A single cell is a living organism. It therefore possesses the requisite characteristics of life that include growth and development, as well reproduction





The Cell Cycle





A cell’s growth and development can be subdivided into two major phases, interphase and mitosis


During interphase the cell grows, carries out its normal functions, and replicates its DNA and many of its organelles


Interphase is divided into three sub phases, G1, S, and G2


In G1 the cell grows and carries out its normal activities


The synthesis (S) phase is the phase in which, in preparation for mitosis, DNA replication occurs
In G2 the replicated DNA is checked for errors

Some errors may be undetectable and the error will be carried on to future generations of the cell


Some errors may be detected and repaired


Detected irreparable errors may precipitate apoptosis programmed cellular death


Interphase ends with G2 and mitosis (M) commences



In preparation for the cell splitting into two identical daughter cells (mitosis), the DNA must be replicated. This happens in the S (Synthesis) sub phase of interphase.







There are three major phases of the S phase: Initiation, Elomgation, and Termination


Initiation. The two complementary DNA

strands are separated, much like unzipping a zipper. Special enzymes, including helicase, untwist and separate the two strands of DNA


Elongation. Each strand becomes a template along which a new complementary strand is built. DNA polymerase brings in the correct bases to complement the template strand, synthesizing a new strand base by base


Termination. Once the two original strands are bound to their own, finished, complementary strands, DNA replication is stopped and the two new identical DNA molecules are complete


The now two replicated strands of DNA are held together by a protein/DNA structure known as a centromere





Mitosis entails the processing, organization, and separating the cell’s genetic material in preparation for cell division into two identical cells

There are four phases of mitosis: prophase, metaphase, anaphase, and telophase.







Prophase


Goal 1: Genetic Material Protection

The first visible cue that a cell has entered the mitotic phase is that the genetic material becomes visible with a light microscope. The DNA molecules are very long. They are functionally shortened as they wrap around the histone proteins. However, at the beginning of mitosis these molecules are additionally condensed and coiled to prevent damage and tangling during processing. When coiled, the chromatin takes on bar-like appearances and are now referred to as chromosomes.

Goal 2: Genetic Material Liberation

During prophase, the nuclear envelope fragments and disintegrates. The genetic material must ultimately be parceled out into the two daughter cells. It therefore must be freed from the confines of the nucleus.

Goal 3: The Beginning of Genetic Material Organization

Centrioles are paired organelles located within a region near the nucleus called the centrosome.


During prophase the replicated centriole pairs are pushed away from each other to opposite sides of the cell as they participate in the organization of microtubules that span the cytosol.


These microtubules form the mitotic spindle. The centromeres of the paired replicated DNA molecules begin to attach to specific spindle fibers.





Metaphase
Goal: Genetic Material Organization
During metaphase, the genetic material attaches to the mitotic spindle along the equator of the cell.

The genetic material attaches to certain spindle fibers, in a specific order via a protein portion of the centromere called the kinetochore.





Anaphase

Goal: Genetic Material Distribution

During anaphase, the mitotic spindles pulls the replicated strands of the chromosomes, known as sister chromatids apart to either side of the cell.



Telophase

Goal: Genetic Material Packaging

Once the sister chromatids have been moved to opposite sides of the cell, telophase begins. There are four major activities of telophase.


Disintegration of the mitotic spindle

Formation of the nuclear envelope around the genetic material

Formation of the cleavage furrow

Uncoiling of the DNA






The cleavage furrow forms as the plasma membrane at two polar sides of the cell are pinched in.
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This process continues in a process called cytokinesis, ultimately splitting the cell into two identical units referred to as daughter cells.