Chapter 10 – Cell Growth and Division

Section 10-1: Cell Growth

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 againThe 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

Mitosis is a phase in the cell cycle, but is also broken up into 4 phases;

prophase, metaphase, anaphase, and telophase

Towards the end of prophase, the nucleous disappears and the nuclear envelope breaks down

Prophase: the first and longest phase of mitosisChromosomes 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

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

The separated chromosomes start to disperse (they aren’t dense any more)

Telophase: the last phase of mitosis

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 cellsCytokinesis 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

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

Neighboring cells help to slow down cell growth so that tissues don’t disrupt each other

Growth factors are important external regulators they help with embryonic development and healing

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