Cells:

The Building Blocks of Life

The Cell Theory
1. All living things are made of cells.
2. Cells are the basic units of life.
3. Cells come only from other cells.


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Parts of the cell cycle:
G1 phase:

* Sometimes called Interphase.

* The formation of two daughter cells from a parent cell in cytokinesis is an energy-consuming process. The time needed by the new cells to replenish their energy causes this "gap".

* After cell energy reserves are restored, the daughter cells begin to grow.
* The G1 phase is the longest phase of the cell cycle. It is during this phase that specialized cells in higher organisms perform their major function.

S phase;


* Synthesis means to make.
* After a period of growth, each cell's original DNA is duplicated in the process of replication.

G2 phase:


* The replication of DNA is another energy-consuming process for the cell. The time taken by a cell to build up its energy reserves produces this second "gap".

                 
       
                     
                                       
   

Mitosis:

The process by which the DNA is divided into two identical sets before the cell divides.

 
                                       
 
 

Early prophase: chromatin coils to form chromosomes, nuclear membrane disappears, and centrioles migrate to opposite poles of the cell.
* Middle prophase: spindle fibers, microtubules of protein, begin developing.

* Polar fibers extend across the cell from centriole to centriole.
* Kinetochore fibers extend from the centromeres of a chromosome to the centrioles.

* Late prophase: protein fibers called asters radiate from each centriole in animal cells.

Kinetochore fibers move the chromosomes to the equator of the cell
The centromere of each pair of chromatids divides, the chromatids separate and are pushed to opposite poles of the cell by the combined efforts of the spindle fibers.
This movement is done in one quick motion.

All spindle fibers disappear, chromatids unwind and stretch to form chromatin, and a nuclear membrane forms around each mass of chromatin.

           
                       
                                       
    Cytokinesis:
* Cytokinesis is the division of the cytoplasm of a parent cell. The cell pinches together roughly in the center.
* You may either consider it the end of Mitosis or the beginning of Interphase.
* While each daughter cell gets identical sets of DNA, the cytoplasm and organelles are only roughly divided equally between the two cells. 		       
                                       
                                 
     

In any fast growing tissue, cells will be found in all phases of the cell cycle.

This is a picture of the very fast growing tissue of an onion root tip.

Notice the different size of the cells and the obvious mitosis phases.

         
                                 
   
Meiosis occurs only in gametes:  
     
Meiosis is the process of nuclear division that reduces the number of chromosomes by half.  

Two nuclear divisions: These are similar to, but different from the single division of mitosis. The process is complicated, providing more opportunity for genetic variation like crossing-over to occur. For our purposes, this summary of events is sufficient.

Meiosis I (1st cell division)

* DNA replication takes place.
* A parent cell produces two daughter cells with one member of each original pair of homologous chromosomes.


Meiosis II (2nd cell division)

* There is no DNA replication.
* The chromatids of each chromosome separate and each daughter cell divides.
* At the end of Meiosis, there are four daughter cells from each parent cell. Each daughter cell has half the number of chromosomes of the parent.

 
               
Mitosis:
* occurs in somatic cells
* one cell division
* DNA is duplicated before cell division
* each parent cell produces 2 daughter cells
* each daughter cell has a "2n" chromosome number 		Meiosis:
* occurs in gametes
* two cell divisions
* DNA is duplicated before the first division, but not the second
* each parent cell produces 4 daughter cells
* each daughter cell has an "n" chromosome number
         
                     

Answer the following questions. Head your paper Unit 3 Day 4, Cells; the Building Blocks of Life.

1. If the cells of almost all living things are essentially the same size, why aren't all living things the same size?

2. In what stage of meiosis does DNA replication take place?

3. The cell cycle has four parts. Describe what happens in the cell cycle using one sentence for each of the four parts.

4. What are the two main differences between mitosis and meiosis?

5. How many daughter cells are produced by a parent cell during meiosis?

6. What is another name for G1 phase?

7. Which type of cell division occurs only in gametes?