WHY DO CELLS DIVIDE?

IT’S ALL ABOUT SIZE

THERE IS AN UPPER LIMIT TO HOW LARGE A CELL CAN BE

WHY DO CELLS DIVIDE? 1.Diffusion

WHY DO CELLS DIVIDE? 1.Diffusion 2.DNA

WHY DO CELLS DIVIDE? 1.Diffusion 2.DNA 3.Surface area-to-volume ratio

WHY DO CELLS DIVIDE? 1.Diffusion –A cell must move all materials needed for functioning via diffusion through cytoplasm

WHY DO CELLS DIVIDE? 1.Diffusion –A cell must move all materials needed for functioning via diffusion through cytoplasm –It takes a fraction of a second for materials to move from the plasma membrane to the center of a typical cell with a diameter of 20 µm

WHY DO CELLS DIVIDE? 1.Diffusion –A cell must move all materials needed for functioning via diffusion through cytoplasm –It takes a fraction of a second for materials to move from the plasma membrane to the center of a typical cell with a diameter of 20 µm –If a cell were 20 cm in diameter, it would take months to move materials from the plasma membrane to the center of the cell

WHY DO CELLS DIVIDE? 2.DNA –A cell must respond to changing conditions rapidly

WHY DO CELLS DIVIDE? 2.DNA –A cell must respond to changing conditions rapidly –A cell frequently needs to produce large amounts of proteins and enzymes

WHY DO CELLS DIVIDE? 2.DNA –The process by which proteins and enzymes are made is complicated DNA makes an RNA copy of the gene to be copied RNA leaves the nucleus and travels to a ribosome RNA directs protein production at the ribosome

WHY DO CELLS DIVIDE? 2.DNA –If there is not enough DNA to rapidly carry out RNA production, the metabolism will slow and the cell will die

WHY DO CELLS DIVIDE? 2. DNA Larger cell = greater metabolism

WHY DO CELLS DIVIDE? 2. DNA Larger cell = Greater metabolism Greater metabolism = More DNA needed to direct metabolism

WHY DO CELLS DIVIDE? 3.Surface area-to-volume ratio –As the size of any object increases, volume increases at a much greater rate than does surface area

WHY DO CELLS DIVIDE? 3.Surface area-to-volume ratio –As the size of any object increases, volume increases at a much greater rate than does surface area –A cell receives all the materials needed for life through its plasma membrane

WHY DO CELLS DIVIDE? 3.Surface area-to-volume ratio –Larger organisms (or cells) need more “stuff”: you require more food than does a two-year-old human

WHY DO CELLS DIVIDE? 3.Surface area-to-volume ratio –Larger organisms (or cells) need more “stuff”: you require more food than does a two-year-old human –As the volume of a cell increases, it needs more and more materials to be passed through the plasma membrane in order to sustain life

WHY DO CELLS DIVIDE? 3.Surface area-to-volume ratio –At some point, there will not be enough surface area on the plasma membrane to absorb the required amount of materials necessary for life

Where do we go from here?

The Cell Cycle Definition: Series of synchronous (sequential) biochemical and physical processes that result in the increase in cell mass, DNA replication, and mitosis. Basis of all growth

G1 (Gap 1) Phase Length of time varies among different cell types and stages of development (typically ~12 hours). This is when proteins for growth and operation are produced. Cells with special operations spend more time here. Accounts for differences in length of cell cycle Rapidly dividing (skin and linings) & embryonic cells spend little or no time in G1. Associated with major cell cycle checkpoints Restriction point

RESTRICTION POINT FIRST DESCRIBED ALMOST 50 YEARS AGO BY ARTHUR PARDEE “POINT OF NO RETURN” IN CELL CYCLE PROGRESSION

S (Synthesis) Phase DNA synthesis/DNA replication occurs. S phase enzymes (enzymes that function in DNA synthesis/DNA replication) are targets of chemotherapeutic agents.

G2 (Gap 2) Phase Relatively poorly defined. Relatively poorly defined. Usually thought to have organelle production Usually thought to have organelle production Increase in cell volume during this time frame between S phase and M phase (mitosis). Increase in cell volume during this time frame between S phase and M phase (mitosis).

Interphase – the basic cell life that includes the majority of the life span of a normal cell.

Prophase – -DNA comes together with protein material to condense into long duplicated threadlike chromosomes ANIMAL CELLS: -Centrioles migrate to the poles -Spindle fibers begin to form

CHROMOSOME Sister chromatids are identical copies of one another

CENTRIOLES AND SPINDLE FIBERS CENTRIOLES SPINDLE FIBERS

Metaphase – -Chromosomes line up on the cell equator -Spindle fibers attach to sister chromatids at the centromere

Anaphase – -Chromosomes separate and migrate to opposite poles -Separation occurs due to shortening of the spindle fibers

Telophase – -Nuclei reform around the migrated chromosome groups PMAT Prophase Metaphase Anaphase Telophase

Cytokinesis – -Cell protoplasmic mass divides into two new cells -Occurs differently in plant and animal cells Plant cells: By forming a cell plate Animal cells: By forming a cleavage furrow that pinches off two new cells

Mitosis in action……..

(h) ( i ) Plant or Animal? Cell equator ? ???? ???

Correct responses (a) Interphase; centrioles (b) Early P rophase (c) Late prophase (d) M etaphase (e) A naphase (f) T elophase (g) Gap 1 (G1) (h) Cleavage furrow (i) Animal cells

Cancer – when it all goes crazy! Initiated & progresses due to genetic alterations Proto-oncogene: ( NORMAL, CELLULAR) genes that encode proteins that function in pathways that promote cell growth (cyclins, CDKs) Oncogene: (mutated proto-oncogene) mutated growth-promoting genes causing enhanced or unregulated growth. Causes hyperplasia & cancer Tumor suppressor gene: genes that encode proteins that inhibit or block cell cycle. Can be altered so that they no longer function, and cause cancer.

The Cell Cycle Definition: Series of synchronous (sequential) biochemical and physical processes that result in the increase in cell mass, DNA replication, and mitosis. Basis of all growth