Chapter 10: Cell Growth and Division

Honors Biology/Chemistry2013

Limits to Cell Growth

• As a cell grows larger:– More demands are put onto the cell’s

DNA.– The cell has more trouble moving

enough nutrients and wastes across the cell membrane.

• A cell’s functions are controlled by its DNA.

• As a cell grows, it usually does not make more DNA.

• If the cell were to grow continuously, it would become too large for the DNA to control...this is called “DNA Overload”.

• Materials such as food, oxygen, waste and water pass in and out of a cell through the cell membrane.

• The rate at which materials can pass through the membrane depends on the cell’s surface area.

• The rate at which food and oxygen are used and waste is produced depends on the cell’s volume.

• To maintain high efficiency, cells maintain a large surface area to volume ratio.

• Imagining that cells are cube-shaped, look at the example below:

Which value increases most rapidy?

How does the SA:V ratio change as the cell grows in size?

Division of the Cell• Before a cell gets too large, it divides

forming two daughter cells.

• The process that forms two daughter cells is called cell division.

Learning Check

• Give 2 reasons why cells divide.

• As a cell increases in size, which increases more rapidly: its surface area or its volume?

• Calculate the surface area to volume ratio of a cube with 3 inch sides.

Cell Division

• Occurs differently in different organisms.

Prokaryotes• Lack a nucleus and have a single

chromosome• Reproduce using binary fission:

– Cells increase their cell mass slightly.– DNA and cell components are replicated.– Each cell divides into 2 new daughter

cells.

Eukaryotes

• Go through a much more detailed cell cycle, and divide using a process called mitosis.

The Cell Cycle

• The time period during which a cell grows, prepares for cell division, and divides to form two daughter cells, each of which then begins the cycle again.

• The cell cycle is broken into 2 large phases:– Interphase: Period of growth and preparation

for division. – M Phase (Cell Division): Mitosis and Cytokinesis

Interphase• Prepares the cell to divide.• Consists of 3 smaller phases:

– G1 : Cells increase in size and synthesize new proteins and organelles.

– S: Chromosomes are replicated

– G2: Many organelles and molecules required for division are produced.

• Once interphase is complete, the cell is ready to enter cell division (M Phase).

M Phase(Cell Division)

Mitosis

• Mitosis consists of 4 smaller phases:– Prophase– Metaphase– Anaphase– Telophase

Prophase• First and longest phase of

Mitosis.

• Chromosomes condense and become visible.

• Centrioles move to opposite sides of the nucleus.

• Spindle appears.

• Nucleolus disappears.

• Nuclear envelope breaks down.

Chromosomes(paired chromatids)

Centromere

Spindle forming

• Chromosomes are made up of DNA and protein.

• Before prophase, they are not visible because their thin strands are spread throughout the nucleus.

• During S phase, the chromosomes are replicated.– Once replication has occurred, each chromosome

consists of 2 “sister” chromatids, which are held together at a centromere.

Metaphase• Second phase of

mitosis.• Chromosomes line up

across the center of the cell.

• Spindles attach to the centromere of each chromosome, connecting them to the centrioles and holding them in place. Spindle

Centriole

Anaphase• Third phase of

mitosis.

• The centromeres split allowing the sister chromatids to separate.

• Spindles pull the sister chromatids to opposite sides of the cell.

Individualchromosomes

Telophase• Final phase of Mitosis. • Chromosomes

unravel

• Nuclear envelopes reform

• Nucleolus reappears

• Spindle begins to break apart.

Cytokinesis• Mitosis is considered to be the

division of the nucleus.• After mitosis, two nuclei with

identical sets of chromosomes are present within the cytoplasm of a single cell.

• Cytokinesis is the division of the cytoplasm, which completes M Phase of the cell cycle.

Cytokinesis• Usually occurs simultaneously with

telophase. • In animal cells:

– The cell membrane is pulled inward until the cytoplasm is pinched in equal parts.

• In plant cells:– A “cell plate” forms midway between the two

new nuclei. The plate will eventually develop into a cell wall dividing the two cells.

Learning Check

• Name the main events of the cell cycle.

• What happens during each stage of interphase?

• What are chromosomes made of?

• At the completion of M Phase (Mitosis and Cytokinesis), two identical daughter cells have formed.

• These two daughter cells restart the cell cycle at G1 of interphase.

Regulating the Cell Cycle

•Experiments show that normal cells will continue to grow until they come into contact with other cells.

•When cell’s come into contact with other cells, they stop growing. This is called contact inhibition.

•This demonstrates that cell growth and division can be turned on and off.

Copyright Pearson Prentice Hall

Contact Inhibition

• Proteins called cyclins regulate the timing of the cell cycle.

• Internal regulators: allow the cell to proceed to the next phase of the cell cycle only when certain processes have occurred inside the cell.– Example: These proteins will not allow a cell to

continue into G2until all chromosomes have been duplicated during S phase.

• External regulators: speed up or slow down the cell cycle depending on events outside of the cell.– Example: Contact inhibition

Uncontrolled Cell Growth• Cancer is a disorder in which the

body’s own cells lose their ability to respond to signals from internal and external regulators.

• These cells divide uncontrollably and form tumors.