sets of chromosomes, they are called diploid cells (2n).

6. Somatic Cell Cycle  The sequence of events by which a somatic cell:  Grows  Duplicates its contents  Divides in two  The cell cycle is divided into two major periods:  Interphase - when a cell is not dividing  Mitotic phase - when a cell is dividing

7. Interphase  Is a time of:  Great metabolic activity  Cell growth  DNA replication.  Consists of three phases:  G1 phase  S phase  G2 phase

8. G 1 phase (8-10 hours)  Lasts 8-10 hours  Newly formed cell:  Is metabolically active  Duplicates its organelles  Grows in size  Time varies – ex. embryonic cells and cancer cells

9. G 0 – a Special Case of G 1  Some cells enter G 1 and do not leave  We say that they are in G 0  G 0 cells are metabolically active, BUT:  They do not replicate their DNA  They do not divide  Examples: neurons and skeletal muscle cells

10. S phase (8 hours)  Lasts about 8 hours  The cell replicates its DNA  Once a cell enters S-phase, it is committed to go through cell division

11. DNA Replication “Recipe” 1. DNA unwinds from histones 2. Helicase separates DNA into its complementary chains 3. DNA polymerase uses each chain as a template to build the other complementary chain 4. DNA polymerase double-checks the new chain 5. Histones reassociate with the DNA Fig 3.31

12. G 2 phase (4-6 hours)  Cell continues to grow and be metabolically active.  It is making its final preparations for cell division.

13. Mitotic Phase of the Somatic Cell Cycle  Consists of a:  Nuclear division - mitosis  Cytoplasmic division - cytokinesis  Produces two identical diploid cells

14. Figure 3.32 Cell division: mitosis and cytokinesis. Nuclear Division: Mitosis Is divided into 4 stages: 1.Prophase 2.Metaphase 3.Anaphase 4.Telophase

15. Prophase 1. 46 Chromatin fibers  46 chromosomes 2. Centrosomes form mitotic spindle 3. Centrosomes move to cell’s opposite poles 4. Nucleolus disappears 5. Nuclear envelope breaks down

16. Metaphase  Mitotic spindle aligns chromosomes’ centromeres along metaphase plate

17. Anaphase 1. Centromeres split 2. Each chromatid (now called a chromosome) is pulled by mitotic spindle to cell’s opposite poles

18. Telophase 1. Nuclear envelopes and nucleoli reappear 2. Mitotic spindle breaks up 3. Chromosomes resume their chromatin form

19. Cytoplasmic Division: Cytokinesis 1. Cleavage furrow begins to form near end of anaphase 2. New cell’s interphase begins when cytokinesis is complete