Cell Division

Name that animal!

How do we go from embryo to fully-developed organism? True or false: Organisms grow and get bigger solely because the cells get bigger. False

Cells have an optimal size If cells grow too large, they aren’t able to exchange materials efficiently. Surface area : Volume Cells like a lot of surface area and want a high SA:V ratio More surface area/membrane allows for more chemical reactions and transport of materials (think cristae)

As the cell grows, the workload for the nucleus becomes too great. DNA Overload As the cell grows, the workload for the nucleus becomes too great.

The solution: Division Rudolf Virchow: “Every cell from a cell.” Multicellular organisms grow and develop from the division of cells. Organisms repair themselves via cell division (cuts, bone fractures, torn ligaments, etc.)

Simple Prokaryotes (bacteria) reproduce via a process called binary fission. Leads to the creation of two identical cells. Bacteria have one, circular chromosome and no nucleus What needs to happen? DNA needs to duplicate

Complex Karyotype: the number and appearance of chromosomes in an organism Humans have 46 chromosomes (23 pairs- one from each parent) Chromosomes are condensed chromatin

Chromosome Structure Chromosomes store DNA and your genes One duplicated chromosome is made of two sister chromatids held together by a centromere Each sister chromatid on a chromosome is identical to each other (same size, shape, and same genes with same traits) Homologous chromosomes have the same size, shape, and genes but different traits (you get one from mom and one from dad)

Mitosis: nuclear division Process of dividing the genetic material in a cell for division Start with one cell and end with two identical cells Must create exact copies of original to maintain genetic continuity Genetic continuity- keeping the same number of chromosomes (don’t want to gain or lose any) Prophase Metaphase Anaphase Telophase and then Cytokinesis

Prophase: 1st Phase Before Mitosis Chromatin condenses into chromosomes Centrioles appear and move to poles Nucleolus disappears Spindle fibers form Before Mitosis

Prometaphase: transition to metaphase Nuclear membrane breaks down Kinetochore forms at each chromatid Microtubules aka spindle fibers attach to kinetochores of chromatids

Chromosomes are moved to middle of the cell by the spindle fibers Metaphase Chromosomes are moved to middle of the cell by the spindle fibers

Sister chromatids separate and move towards opposite ends Anaphase Sister chromatids separate and move towards opposite ends

Chromosome movement Microtubule shortens at kinetochore end rather that at centriole end. Motor protein walks chromosome towards centriole in a process that requires ATP

Telophase: opposite of prophase Chromatin condenses into chromosomes Centrioles appear Nucleolus and nuclear membrane disappear Spindle fibers appear Chromosomes spread out as chromatin Centrioles disappear Nucleolus and nuclear membrane reappear Spindle fibers disappear

Telophase & Cytokinesis Two nuclei appear Cytoplasm splits into two cells “Cytokinesis” = cell movement

Differences with Cytokinesis Animal Cell Plant Cell Animal cells are pinched with a cleavage furrow (microfilament) that splits the cell in half Cell wall prevents plant cells from pinching Instead, a cell plate forms in the middle of the cell

Can the daughter cells divide again as they are? Summary Can the daughter cells divide again as they are?

Cell Cycle Mitosis is the shortest part of the cell cycle The larger portion of the cycle is Interphase (nondividing) which consists of three phases G1 – after mitosis, cell must make more organelles to perform functions S – synthesis of DNA (single chromosome to duplicated) G2 – cell makes more organelles & molecules to prepare for division G0 – cell stops dividing (ex. Nerve cell)

Cell cycle regulation

Is it ideal for an organism’s cells to keep dividing forever? Energy expensive Not enough space When should cells stop dividing? Regulation

Cells stop growing when they contact one another How would this apply to a cut? Or to a developing organism?

CELL DIVISION GENES EXAMPLE: Cell division genes can be ________ in case of injury. Cells near injury are stimulated to divide to heal and replace damaged/missing cells and shut off when the repair has been made. turned on

CELL DIVISION GENES Some cells divide frequently (some human skin cells divide once/hour) Some cells divide occasionally (liver cells divide about once/year) Some cells don’t divide once they form (nerve cells)

CELL CYCLE REGULATORS In early 1980’s scientists discovered a protein in dividing cells that caused a ______________to form in _______________ cells Mitotic spindle NON-dividing

CELL CYCLE REGULATORS Levels of this protein rose and fell with the cell cycle so it was named __________ because it seemed to control the cell cycle. A whole family of CYCLINS have since been discovered that regulate the _____________________ in EUKARYOTIC CELLS CYCLIN TIMING of CELL CYCLE

OTHER REGULATORS INTERNAL ______________ REGULATORS Proteins that respond to events inside the cell. Allow cell cycle to proceed only if certain processes have happened EX: Cell can’t enter mitosis until all the chromosomes have been copied

Three Cell Cycle Checkpoints 1st checkpoint: at the end of G1 (checks that the cell has doubled in size) 2nd checkpoint: at the end of G2 (checks that the cell can divide) 3rd checkpoint: at the end of metaphase (checks to see that replicated chromosomes are attached to spindle fibers)

OTHER REGULATORS EXTERNAL ______________ REGULATORS Proteins that respond to events outside the cell. Signals tell cell to speed up or slow down the cell cycle EX: Growth factors stimulate cells to divide Especially important during wound healing and embryo development

EXTERNAL REGULATORS Molecules on the surface of neighboring cells act as signals to slow down or stop the cell’s cycle. These signals prevent excessive growth and keep tissues from disrupting each other.

Cancer cells have lost control of their cell division genes SEM Image by: Riedell CHO (Chinese Hamster Ovary) cells in culture

NO CONTACT INHIBITION Cancer cells don’t stop when they touch nearby they just keep growing! That’s what makes a tumor. See a video http://www.exn.ca/news/images/2000/08/02/20000802-cancer.jpg

Cancer cells Carcinogens Don’t stop dividing Like a “car with no brakes” Can spread to new places (METASTASIS) ______________ are substances that can damage DNA and cause cancer Ex: Cigarette smoke (OR CHEW), Radiation, chemicals in environment, even viruses, Carcinogens

Cancer cells CELL CYCLE p53 Cancer is complicated and can have many causes, but all cancers have one thing in common . . .… They have lost control over their _____________. Many cancers cells have a damaged or defective gene called _____, so they can’t respond to normal cell signals to control their growth. CELL CYCLE p53