Material must be supplied by the cell by moving things in and out of the cell membrane.  The amount of cell membrane is equal to the surface area of the cell.  As the cell gets larger, the ratio between the surface area and the volume of the cell does not stay constant.  As the surface area doubles, the volume is squared.  In a fairly short time, the cell’s volume becomes too large to be supplied by the cell membrane, so the cell must divide (become two cells, each with half the volume of the original cell

            Cells that divide go through a life cycle called the cell cycle .   The cell cycle consists of 4 stages, G₁, S, G₂ and M. 

            G₁ – There is general growth of the cell, organelles and proteins are made.

                     Most of a cell’s life is spent in this phase.  If a cell gets too large and

                     needs to divide, it goes on to the other stages.  Some cells in a multicellular

                     organism do not divide and they stay in this phase

            S – During this phase, the DNA is replicated (synthesized) so a copy of the

                   DNA can be passed on the new cell

            G₂ – Because the cell is committed to divide at this point, organelles needs for

                    successful cell division are made

            M – The M stands for mitosis.  This is actually a process where the cell nucleus is

                    replicated, there are times when a cell nucleus is replicated but the cell does

                    not actually divide.  The process of one cell becoming two cells is called  

                    cytokinesis

            There are 4 phases of mitosis.  Some books include a phase called interphase, but this is defined as the time between the processes of mitosis, so interphase actually is everything in the cell cycle but mitosis.

            Prophase – The working form of DNA in the cell is called chromatin.  In this

                                    form, the DNA is unwound and is spread all over the nucleus.  In

                                    this form, it would be impossible to manage, so the DNA

                                    condenses into tightly wound structures called chromosomes. 

                                    Also during this phase the nuclear membrane disappeares

            Metaphase – During metaphase, the chromosomes line up in the center of the cell

            Anaphase – During anaphase, the chromosomes are pulled to opposite sides of

                                    the cell with one complete set of chromosomes on each side.  If

                                    the cell is going to divide, a division furrow will be formed where

                                    the cell is going to divide

            Telophase – During telophase, everything that happened in prophase is reversed.

                                    The chromosomes uncoil back into chromatin, and the nuclear

                                    membrane reforms.  If the cell is going to divide it occurs during

                                    telophase.  In plant cells, a new cell wall begins to form between

                                    the two cell, but before it is fully formed, it is referred to as a cell

                                    plate.