CHAPTER 10 CELL GROWTH AND DIVISION (page241) . *-Cells cannot grow by simply increasing in size because of a limiting factor, the surface to volume ratio .
CHAPTER 10 CELL GROWTH AND DIVISION (page241)
*-Cells cannot grow by simply increasing in size because of a limiting factor, the surface to volume ratio.
*-As a cell enlarges, its volume increases several hundred times faster than its surface area, so the ratio quickly becomes very small.
This results in:
inefficiencies in the cell's internal processes, slowing its growth.
** Cell division solves this problem.
CELL DIVISION- the process by which a cell divides into 2 daughter cells.
In most cases an organism grows because it produces more and more cells.
*The cells of a baby and an adult are the same size, but in the adult there are more of the cells.
LIMITS OF CELL GROWTH
* The most important factor that limits the size of the cell is…its ability to transport materials efficiently into and out of the cell.
**- As a cell increases in size, it becomes increasingly more difficult to transport materials efficiently...this is because the volume of the cell or the amount of internal contents, increases at a much faster rate than the surface area or the size of the cell membrane.
**- Ratio- a comparison between 2 variables expressed mathematically as division.** The surface to volume ratio is obtained by dividing thesurface area by the volume.
* The ratio becomes smaller as the cell increases in size.... so this means that the volume or internal contents of the cell is increasing at a faster rate than the area of the surface
SEE figure 10-2 to illustrate this fact…page 243
A larger cell will have a more difficult time getting nutrients in and wastes out... This is why cells do not grow much larger even if the organism itself does grow larger.
*- Cells reach a point where they do not grow larger even if the organism itself DOES grow larger… WHY? Because they have trouble getting nutrients in and wastes out!
**- DNA is stored in a cells nucleus (in eukaryotic cells) ...When the cell is small... copies of DNA that are stored in the nucleus are able to produce enough mRNA to make all the proteins it needs.
* -However, when a cell increases in size, it does not make extra copies of DNA.
(If the cell grew without limits this would cause an information crisis).
**- In time, the cell's DNA may no longer be able to make enough RNA to supply the increasing needs of the growing cell… so the cell must slow down its growth.
10-2 CELL DIVISION: CELL CYCLE, MITOSIS AND CYTOKINESIS PAGE 244
Cell division involves 2 major stages: mitosis and cytokinesis. (see page245 figure 10-4)The cell cycle is the period from the beginning of one mitosis to the beginning of the next.
MITOSIS- the process by which the nucleus of the cell is divided into 2 nuclei, each with the same number and kinds of chromosomes as the parent cell.
In mitosis the cell is actively dividing.
Mitosis itself is actually divided into four stages: prophase, metaphase, anaphase, and telophase. During interphase(*the majority of the cell’s life cycle!), other cell processes occur.
Interphase-the GROWTH period of a cell in which chromosomes are duplicated. Interphase is divided into three stages known as G1, S, and G2.…
During interphase,cell growth and development occurs, DNA is made and the cell produces some of the material needed for the next cell division.
Mitosis requires only a small percentage of time, so the cell is usually in interphase.
CYTOKINESIS- the stage of (m phase) cell division in which the cytoplasm divides, and forms 2 distinct cells.
- Chromosomes contain the genetic information (DNA) that is passed from one generation to the next…
Chromosomes are only visible during cell division (mitosis).
Mitosis evenly distributes chromosomes.
- In eukaryotic cells, genetic information is carried in the form of DNA. The DNA together with protein forms chromatin.
Chromatin appears as dark places in the nucleus.
Since the DNA is actually many times longer than the chromatin, it is folded so that it can fit into the nucleus.
DNA coils around special proteins called HISTONES, to form bead-like structures called nucleosomes.
The tightly packed nucleosome is important and is separated during mitosis.
The cells of each species contain their own specific number and kinds of chromosomes.
- A human cell entering mitosis contains 46 chromosomes, each which consists of 2 chromatids.
SEE figure 10-3 page 244
**Chromosomes contain 2 chromatids which are often called sister chromatids.Chromatids are attached at an area called the CENTROMERE.Centromeres are usually located near the middle of the chromatids.
CELL CYCLE(already defined as the period of time from the beginning of one mitosis to the beginning of the next).
