The Cell cycleCHAPTER 8 PART 2

Cell GrowthAs organisms grow, do they grow because cells

get larger and larger or because more cells are produced?

Cells divide instead of growing larger for two reasons:As a cell grows larger, there are more demands

placed on its DNA.As a cell grows larger, the cell has more trouble

moving enough nutrients and wastes across the cell membrane.

Deoxyribonucleic Acid (DNA) is the genetic information of a cell. In eukaryotes it is found in the nucleus.

A cell typically only has one copy of its’ DNA. Imagine going to the library to use the computers.

As more and more students arrive, there are not enough computers for everyone.

All materials enter and/or leave the cell through the cell membrane.

The rate at which things can pass through the cell (through active or passive transport) is dependent on the surface area of that cell.

The rate at which resources are used up and waste leaves the cell is a dependent on the volume of that cell.

You have to look at the ratio of surface area to volume. The higher it is, the more efficient the cell is. Surface area is calculated as length x width x # of sides.

Let’s look at three cells.If a cube has height of 1 cm what is the surface area? ______.

What is the volume? ______ Therefore the ratio is ______.If a cube has height of 2 cm what is the surface area? ______.

What is the volume? ______ Therefore the ratio is ______.If a cube has height of 3 cm what is the surface area? ______.

What is the volume? ______ Therefore the ratio is ______.

If you think of a town it’s like cars on a main street. If the number of cars is increased, but the road isn’t widened, it becomes more and more crowded.

Cell DivisionBefore a cell grows too large, it will divide into two

“daughter cells”.The process of division is called Cell Division.Before a cell can divide, it must replicate all of its’

DNA. It will begin with one copy and end with two copies.

This replication gives one copy of DNA to each daughter cell.

This also raises the surface area to volume ratio of each cell.

The Cell CycleProkaryotes can just divide into two cells, but

eukaryotes must go through a lot of preparation. Cell division is in three sections:Interphase-The cell grows and replicates DNA.Mitosis-The nucleus divides.Cytokinesis-The cytoplasm and the rest of the cell

divide.

In eukaryotes, DNA is carried in the form of chromosomes. How many chromosomes are present depends on the type of organism.Fruit flies have 8 chromosomes.Humans have 46 chromosomes.

Chromosomes can only be seen in most cells during mitosis.

Before cell division each chromosome is replicated and has two different “sister chromatids”. When the cell divides, the chromosome splits and each daughter cell gets a chromatid.The chromatids are connected in an area called a

centromere.

Cells are always in the cell cycle. It consists of 4 main phases:Mitosis and cytokinesis are in M phase.Two “Gap” phases where the cell grows. They are called G1 phase and G2 phase.

DNA replication occurs in S phase.

Events of the Cell CycleFirst, in G1 phase, cells grow and make new

organelles and proteins.Second, in S phase, the chromosomes are

replicated. Usually a cell proceeds to division if it enters S phase.

Third, in G2 phase, many of the organelles and molecules needed for cell division are produced. It is the shortest of the 4 phases.

All three of these phases take place in Interphase.

MitosisThe fourth stage is mitosis. It consists of 5

stages:ProphasePrometaphaseMetaphaseAnaphaseTelophase

After these cytokinesis occurs.

In prophase, the DNA condenses into chromosomes. The centrioles separate and a spindle begins to form.

In prometaphase the nuclear envelope

breaks down and the centrioles begin to stretch out their spindles to find the chromosomes.

In metaphase the chromosomes line up in the middle of the cell and a spindle fiber connects to each centromere.

In anaphase the sister chromatids are pulled

apart towards opposite ends of the cells.

In telophase the chromosomes gather at

opposite ends of the cell and revert to their usual form (chromatin). Two new nuclear envelopes begin to form.

Totally lame mitosis song!

CytokinesisThe cytoplasm pinches in half and two daughter

cells are formed. Each daughter cell has a duplicate set of chromosomes.In animal cells the cell membrane pinches inward

until two equal parts are formed.In plant cells a structure known as a cell plate

forms between the nuclei and develops into a separating membrane. The cell wall then forms and spreads outwards.

Controlling the Cell Cycle Proteins called cyclins regulate the timing of the cell cycle in

eukaryotic cells. They can be divided into two groups:

Internal Regulators are proteins that respond to events inside of the cell. For example: making sure that the cell does not proceed to G2 phase or mitosis unless all of the chromosomes have been replicated.

External Regulators are proteins that respond to events outside of the cell. For example: if there is not enough space outside of a cell for others to exist, the cell may not enter or proceed past G1 phase.

This is also why when a scab forms over a cut it does not rise much above the surface of the skin. It limits new cells to within a certain distance of the others.

G0 phaseG0 phase is when a cell is in a dormant stage.

This occurs for some cells such as heart or nerve cells which are not replicated and typically survive for a long time.

It can also occur for cells that do not have enough resources to enter cell division.

It typically occurs before cytokinesis or after G1 depending on the type of cell.

Uncontrolled Cell Growth In uncontrolled cell growth, the checkpoints are

bypassed or do not work correctly and the cells grow and divide whether or not they are ready.

Cancer is when some of a body’s cells lose the ability to control growth.The masses of cells formed by the uncontrolled growth are

called tumors. These can damage surrounding tissues.When cancer metastasizes, cancer cells break loose from the

tumor and spread throughout the body affecting other areas of cells.

The loss of control can be caused by many different factors depending on the type of cancer such as:TobaccoRadiation ExposureViral InfectionsDamage to the p53 gene which halts the cell cycle until all

the chromosomes have replicated.

Tumors can be:Benign: not usually caused by cancer. It does not spread to

surrounding healthy cells or tissue.Malignant: cancerous cells that can invade and destroy

surrounding healthy cells or tissue.