The Cell cycle CHAPTER 8 PART 2

Cell Growth  As 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 Division  Before 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 Cycle  Prokaryotes 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 G 1 phase and G 2 phase.  DNA replication occurs in S phase.

Events of the Cell Cycle  First, in G 1 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 G 2 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.

Mitosis  The fourth stage is mitosis. It consists of 5 stages:  Prophase  Prometaphase  Metaphase  Anaphase  Telophase  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!

Cytokinesis  The 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 G 2 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 G 1 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.

G 0 phase  G 0 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 G 1 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:  Tobacco  Radiation Exposure  Viral Infections  Damage 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.