Interphase: This part if the cells life cycle occupies most of the cells "life". The mitosis portion of the cycle may last only 10 - 20 hours while some cell can live for days or weeks.

Parts of Interphase:

G1phase: This part of the cell cycle is where the cell spends most of its functional life. This is the time when the cells are performing their assigned tasks, metabolizing, synthesizing etc. At some point in the cycle something triggers the cell to begin a cell division event. There are many stimuli which can cause the cell to require a cell division:

- To replace dead or dying cells,

- To produce more cells to enlarge the organism (growth and development)

- Reproduction, i.e. to increase the number of unicellular organisms.

- (to reduce the surface to volume ratio) - as the cell (unicellular organism) becomes too large its surface area grows slower than its volume therefore it becomes less able to absorb sufficient nutrients. Therefore it divided itself in two and the two new daughter cells are each much smaller and they tend to grow.

NB In order for the two new daughter cells to function as the parent cell did (or to have all the characteristics of the parent cell) the material of heredity (DNA) must be copied. With one copy for each new daughter cell.

S-phase: This is known as the synthesis phase. Here the DNA molecules are copied or replicated, therefore we go from single stranded DNA in G₁ phase to double stranded DNA in G₂.

Gap 2 or G₂ Phase

The cell is preparing for the actual division events (much protein synthesis). For example tubulin the protein of which microtubules are created is synthesized. This tubulin will be used to manufacture the microtubules of the spindle apparatus in prophase of mitosis.

G₂ completes interphase now the cell is prepared for mitosis.

In eukaryotic cells DNA is a very long double helix shaped molecule comprised of many nucleotides linked end to end. During S phase these strands of DNA are replicated or copied. The exact mechanism of DNA replication was not determined until 1952 when the structure of DNA was deduced by James Watson and Frances Crick.

Aside: Historical Respective

Early investigators into mitosis did know that the nuclear material, (they didn't even know at this point what chemicals made up this mater chromatin) changed during mitosis. They saw these chromosomes condense and become more easily stained. As well it was obvious that within a species all cells had the same number of chromosomes.

They also saw that during metaphase each chromosome seemed to split into two, longitudinally and the two halves of the chromosomes separate from each other and move to opposite poles of the dividing nucleus.

The observation of such people as Flemming (1882) showed a very complex process of cell division. Why? Why was such a complex process necessary? Why do cells and nucleus simply divide in half?

Aside: W. Roux's Theory (1883)

Roux speculated that simple cell division would not be sufficient if material in the nucleus was heterogeneous, if it consisted of innumerable particles each of which has a different genetic capacity. Therefore, the only possible method by which this material can be divided in such a way that each particle was represented in each daughter cell. This method is to arrange all particles serially like beads on a string longitudinally so that each chromatin globule is divided into two halves, so tat from each string of globules two adjacently placed strings result.

A mechanism for equal division, Roux's major thesis is of course precisely the modern interpretation of mitosis. But his idea was ignored.

NB: Roux's ideas was purely speculation but he asked the why question, i.e. what selective advantage was thereto such a complicated process?

these chromosomes become the center of interest all investigation in the mid 1880"s became center on "what do the chromosomes do during cell division?

The necessity for such a complicated process of mitosis events did not become clear until the structure and function of chromosomes was determined. Let us look at all division as the scientists of the mod 1880's did, from a purely mechanistic point of view.