The main phases of the cell cycle are the M phase (mitosis & cytokinesis), and the G1, S, and G2 phase which occur during interphase.
Interphase is the longest phase of the cell cycle.G1 or GAP 1 phase: a period of activity in which cellular growth and development take place.
S phase or DNA synthesis phase: DNA replication takes place and several proteins are made.In the G2 or Gap 2 phase: Organelles and other materials required for cell division are made.
During interphase, the nucleus is actively making mRNA in order to direct cell activities.
Interphase seems to be a quiet phase but intense activity is still going on even though cellular division is NOT occurring.
During the cell cycle, proteins are made, DNA is copied, ATP is made and used.
CENTRIOLE- 2 tiny structures involved in mitosis that are located in the cytoplasm near the nuclear envelope.
STAGES OF MITOSIS:
1. PROPHASE- the longest phase of MITOSIS (NOT THE LONGEST PHASE OF THE CELL CYCLE). Chromosomes become visible as a result of coiling the chromatin
-Centrioles separate from each other and move to each end of the cell.The condensed chromosome becomes attached to fibers in the spindle at a point near the centromere of each chromatid.
The spindle helps move the chromosome apart. It develops from the centriole.At the end of prophase, the coiled chromosomes become tighter, the nucleolus disappears, and the nuclear envelope breaks down.
2. METAPHASE- the second phase of mitosis, and also the shortest phase of mitosis. -Chromosomes line up across the center or equator of the cell.Microtubules connect the centromere of each chromosome to the poles of the spindle fibers.
3. ANAPHASE- the third stage of mitosis; -The centromeres that join the sister chromatids split and allows the sister chromatids to move to the opposite poles of the cell.
Anaphase ends when the movement of the chromosomes stops.
4. TELOPHASE- The 4th and final phase of mitosis. -Chromosomes uncoil into a tangle of chromatin.-Nuclear envelope forms around each bundle of chromatin to form a nucleus.
-A nucleolus becomes visible in each nucleus. -This is the last of the mitosis phases.
* Cell division has a second part… CYTOKINESIS.
Cytokinesis quickly follows mitosis, this is the division of the cytoplasm into 2 individual cells.
Cytokinesis can take place in a number of ways... *The cell membrane moves inward and pinches the parent cell into 2 identical daughter cells (animal cell).* In plants a cell plate forms half way between each nuclei and gradually develops a separating membrane which will become a cell wall.
10-3 Regulating the Cell Cycle
The rate at which a cell divides is determined by
1.)the nature (type) of the cell itself as well as
2.)the cells circumstances.
It may vary from as little as once every 30 minutes to virtually never.
EX:Cells at the site of an injury to the skin will undergo rapid division to promote the process of healing.
EX:Cells of the heart or nerve cells divide very little if at all.
In single celled organisms, such as bacteria, cell division may occur as frequently as every 20 minutes. In multicellular organisms, different tissue types differ in frequency of growth and division.
Escherichia coli or E. coli is a type of bacterium that divides every 30 minutes...
** Even though they divide rather rapidly, they quickly use up the available nutrient supply and the rate of growth slows.Ideal conditions for this type of growth can never be maintained for very long.
CONTROLS ON CELL GROWTH PG 250
Cells seem to possess internal controls over the process of cell division. As already mentioned cells of the heart and nervous system rarely divide.
**- When isolated cells are cultured, they continue to grow until they make contact with other cells.Cells on the edge of an injury site divide rapidly to aid healing.
When the healing process is near complete, the rate of cell division slows down, and everything returns to normal.
Much of the recent research (1998) on cancer focuses on the mechanism of cell growth and division.
Cancer is caused by the uncontrolled multiplication of groups of cells.
Ovarian Cancer Cells…
Breast Cancer Cells
**- Because the rate of multiplication of cells, along with the cell's function, is controlled by the genetic code contained in DNA,the cancerous growth must be a result of genetic change.
**The genetic code controls the rate of cell multiplication & the cells function. So cancerous growth must be a result of genetic change…a change in DNA.
Cancerous cells do not stop growing. They have lost their ability to control their own rate of growth.
The part of DNA that controls cell growth & division has somehow been changed.