Cell Growth and Division

Cell Growth• Limits to growth– Stress on DNA – Difficulty moving nutrients/wastes

across membrane• Ratio of surface area to volume• Volume increases at a faster rate than

surface area

Cell Division

• Before a cell becomes too large, it divides, producing 2 daughter cells– Each daughter cell is an exact replica of the

parent cell

• Before the cell divides, the DNA is replicated, so each new cell will have the same genetic information as the parent cell

Cell Division

• 2 stages (eukaryotes)– Mitosis- division of the cell nucleus– Cytokinesis- division of the cytoplasm

• Asexual reproduction (esp for unicellular organisms)

• Also used for new cells as an organism grows and develops

Chromosomes

• Threadlike structure within the nucleus containing the genetic information (DNA) that is passed from one generation of cells to the next

• Cells of every organism have a specific number of chromosomes– Fruit flies = 8; Humans = 46; carrots = 18

Chromosomes

• Chromatin- granular material visible within the nucleus; consists of DNA tightly coiled around proteins

• Chromatid- one of two identical “sister” parts of a duplicated chromosome

• Centromere- an area where the chromatids are attached to one another

The Cell Cycle

• 2 major phases– Interphase– Mitosis

Interphase

• 3 phases– G1 phase= cells do most

of their growing• Increase in size and

synthesize new proteins and organelles

– S phase= chromosomes are replicated and the synthesis and DNA molecules takes place

• Usually if a cell enters S phase and begins replication, it completes the rest of the cycle

– G2 phase= many of the organelles and molecules required for cell division are produced

• Shortest of the 3 phases of interphase

Mitosis

• Divided into 4 phases– Prophase– Metaphase– Anaphase– Telophase

• Followed with Cytokinesis

• Depending on cell- may last a few minutes to several days

Prophase• 1st and longest phase

of mitosis• Events

– Chromosomes become visible– Centrioles separate and move to opposite sides

of the cell– Chromosomes become attached to fibers in the

spindle at the centromere– Chromosomes coil more tightly– Nucleolus disappears – Nuclear envelope breaks down

Metaphase• Often lasts only a

few minutes

• Events– Chromosomes line

up across the center of the cell

– Microtubules connect the centromere of each chromosome to the two poles of the spindle

Anaphase

• Centromeres split

• Sister chromatids separate and move to opposite poles

• Anaphase ends when chromosomes stop moving

Telophase

• Chromosomes begin to disperse into a chromatin

• Nuclear envelope re-forms around each cluster of chromosomes

• Spindle begins to break apart

• Nucleolus becomes visible

Cytokinesis

• Occurs at the same time as telophase

• Animal cells:– Cell membrane is drawn inward

until the cytoplasm is pinched into 2 nearly equal parts

• Plant cells:– Cell plate forms midway between

the divided nuclei– Cell wall begins to appear in the

cell plate

• Result? 2 new identical cells

Controls on Cell Division

• Effects of controlled cell growth can be seen by placing some cells in a petri dish containing nutrient broth

• Cells grow until they form a thin layer covering the bottom of the dish

• Cells stop growing when they come into contact with other cells

• If cells are removed, the remaining cells will begin dividing again

• Something can turn cell division on or off

Regulating Cell Growth

• Cyclins- proteins that regulate the timing of the cell cycle in eukaryotic cells– Internal regulators: proteins that respond to

events inside the cell• i.e. make sure all chromosomes have been

replicated; make sure all chromosomes are attached to the spindle before entering anaphase

– External regulators: proteins that respond to events outside the cell

• i.e. embryonic development; wound healing

Uncontrolled Cell Growth

• Cancer- disorder in which some of the body’s own cells lose the ability to control growth

• Cancer cells do not respond to the signals that regulate the growth of most cells

• P53 gene halts the cell cycle until all chromosomes have been properly replicated– A damaged or defective p53 gene causes the cells to

lose the information needed to respond to signals that would normally control their growth