Mitosis. Cell Size Limits Why aren’t we just one giant cell??

Mitosis

Cell Size Limits

• Why aren’t we just one giant cell??

3 limitations to cell size

1. Diffusion- - is fast and efficient over short distances but

slow and inefficient over long distances - cells would die long before nutrients could

reach the organelles

3 limitations to cell size

2. DNA limits cell size- cells need A LOT of proteins to perform critical

functions BUT there is a limit to how quickly DNA can be made into proteins.

3 limitations to cell size

3. Surface area-to volume ratio- as a cell’s size increases, its volume increases

much faster than its surface area

Reasons for cell division

1. maintaining optimum cell size2. growth in multicellular organisms3. asexual reproduction of single-celled

organisms4. to replace dead cells

Chromatin vs. Chromosome

• Chromatin = long strands of DNA wrapped around proteins (tangled up spaghetti)

• Chromosome = coiled strands of DNA that

look more like rope or X’s; becomes like this right before cell division

Cell cycle

• G1→G2 = interphase• S phase = DNA replication• G1 & G2 = growth 1 & 2, cell is growing and

preparing for division• Which phase does a cell spend most of its

time in?

Mitosis

• Mitosis = process in which the nucleus of a cell is divided into 2 nuclei, each with the same number and kinds of chromosomes as the parent cell (also called asexual reproduction)

Interphase

• First step of the cell cycle.

• Interphase is the preparation phase.

• Three stages: G1, Interphase and G2.

• What happens during interphase?

Mitosis

• Mitosis is the second step of the cell cycle (asexual reproduction)

• It is broken into four phases: prophase, metaphase, anaphase and telophase.

Prophase

• First stage of mitosis.• Nuclear envelope

begins to dissappear• Chromatin coils up and

becomes X-shaped chromosomes

Metaphase

• Chromosomes (X’s) line up in the middle of the cell.

• Spindle fibers attach to the centromere on the chromosome.

Anaphase

• Spindle fibers begin to pull the chromosomes (X’s) apart.

• Chromatid (V’s) now move toward opposite ends

Telophase

• Chromatid (V’s) reach the poles.

• Nuclear envelope begins to reappear around the chromatid

• In plant cells a cell plate appears down the middle to divide the cell.

• In animal cells a cleavage furrow appears to separate the cell.

Cytokinesis

• Last stage of the cell cycle.

• Cell has separated creating two new identical daughter cells.

Cytokinesis

• Cytokinesis= the process by which the cytoplasm divides, thus forming 2 distinct cells

Results of Mitosis• 2 identical daughter cells• In unicellular organisms that is how it reproduces• In multicellular organisms, cell growth and

division results in a group of cells that work together (all human cells undergo mitosis except for our sex cells). Skin cells undergo mitosis the most.cells tissue organ organ system ORGANISM

Cancer

• Enzymes control each phase of the cell cycle.

• Cancer = result of uncontrolled cell division

• Extra cells form masses called tumors

• Causes of cancer are both genetic and environmental (smoke, exposure to UV rays, viral infections)