Download Cell cycle and mitosis

The Cell Cycle
I. The Role of Cell Division
• Cell division is an essential process in the growth and repair of
multicellular organisms.
• Cell division is also necessary for the process of reproduction in simple
organisms.
• Cell division produces the cells that are precursors to the process of sexual
reproduction.
II. Genetic Organization: The Control of Cell Division
a) The genetic material of a given organism is collectively
referred to as their genome.
b) There exists several levels of organization within an organism’s
genome.
c) The nucleus (eukaryotes) is a membrane bound organelle that
contains the genetic information in the form of chromatin, highly
folded ribbon-like complexes of deoxyribonucleic acid (DNA)
and a class of proteins called histones.
• The DNA of chromatin is wrapped around a complex of histones making
what can appear in the electron microscope as "beads on a string" or
nucleosomes.
d) When a cell divides, chromatin fibers condense and become very highly
folded.
e) The condensed chromatin fibers which have a cylindrical shape are
now considered to be chromosomes. They are visible using a light
microscope.
f) DNA or deoxyribonucleic acid is a large molecule structured from
chains of repeating units of nucleotides.
g) Nucleotides of DNA are made of the five carbon sugar deoxyribose, a
phosphate group, and one of four different nitrogen-containing bases
abbreviated adenine (A), thymine (T), guanine (G), and cytoseine (C).
h) It is the sequence of the nucleotides in the DNA molecule that
imparts the genetic traits to individuals.
i) The individual segments of the DNA strand that impart character traits to
individuals are known as genes.
j) Prior to the onset of cell division (mitosis and meiosis), the cell must
replicate its DNA..
III. The Cell Cycle
• The Cell Cycle is the sequence of growth, DNA replication, growth and cell
division that all cells go through.
Cytokinesis
a) Events in the Cell Cycle
1. The majority of the time in a given cell’s life span is
spent preparing for cell division. This time is called
interphase.
2. In the G1 phase (gap one) of interphase cells acquire ATP
and increase in size.
3. Cells undergo DNA Synthesis (replication of the original
DNA molecules, making identical copies, one "new molecule"
eventually destined for each new cell) which occurs during the S
phase.
4. The cell undergoes a second growth and energy acquisition stage, the
G2 phase. During this phase proteins essential to cell division are
synthesized.
5. The cell then undergoes the process of cell division and
cytokinesis (cell splitting).
IV. Prokaryotic Cell Division
a) Prokaryotes are much simpler in their organization than are eukaryotes.
b) Most prokaryotic genes are found on a single chromosome that consists of
a circular DNA molecule which is attached to inner cell membrane.
c) Prokaryotes reproduce by a process known as binary fission.
d) Binary Fission results in two prokaryotes that are theoretically identical to
each other.
Prokaryotic Binary Fission
E.Coli undergoing Binary Fission
V. Eukaryotic Cell Division: Mitosis
a) Mitosis is the process of forming (generally) identical daughter
cells by replicating and dividing the original chromosomes.
b) Eukaryotic chromosomes occur in the cell in greater numbers than
prokaryotic chromosomes.
c) The condensed replicated chromosomes have several points of
interest.
Replicated Chromosomes (sister chromatid)
d) The kinetochore is the point where microtubules of the spindle apparatus
attach.
e) Replicated chromosomes consist of two molecules of DNA (along with
their associated histone proteins) known as chromatids. .
f) The area where both chromatids are in contact with each other is
known as the centromere (the kinetochores are on the outer sides of the
centromere).
g) Remember that chromosomes are condensed chromatin (DNA plus
histone proteins).
h) Steps of Mitosis
NOTE:
Interphase is a
precursor to
mitosis in which
the cell prepares
for division.
• The genetic material is uncoiled and considered chromatin.
• Interphase includes the G1, S, and G2 phases of the cell cycle.
1) Prophase
• Chromatin condenses and the nuclear envelope dissolves.
• Centrioles (if present) divide and migrate toward the poles of the cell.
• Kinetochores and kinetochore fibers form, and the spindle forms
ProMetaphase
2. Metaphase:
• Nuclear membrane is completely broken down.
• The sister chromatid (replicated chromosomes) are aligned in the cell’s
equatorial plane.
• Spindle fibers extend from the kinetochore of the centrosome to the
centromere of the replicated chromosomes.
3. Anaphase
• Anaphase begins with the separation of the centromeres, and the
pulling of chromosomes (we call them chromosomes after the
centromeres are separated) to opposite poles of the spindle.
4. Telophase
• Telophase is when the chromosomes reach the poles of their
respective spindles.
• The nuclear envelope reforms.
• Chromosomes uncoil into chromatin.
• The nucleolus (which had disappeared during Prophase) reform.
• Where there was one cell there are now two smaller cells each
with exactly the same genetic information.
Telophase
Early
Late
• Cytokinesis is the process of splitting the daughter cells apart. Note that
certain cells undergo cytokinesis while others do not.
VI. The Control of Cell Division: Mitosis
a) The rate of cell division is specific to the type and purpose of
the cell.
b) Physical and chemical variables affect the rate of cell division.
• Physical variables include temperature, light, and
density.
• Chemical variables include nutrient concentration,
salinity, pH, and specific chemicals called growth
factors.
c) Growth factors directly affect the rate of cell division by binding to the
cell membrane surface receptors which in turn triggers the expression of
regulatory genes. An example includes PDGF (platelet-derived growth
factor) which stimulates the production of fibroblasts when tissue injury
occurs.
d) The G1 phase of the cell cycle is the crucial time in regulating cell division.
This is called the restriction point.
e) When the cell is in its nondividing state (which consequently is the
majority of time for most cells) it is considered to be in the Go
phase.
f) It is essential to note progression of the various steps in cell
division is dependent upon the successful completion of the previous
one.
g) The molecular clock that controls the pace of the cell cycle is guided
by regulatory chemicals called protein kinases.
h) Protein kinases are enzymes that activate proteins as they transfer a
phosphate group to them. This process will control the transition from one
phase of the cell cycle to another.
i) Cyclins are another class of regulatory proteins that regulates the activity
of the protein kinases.
j) A protein kinase that aids in controlling the cell cycle is active only when it
is bound to a particular cyclin molecule.
k) These classes of kinases are known as cyclin-dependent kinases or Cdks.