Chapter 10
Cell Cycle
A. Introduction
 The process of Cell growth, division and replication is known as the cell
cycle.
 Traditionally, the cell cycle has been divided into 4 stages: G1 phase, S phase,
G2 Phase, and M Phase.
 The Cell cycle is an endless repetition of mitosis, cytokinesis, growth, and
chromosomal replication.
M = Mitosis, S = Synthesis (replication) of DNA, G1 and G2 = gap 1 and gap 2
 Organelles and DNA must be apportioned in the appropriate numbers when a
cell divides.
B. Chromosome Structure
 All Eukaryotic cells contain DNA in the form of distinct chromosomes.
 Human somatic (body) cells have 46 chromosomes.
 During division, a copy of each of the 46 chromosomes is inherited by each
new cell.
 A gene is made up of a segment of DNA.
 A chromosome is made up of many genes.
 The DNA in the chromosome is wrapped around histone and non-histone
proteins.
 Before DNA replicates, there is only one double stranded helix of DNA in
each chromosome.
1. Chromatid
 After DNA replication, there are two identical DNA connected by a structures
known as a centromere.
 Each single DNA helix is called a chromatid.
2. Centromere (Kinetochore)
 After DNA synthesis, the chromosome is made up of two identical chromatids
connected by a centromere.
 These chromatids are called sister chromatids because they are identical in
gene make-up.
C. Interphase
1. G1 Phase or the Gap 1 Phase
 The chromosomes unwind in the G1 Phase forming chromatin.
 The cell sysnthesizes the necessary enzymes and proteins needed
for cell growth.
 DNA consists of a single unreplicated helix.
 In the G1, the cell may be growing, active, and performing many
intense biochemical activities.
2. S Phase or he Synthesis Phase
 DNA and chromosomal proteins are replicated.
 This phase lasts a few hours.
3. G2 phase of the Gap2 Phase
 Occurs between synthesis and mitosis.
 The mitotic spindle proteins are synthesized which is used to
move the chromosomes in mitosis.
D. REGULATION OF THE CELL CYCLE
 Different types of cells take different amounts of time to complete the cell cycle.
 Some cells, like nerve cells, do not go through the cell cycle and no longer divide.
 If cells divide too rapidly, it disrupts the function of the tissue and is referred to as
cancer.
 It is important that cells divide only upon reaching a certain size.
 Depletion of nutrients, changes in temperature, and pH can stop a cell from
growing and dividing.
 The density or the touching other cells can control cell division.
D. THE CONDENSED CHROMOSOMES
 The chromosomes begin to condense after the S phase of the cell cycle.
 Each chromosome consists of two copies of the same chromatid.
 Each chromatid is joined together by a constricted area common to both
chromatids.
 This region of attachment is known as the centromere.
E. MITOSIS
 Duplication and division of the nucleus.
 The M stage (Mitosis) has five phases; Prophase, Prometaphase, Metaphase,
Anaphase, and telophase.
 Another process divides the cytoplasm and organelles -Cytokinesis
F. PROPHASE
 Chromosomes condense; supercoiling of the chromosomes.
 The chromosomes get more compact and become visible.
 Nucleoli disappear.
 Each chromosome is made up of two sister chromotids and these two chromatids
are held together by a centromere (kinetochore).
G. PROMETAPHASE
 Nuclear envelope Fragments.
 Microtubules of the spindle can now invade the nucleus and interacts with the
chromosomes.
 The two centrioles that were fairly close together move the the opposite poles of
the nucleus.
 Spindle fibers extend from the poles to the equator.
H. METAPHASE.
 The chromosomes line up on a plane called the metaphase plate.
 This lies in the middle of the spindle apparatus and is perpendicular to its axis.
I. ANAPHASE
 Anaphase begins when the centromeres divide and the spindle apparatus starts
pulling the centromeres (kinetochores) to the opposite poles.
 The daughter kinetochores move apart dragging the chromosomes ( each now
a single strand) to the poles.
 Two cells begin to form.
J. TELOPHASE
Telophase is the reverse of Prophase , but there are now two nuclei instead of one.
 Chromosomes decondense
 Nuclear membrane reappears.
 Spindle Fibers become disorganized.
 The cell pinches in the middle, beginning the formation of the two cells.
K. CYTOKINESIS; DIVISION OF THE CYTOPLASM
1. Animal cells
 Cytokinesis usually begins with an cleavage furrow at the metaphase as an
indentation in the surface of the cell.
 It looks as though the cell membrane were being pulled toward the middle, as
if a thread were being wrapped around the cell and being tightened.
2. Plant cells
 At the time of telophase, small membraneous vesicles forms a double
membrane, which is called the cell plate.
 The cell plate forms across the midline of the plant cell where the old
metaphase plate was located.
L. FUNCTION OF MITOSIS
Cell division consists of mitosis (nuclear and chromosomal events) and cytokinesis (cell
membrane and cytoplasm events). Mitotic cell division serves organisms in 2 ways.
1. Single cell organisms
Mitosis allow for and increase in the population. This is a form of asexual
reproduction. There is no exchange of genes between individuals. The colony will
be made up of individuals with genes what are identical to the founder, called clones.
2. Multicellular Organisms
a. Mitosis and cytokinesis allow for an organism to grow in size while
maintaining the surface area volume ratio of its cells.
b. Mitosis and cytokinesis allow for specialization of cell types through cell
differentiation.
c. Mitosis and cytokinesis that are dead or damages allow cells to be
replaced.
3. Abnormal Cell Division
a. Cancer cells
Cancer cells do not respond to normal cell division controls. They divide
excessively and ignore density-dependent inhibition.
Cancer cells can go on dividing indefinitely. Most mammalian cells will
divide about 20-50 times before many stop, age and die. However, there is a
line of laboratory maintained cancer cells cells that have been dividing since
1951.