Chapter 10 – Cell Growth and Division Section 10-1: Cell Growth

There are 2 main reasons why cells divide instead of getting bigger; Organisms become larger because they produce more cells, not because the cells themselves get bigger There are 2 main reasons why cells divide instead of getting bigger; DNA & nutrient and waste movement

1. The larger the cell = the more demands on the cell’s DNA The cell only has so much DNA and when the cell is small it can meet all of its needs More DNA is usually not created even if a cell gets larger, so the initial amount of DNA isn’t enough to meet the larger needs

2. The larger the cell = the more trouble moving nutrients and waste across the cell membrane Exchange of food, water, waste, and oxygen depend on the cell membrane (the surface area) The rate at which cells use food and produce waste depend on the cell’s volume, not the surface area

Volume increases faster than surface area does, so eventually if the cell got large enough, there would be more volume than surface area and it wouldn’t be able to regulate food and waste properly.

Before a cell becomes too large to function, the cell divides forming two “daughter” cells Cell division = the process of dividing to form two “daughter cells” Before division, a cell will copy all of its DNA to make sure the two new cells have the same DNA

Section 10-2: Cell Division

In cell division, each daughter cell needs to have the same contents  this takes some prep work In prokaryotes, cell division is pretty simple: just separate the cell contents into two equal parts

In eukaryotes, cell division is more complex and happens in two main stages; mitosis and cytokinesis Mitosis occurs in the nucleus of the cell It is the source of new cells when an organism grows and develops Cytokinesis occurs in the cytoplasm

Chromosomes are important when it comes to passing on DNA to daughter cells Chromosomes are made up of DNA  it carries the cell’s genetic code The number of chromosomes present is important, each type of organism has a specific number of chromosomes

Normally, DNA is spread through the nucleus, but during cell division chromosomes condense Before cell division happens, each chromosome is replicated  this means that each chromosome consists of two “sister” chromatids

Since “sister” chromatids contain the same information, during cell division the “sister” chromatids separate into 2 different cells Centromeres = what keep the chromatids attached to each other before division

The cell cycle is the series of events that cells go through as they grow and divide During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells, which both start the cycle over again The cell cycle consists of 4 phases; M phase, G1 phase, S phase, and G2 phase

1. M phase: mitosis and cytokinesis take place 2. G1 phase: the cell experiences a good deal of cell growth (an increase in size, new organelle production) 3. S phase: chromosomes are replicated (synthesized) 4. G2 phase: the shortest phase, the cell prepares for mitosis to happen The G1, S, and G2 phases all make up interphase = a period in between cell divisions

prophase, metaphase, anaphase, and telophase Mitosis is a phase in the cell cycle, but is also broken up into 4 phases; prophase, metaphase, anaphase, and telophase

Prophase: the first and longest phase of mitosis Chromosomes become visible as they start to condense Centrioles separate and take a position on opposites sides of the nucleus Spindle fibers start to form The centromeres of each set of chromotids attach to the spindle fibers Towards the end of prophase, the nucleous disappears and the nuclear envelope breaks down

Metaphase: the second phase that may only last a few minutes Chromosomes line up across the center of the cell

Anaphase: the third phase of mitotis The centromeres connecting sister chromatids split  the sister chromatids can now separate and become their own chromosome The chromosomes move away from each other and move toward the centrioles When the chromosomes stop moving, anaphase ends

Telophase: the last phase of mitosis The separated chromosomes start to disperse (they aren’t dense any more) The spindle breaks apart A nuclear envelope starts to form around each cluster of chromosomes

Cell division and the cell cycle end with cytokinesis Cytokinesis = the division of the cytoplasm of the cell to create the two new cells Cytokinesis usually happens at the same time as telophase

In animal cells, the cell membrane gets pinched in and creates 2 cells In plant cells, a cell plate forms between the nuclei  it eventually turns into a separating membrane

Regulating the Cell Cycle Section 10-3: Regulating the Cell Cycle

Cell growth and division are controlled very carefully in the human body Not all cells move through the cycle at the same rate, and some developed cells do not even undergo division

Controls for cell division can be turned on and off Cells grown in a culture will continue to grow until they come into contact with another cell  at that point they stop growing If some of the cells are removed, the cells respond by growing again to fill in the gaps The idea of control can be seen in the human body  when you get a cut, your cells respond by rapid growth to fill in the cut, or heal

Cyclins regulate the timing of the cell cycle in the eukaryotic cell Cyclins are proteins that rise and fall in numbers, depending on where a cell is in the cell cycle If you inject cyclin into a nondividing cell, it will start to divide

More recently, dozens of other proteins were found that regulate the cell cycle They are separated into two categories; those that occur inside the cell and those that occur outside the cell

Internal regulators are found inside the cell and basically serve to double check each step of mitosis

External regulators are found outside the cell and help speed up and slow down the cell cycle Growth factors are important external regulators  they help with embryonic development and healing Neighboring cells help to slow down cell growth so that tissues don’t disrupt each other

Cancer cells do not respond to the signals that regulate the growth of most cells Cancer cells divide uncontrollably and form tumors The control of the cell cycle is broken down by something (anything from smoking, to excessive radiation, to viral infections)

A lot of cancer cells have a defect in a gene called p53  it normally stops the cell cycle until all chromosomes have been replicated Cancer is an example of why the cell cycle is so important