Continual cycle of growth and replication.
- G1: intial growth phase
- S phase: during which the DNA is replicated
- G2: second growth phase
- M phase or mitosis: during which the cell divides its organelles, cytosol, and replicated DNA among two indentical daughter cells
The portions of the cycle from G1-G2, when the cell is not dividing.
Phases of Mitosis
(1) The nuclear membrane starts to degenerate, and the DNA condenses so individual chromosomes are visible. Also during this stage, we see the formation of a structure called the mitotic spindle.
- chromosomes condense to prevent tangling
- nuclear membrane temporarily disassembles
- spindle fibers form the mitotic spindle
(2) In metaphase we see the chromosomes line up along the central portion of the cell. Microtubules called spindle fibers emanate from the mitotic spindle and attach to the center of each pair of chromosomes.
(3) During anaphase we see the chromosomes start to move toward opposite pole of the cell as the spindle fibers shorten. In addition, a process called cytokinesis begins, during which the cytoplasm is divided up among the two cells.
(4) In the final phase of mitosis, a divot forms between the two cells, called a cleavage furrow, that will pinch the cell into two identical daughter cells. In addition, during this stage the nuclear membranes begin to reassemble, the mitotic spindle becomes less visible, and cytokinesis is completed.
Chromatin is the term used to describe the
physical state of DNA and associated proteins in
an interphase cell. The nucleus contains many
linear strands of chromatin. Each long strand of
chromatin is a complete DNA molecule called a
Chromatin gives the nucleus a stippled
Human somatic cells
have 23 pairs of chromosomes,
(46 chromosome total)
containing genes inherited
from both parents.
All 46 chromosomes are replicated
during S phase of the cell cycle, in
preparation for cell division.
During DNA replication,
DNA is copied along each
chromosome, from one end of the DNA
molecule to the other end.
Each sister chromatid is
a very long piece of DNA!
During cytokinesis the parent cell forms a cleavage furrow, pinching the
parent cell into two separate daughter cells.
Cytokinesis (cytoplasmic division) is not mitosis (nuclear division), but it
coincides with late mitosis. Cytokinesis begins during late anaphase or