Cell Growth & Division

1. Cell Growth & Division Chapter 5

2. The Cell Cycle • Eukaryotic cells undergo a predictable cycle of growth and division • This Cell Cycle has 4 main stages • G1 – growth • S – synthesis of DNA • G2 – more growth • M – mitosis

3. The Cell Cycle • G1 – the cell carries out its normal functions - GROWTH • The cell will also increase in size and organelles increase in number • Cells must pass a critical checkpoint in order to proceed to the next stage of the cell cycle

4. The Cell Cycle • S – synthesis of DNA • The cell will make a copy of its DNA • The DNA is arranged loosely inside the nucleus during the S stage • At the end there are 2 exact copies of the DNA

5. The Cell Cycle • G2 – additional growth occurs • Normal functions continue • Preparing for the M stage (mitosis) of the cycle • Must pass a critical checkpoint, just like in G1, in order to proceed to Mitosis

6. The Cell Cycle • M – mitosis – cell division • The nucleus divides in mitosis • The nuclear membrane dissolves • DNA condenses and separates • Two new nuclei form • Cytokinesis – the process that divides the cytoplasm into 2 new cells

7. The Cell Cycle • Cells all go through the cell cycle, but at different rates • The S, G2 and M stages in a human take about 12 hours. • The G1 stage varies widely with the type of cell • Skin cells live about 2 weeks

8. The Cell Cycle • Red blood cells live about 4 months • Liver cells live about 300-500 days • Intestine (internal lining) cells live about 4-5 days • Intestine (muscle & other tissue) cells live about 16 years!

9. The Cell Cycle • Some cells do not divide or divide very rarely (neurons, for example) • These cells enter a stage called G0. • Cells in G0 continue to carry out normal functions, but do not proceed to the S, G2 and M stages

10. The Cell Cycle • Cell size is limited – they cannot be too small (the organelles won’t fit!) or too big (the cell would be inefficient) • The upper limit on the size of a cell is because of the cell “surface area to volume ratio”

11. The Cell Cycle • As a cell grows, its volume and surface area both increase • However, the volume increases faster than the surface area • The surface (cell membrane) of the cell is very important in getting materials in & out, so…

12. The Cell Cycle • If a cell has a large volume & not enough surface area, the cell could not get enough needed materials to the entire cell quickly enough On page 137 in the textbook, there is a similar diagram

13. The Cell Cycle • Some cells must be large – for instance a neuron that needs to reach from your brain to your leg • This cell is not shaped like a sphere or cube, but instead is long & thin, making the surface area increase but with very little volume increase

14. The Cell Cycle • Cells have to regulate the amount of growth that occurs & the size of the cell before it divides • Cells that are more than twice as large as the original will produce daughter cells that are too big • Cells less than double the size will produce daughter cells that are too small

15. The Cell Cycle • Let’s take an in-detailed look at the last parts of the cell cycle – Mitosis & Cytokinesis • The cell must make sure that each new daughter cell has the exact same DNA as the other, if not, things could be disastrous!

16. Mitosis • Each human body cell has 46 chromosomes • A chromosome is a condensed strand of DNA that contains many genes along with other regulatory info • Chromosomes are tightly coiled DNA, wrapped around proteins

17. Mitosis • Page 139 in your text shows the organization of DNA from the basic strand to the tightly coiled chromosome (Fig 5.5) • Histones are proteins that DNA wraps around at regular intervals

18. Mitosis • Chromatin is the loose form of DNA (not as coiled as chromosomes, think spaghetti) • Chromosomes look like an “X” in which the left & right sides are identical • One side of the “X” is called a chromatid.

19. Mitosis • Sister chromatids are held together at the centromere • The end of the DNA molecules form telomeres which are repeating nucleotides & do NOT code for genes • A portion of each chromosome is lost each time a cell divides, & it must be from the telomeres!

20. The Phases of Mitosis • Interphase – G1, S & G2 stages • Prophase – 1st official phase of mitosis, chromatin condenses, nuclear envelope breaks down, centrioles move to opposite sides of the cell, spindle fibers (microtubules) grow out of the centrioles

21. The Phases of Mitosis • Prophase Pictures

22. The Phases of Mitosis • Metaphase – the chromosomes line up in the middle of the cell • Spindle fibers attach to each chromosome on either side

23. The Phases of Mitosis • Anaphase – The chromatids begin to separate to opposite sides of the cell • Cytokinesis will usually begin in late anaphase Anaphase “bananaphase”

24. The Phases of Mitosis • Telophase – the last phase of mitosis • 2 new nuclear membranes start to form • Chromosomes begin to uncoil • Spindle fibers fall apart • The new cells start to pinch off

25. The Phases of Mitosis • Photos of Telophase

26. Regulation of the Cell Cycle • Both internal & external factors regulate the cell cycle • External factors come from outside the cell – messages from OTHER nearby cells or even from distant cells in the organism’s body • Internal factors come from inside the cell & include many different molecules found in the cytoplasm

27. Regulation of the Cell Cycle • External factors can be physical or chemical signals. • A physical signal could be cell-to-cell contact • If a cell is normal, once it touches another cell, it will stop growing/dividing

28. Regulation of the Cell Cycle • Growth factors are chemical signals that stimulate cell growth • Some hormones may also stimulate cell growth • External factors stimulate internal factors • Kinases and cyclins are 2 internal factors

29. Regulation of the Cell Cycle • Kinases are enzymes that are activated by cyclins to help regulate the cell cycle and move from stage to stage

30. Regulation of the Cell Cycle • Apoptosis: programmed cell death • Some cells are supposed to die at certain times • Genes are activated that produce self-destructive enzymes

31. Regulation of the Cell Cycle • Apoptosis occurs in developing babies between the fingers before birth • In the womb, babies have webbed fingers • At birth, babies do not have webbed fingers because those cells die

32. Regulation of the Cell Cycle • Sometimes, cells grow uncontrollably • This is commonly referred to as cancer • Cancer cells do not stop growing & dividing when they touch other cells, resulting in tumors

33. Regulation of the Cell Cycle • If a tumor is benign, the cells stay clustered together • If a tumor is malignant, the cells break off (metastasize) and spread to other parts of the body • Cancer cells take nutrients away from healthy cells

34. Regulation of the Cell Cycle • Cancer cells also do not perform regular cell functions • Cancer tumors can also put pressure on internal organs and cause them to not work properly • Carcinogens are cancer causing agents that can be environmental or even viruses

35. Regulation of the Cell Cycle • Cancer treatment often involves radiation therapy or chemotherapy • Radiation therapy uses radiation to shrink and kill cancer cells • Chemotherapy uses chemicals to kill the cells

36. Regulation of the Cell Cycle • Cancer treatments can also cause unwanted side effects because they kill healthy cells as well as the cancer cells • Radiation is localized (one spot) & chemotherapy is systemic (all over the body)