Limits to Cell Growth The 2 reasons cells divide rather than continue to grow indefinitely are- The demands on the cells DNA become too great and the DNA may “overload” (refer to the library analogy) The rate of food/oxygen consumption and waste production is directly related to the cells volume. This exchange occurs through the cell membrane, (surface area). The surface area of a cell does not increase at the same rate as the volume does if the cell simply continues to grow.

Ratio of Surface Area to Volume Cube 1cm x 1cm x 1cm has a Surface area of 6cm2 and a volume of 1cm3. Ratio is 6:1 Cube 2cm x 2cm x 2cm: Surface area=24cm2, Volume of 8cm3. Ratio is 3:1 Cube 3cm x 3cm x 3cm: Surface area = 54 cm2, Volume of 27cm3. Ratio is 2:1 Bottom line: as the cube (cell) increases in volume, the surface area does NOT keep up accordingly. This causes a big problem when the cell relies on the cell membrane for materials exchange. Refer to the Town/highway analogy

Surface area to volume ratio

Cell Division When a cell divides into 2 cells, this is called cell division and produces 2 “daughter” cells Before this happens, all of the DNA must be copied to provide the new cell with information Cell division also solves another problem. That it keeps the surface area/volume ratio to a reasonable level.

Chromosomes Chromosomes are made up of DNA, and are responsible for all of a cells genetic information The cells of every organism have a specific number of chromosomes. (cells of fruit flies-8 chromosomes, cells of carrots-18 chromosomes, cells of humans-46 chromosomes) Prior to cell division, each chromosome is replicated. During this process, the chromosomes become visible and consist of 2 identical “sister” chromatids attached to a centromere

Sister Chromatids attached to a centromere

Cell Cycle-the series of events that a cell goes through as they grow and divide During the cell cycle, the cell grows, prepares for division, and divides to form 2 daughter cells, each of which goes through the same steps.

Interphase G1 phase-the cells does most of it’s growing in G1 In G1, cells increase in size and synthesize new proteins and organelles Next is the S phase. In the S phase, chromosomes are replicated and the synthesis of DNA occurs G2 phase follows after S phase is completed. G2 is the shortest of the parts of Interphase. During G2, many organelles and molecules required for cell division are produced.

Mitosis-divided into 4 phases Phase 1-Prophase Prophase-first and longest phase of mitosis (50-60% of the time of mitosis is spent here) Centrioles become visible Centrioles lie in a region called the centrosome that helps in organizing the spindle. The spindle helps separate choromsomes At the end of prophase, the chromosome become more tightly coiled, the nucleolus disappears and the nuclear membrane breaks down

Metaphase-2 nd phase of mitosis Metaphase lasts only a few minutes Chromosomes line up across the center of the cell Microtubules connect the centromere of each chromosome to the poles of the spindle

Anaphase-3 rd phase of mitosis Centromeres that join sister chromatids separate, allowing chromatids to separate and become individual chromosomes Chromosomes continue to separate into 2 groups near the poles of the spindle Anaphase is over when the chromosomes stop moving

Telophase-4 th and final mitotic phase Chromosomes begin to disperse Nuclear envelope re-forms around each chromosome cluster Spindle begins to break apart Nucleolus becomes visible in each new daughter cell nucleus Mitosis in complete, but cell division is not complete

Cytokinesis The cytoplasm of the cell must separate The cytoplasm is “pinched off” in 2 nearly equal parts In plant cells a cell plate forms midway between the 2 new nuclei

How long does cell division take? Sea urchin cells-2 hours Many plant & animal cells-8-10 hours Bean cells-19 hours Mouse cells-22 hours Human nerve and RBC never divide

How long do cells live? Lining of esophagus:2-3 days, (can divide) Lining of small intestine:1-2 days, (can divide) Lining of large intestne:6 days (can divide) RBC: Less than 120 days (cannot divide) WBC: 10 hours to decades (cannot divide) Smooth muscle: long lived (can divide) Cardiac (heart) muscle: long lived (cannot divide) Skeletal muscle: long lived (cannot divide) Neuron(nerve cell) long lived (most do not divide)

Controls of Cell Division Cyclins are proteins in cells that regulate the timing of the cell cycle in eukaryotic cells Internal regulators are proteins that respond to events inside the cell External regulators are proteins that respond to events outside the cell

Uncontrolled cell growth Certain cells do not respond to the signals that regulate cell growth. Tumors are masses of cells that can damage surrounding tissue. Cancer may result when cells break loose from tumors, spread throughout the body, disrupting normal activities, causing serious problems potentially.

Normal cells compared to cancer cells

Stem Cells All cells in the human body come from one original cell The first few daughter cells can potentially differentiate into any type of body cell Most cells become differentiated prior to birth Once a cell becomes differentiated, it cannot change what it is (skin cells remain skin cells, etc) Non-specialized cells that can become other types of cells are called stem cells Stem cells can be used to grow other cells needed for repair in the body =CLT8wqup8qACFQHX5wodKQ5IIw =CLT8wqup8qACFQHX5wodKQ5IIw

Stem Cells