Cell Growth

Limits to Cell GrowthCells divide for two main reasons:•DNA overload•Exchanging materials gets too difficult

– DNA overload – as a cell increases in size, it usually does not make extra copies of DNA and this places more demands on the DNA. If a cell increases with unlimited growth, an information crisis would occur.• The cell would no longer be able to serve the needs of

the growing cell.

– Exchanging Materials• The rate at which food and oxygen are used up and

waste products are produced depends on the cell’s volume. The larger the cell, the more quickly it uses food and oxygen.• The rate at which the exchange of materials takes place

depends on the surface area of the cell membrane. The larger the cell grows, the more difficulty it has moving materials across its cell membrane.

– Ratio of Surface Area to Volume• Volume increases more rapidly than does the surface

area.• If a cell got too large, it would be more difficult to get

sufficient amounts of oxygen and nutrients in and wastes out.• Large surface area to volume ratio is most beneficial

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

two “daughter” cells. This is called cell division.–Before a cell divides, the cell replicates, or

copies, it’s entire DNA.– This replication of DNA solves the problem of

information storage because each daughter cell gets one complete set of genetic information.

Cell Division

Key Concepts:• What are the main events of the cell cycle?• What are the four phases of mitosis?

There are two main stages of cell division in eukaryotic cells.

• Mitosis• Cytokinesis

• Mitosis is classified as asexual, since the cells produced (two daughter cells) are genetically identical to the parent cell (identical number of chromosomes).

• Mitosis is also a source of new cells when a multicellular organism grows and develops.

Cell Division

In eukaryotic cell, the genetic information of the cell is carried on chromosomes. Chromosomes are made of DNA.

Just before cell division, the chromosomes replicate, or copy themselves, creating identical sister chromatids, joined by a centromere.

Chromosomes

– The genetic information carried on from one generation of cells to the next is by chromosomes.

– Chromosomes are made up of DNA and protein.– Human cells have 46 chromosomes.– Most chromosomes are not visible in most cells

except during cell division.

Chromosomes (cont.)

– At the beginning of cell division, the chromosomes condense into visible structures.

– Before cell division occurs, each chromosome replicates.

– Each chromosome consists of two identical “sister” chromatids separate from each other.

– Each pair of chromatids is attached to an area called the centromere.

The Cell Cycle

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

Interphase

– The cell cycle consists of four phases of which the first three are grouped together and called INTERPHASE (G1, S, G2) followed by M phase (mitosis).• G1 phase (gap 1)• S phase (synthesis)• G2 phase (gap 2)• M phase

Events of Interphase– G1 phase: period of activity in which cells do most

of their growing. Cell increases in size and synthesize new proteins and organelles.

– S phase: chromosomes are replicated: the synthesis of DNA molecules takes place. Key proteins made.

– G2: shortest phase of three phases of Interphase. Many of the required organelles and molecules required for cell division are produced.• When the events of the G2 phases are completed, the

cell is ready to enter the M phase and begin the process of cell division.

MitosisVideo Review

Mitosis

– There are four phases: prophase, metaphase, anaphase, and telophase.

– Depending on the type of cell, the four phases may last from a few minutes to several days.

Prophase

– Prophase• The first and longest phase of mitosis.• The chromosomes become visible.• The centrioles, two tiny structures located in the

cytoplasm near the nuclear envelope, separate and take up positions on opposite sides of the nucleus.• Spindle fibers begin to form.• The nuclear envelope breaks down and the

nucleolus disappears.

Metaphase• Often lasts only a few minutes.• The chromosomes line up across the center of the cell.• Microtubules connect the centromere of each

chromosome to the two poles of the spindle.

Anaphase• The third phase of mitosis.• The centromeres that join the sister chromatids split,

allowing the sister chromatids to separate and become individual chromosomes. The chromosomes move until they have separated into two groups near the poles of the spindle. Anaphase ends when the chromosomes stop moving

Telophase• The fourth and final phase of

mitosis. The chromosomes begin to disperse into a tangle of dense material.

• A nuclear envelope re-forms around each cluster of chromosomes.

• The spindle begins to break apart, and a nucleolus becomes visible in each daughter nucleus.

• Mitosis is complete.

Mitosis

• The 4 phases of mitosis are, from beginning to end: prophase, metaphase, anaphase, telophase.

• Remember… PMAT! or… The entire cell cycle is abbreviated IPMAT! (but Interphase isn’t part of mitosis)

Cytokinesis

– The division of the cytoplasm occurs.– Cytokinesis usually occurs at the same time as

telophase.– In animal cells, the cell membrane is drawn

inward until the cytoplasm is pinched into two nearly equal parts.

– In plant cells, a cell plate forms midway between the divided nuclei. The cell wall then begins to appear in the cell plate.

Video

Mitosis

• Mitosis produces 2 identical daughter cells from each parent cell.

• If a parent cell had 4 chromosomes, how many chromosomes would each of the 2 new daughter cells have?

• Human cells have 46 chromosomes. When a muscle cell undergoes mitosis, how many chromosomes will each daughter cell it produces have?

Regulating the Cell Cycle

• How is the cell cycle regulated?• How are cancer cells different from other

cells?

Controls On Cell Division

– Cells in a Petri dish will continue to grow until they come in contact with other cells.

– If the cells are removed from the center of the Petri dish, the cells bordering the empty space will begin dividing. The controls on cell growth can be turned off and on.

– When you have an injury such as a cut in the skin or a break in a bone, cells will begin to divide rapidly. This produces new cells that will help in the healing process.

Cell Cycle Regulators– It was discovered that cells in mitosis contained a protein that when

injected into a non-dividing cell, would cause mitotic spindle to form.– Cyclin and kinase are proteins that seemed to regulate the cell cycle.– Cyclins regulate the timing of the cell cycle in eukaryotic cells.– Internal Regulators

• Proteins that respond to events inside the cell.• Allow the cell cycle to proceed only if certain processes have happened inside

the cell.• For example, several regulatory proteins make sure that a cell does not enter

mitosis until all chromosomes have been replicated.– External Regulators

• Proteins that respond to events outside the cell.• Direct cells to speed up or slow down the cell cycle.• Growth factors are important external regulators.• They stimulate the growth and division of cells.

Uncontrolled Cell Growth– Cancer – 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.– They divide uncontrollably and produce masses called

tumors.– The control over the cell cycle has broken down.– Some causes of cancer include smoking tobacco, radiation

exposure, and even viral infections.– A large number of cancer cells have a defect in a gene

called p53, which normally halts the cell cycle until all chromosomes have been properly replicated.

– Cancer is a disease of the cell cycle.