Cell Division and Mitosis

Limits on Cell Size

• Cell size is restricted by the surface-to-volume ratio.– When a cell grows, its volume increases

more rapidly than its surface area.– A cell that is too large will not be able to

move materials into and out of the cell.

• When this limit is reached the cell can either stop growing or divide.

Before Cell Division

• Interphase: the cell grows and the chromosomes make copies of themselves.

• This process goes in 3 stages called G1, S, and G2.

• About 90% of the time spent in the cell cycle is during interphase.

• 10% of the Cell cycle is mitosis.

Phases of Interphase

• Much of the time spent in interphase, the cell is carrying out its normal functions for the organism.

• G1= Gap 1 phase, cell grows and organelles replicate.

• S= DNA synthesis, genetic material duplicates

• G2= Chromosomes remain loosely packed and the cell prepares for mitosis.

Interphase

• Nuclear envelope is fully intact.

• Nucleolus is present.• DNA takes the form

of chromatin.

Cell Division

• Cells divide in two steps:–Mitosis: division of nucleus–Cytokinesis: division of the cytoplasm

Prophase

• The DNA condenses into chromosomes.

• The nuclear envelope disappears.

• The centrioles begin to produce a network of protein filaments called the spindle apparatus.

Prophase(Animal Cell)

Chromosomes• Chromosomes are made DNA, just like

chromatin.

• Duplicated chromosomes are composed of two identical sister chromatids.– They are joined at the centromere.

Mitosis

• In mitosis, the chromosomes cannot move around on their own, so the spindle apparatus moves them.

• Construction of the spindle apparatus is the sole job of the centrioles.

Metaphase

• The spindle apparatus lines up the chromosomes along the center of the cell.

Metaphase(Animal Cell)

Anaphase

• The sister chromatids are separated and pulled to opposite ends of the cell.

Anaphase(Animal Cell)

Telophase

• Nucleus separation is complete.

• A nuclear envelope forms around each new nucleus.

• In plants, a cell plate begins to form. This will eventually become a cell wall.

Telophase(Animal Cell)

Cytokinesis

• The cell plate develops into a new cell wall and plasma membrane.

• The cytoplasm of the two cells is completely separated.

Cytokinesis(Animal Cell)

cleavage furrow

Virtual Onion Root Tip Mitosis Lab

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The Cell Cycle Control System

• The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock

• The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

• For many cells, the G1 checkpoint seems to be the most important one.

• If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the other phases and divide.

• If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase

• Density-dependent inhibition causes cells to stop dividing when they run out of space.

• Anchorage dependence causes cells to only divide when they are attached to a layer of connective tissue.

Loss of Cell Cycle Controls in Cancer Cells• Cancer cells do not respond normally to the

body’s control mechanisms• Cancer cells form tumors, masses of

abnormal cells within otherwise normal tissue

Loss of Cell Cycle Controls in Cancer Cells• If abnormal cells remain at the original site,

the lump is called a benign tumor.

• Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form more tumors.