The Cell Cycle

1. The Cell Cycle

2. Cell Division • Cell Division • Cells divide in order for us to grow • As a cell grows, its volume increases more rapidly than its surface area • When the surface area-to-volume ratio is too small, the cell cannot move materials in and out of the cell at a sufficient rate or in sufficient quantities. • Why are we made of millions of small cells instead of just one big cell?

3. Surface Area to Volume Ratio

4. 3 Phases of the Cell Cycle • Interphase • G1 • S phase • G2 • Mitosis • Prophase • Metaphase • Anaphase • Telophase • Cytokinesis

5. Cell Life Span

6. Purpose of the Cell Cycle: • To make 2 genetically identical daughter cells from a single parent cell

7. Interphase • The "holding" stage or the stage between two successive cell divisions. • Some 90 % of a cell's time in the normal cell cycle may be spent in this phase C = chromatin I = nucleolus

9. InterphaseGap 1 (G1) • Cell carries out its normal cell functions • Cell increases in size (grows) • Organelles increase in number • A cell spends most of its time in this stage, although the length of time varies by cell type.

10. InterphaseSynthesis (S) • Synthesis—combining of parts to make a whole • Cell makes a copy of its DNA (nucleus) • By the end of the S stage, the cell nucleus contains two complete sets of DNA

11. InterphaseGap 2 (G2) • Additional growth occurs • Checkpoint—everything must be in order before the cell can move on and go through mitosis and then cell division

12. Mitosis (M Stage) • Mitosis—division of the cell nucleus and its contents • Nuclear membrane dissolves • The duplicated DNA condenses • Two new nuclei form

13. MitosisProphase • Loose chromatin condenses into tightly coiled chromosomes • The nuclear envelope breaks down • Centrioles begin to move to the opposite poles and spindle fibers form

14. Condensing of Chromosomes • One chromosome = one continuous strand of DNA • DNA wraps around proteins called histones, forming chromatin • Chromatin compacts further, forming a chromatid • Two identical chromatids are called sister chromatids • Sister chromatids are held together at the centromere • Telomeres are located at the ends of the chromatids • Humans have 23 pairs of chromosomes (46 total)

15. Sister Chromatids Telomeres—regions of repetitive DNA at the end of a chromosome, which protects the end of the chromosome from destruction

16. Progeria

17. MitosisMetaphase • Spindle fibers attach to each chromosome • Sister chromatids are aligned along the equator by the spindle fibers • In humans, 46 sister chromatids line up in the middle of the cell

18. MitosisAnaphase • Chromatids separate to opposite sides of the cell • Pulled by spindle fibers

20. MitosisTelophase • A complete set of identical chromosomes is positioned at each pole of the cell • The nuclear membranes start to form • Chromosomes begin to uncoil • Spindle fibers fall apart

21. Cytokinesis • Cytokinesis—division of the cytoplasm • Results in two daughter cells • Both new cells are genetically identical to the original parent cell

22. Cytokinesis Differs in Animal and Plant Cells • In Animal cells, the membrane forms a furrow and pinches closed • In Plant cells, the membrane cannot pinch inward because of the cell wall. Instead, a cell plate forms between the two new nuclei

23. At the End of the Cell Cycle: • Final Products are two identical cells • Smaller in size (larger surface area to volume ratio) • Each new cell has 46 chromosomes • Organelles get divided up between the two cells during cytokinesis • Not always exactly evenly divided • The cell can make more organelles if needed