Chap. 14: Cellular Reproduction
Ø Cell theory:
1. All organisms are composed of one or more cells.
2. Cell is the structural unit of life.
3. New cells originate only from other living cells.
Ø Cell Division:
Mother cells è Daughter cells (Cellular reproduction)
Ø Cells are constantly being produced to replace old cells. (2.5 x 106/s).
Ø Cell cycle:
Cells divide through certain defined stages. The stages through
which a cell passes from one cell division to the next constitute a
cell cycle.
Ø Cell cycle can be divided into two major phases based on cellular
activities observable by light microscope.
è M phase and Interphase
Ø
M phase include: A period during which cells divide.
- Mitosis(有絲分裂): A process during which duplicated
chromosomes are separated into two nuclei.
- Cytokinesis (細胞質分裂): A process during which the entire cell
divides into two daughter cells.
- Short time duration (30 – 60 minutes)
- Synthesis of macromolecules is largely shut down.
- Sub-divided into several sub-phases.
Ø Interphase: A period during which cells prepare for cell division.
- Time duration long and may vary depending on conditions
(hours or days of never).
- Actively synthesizing macromolecules (DNA, protein etc).
- Defined and regulated biochemical activities during interphase.
- Sub-divided into G1, S and G2 phases.
The oversll rate of RNA and protein synthesis
is relatively constant through interface
Ø The overall rate of RNA and
protein synthesis is relatively
constant through interface
Ø Use radioactive pulse chase
experiment to define the
activities in each phase
G1 Cycle: Cell growth,
S
protein synthesis
cycle: DNA synthesis
G2: Cell growth,
prepare for mitosis
Radioactive pulse chase experiment
Ø How long does it takes for the cells which incorporate radioactive T
to get to M phase to determine the length of G2 phase.
Ø From this type of experiments one can defined the length of each
period and the nature of the biochemical processes which take place
in each phase.
Procedures
1. Grow Hela cell with 3H-T
(hot T) for 30 min
(pulse Labeling)
2. Grow cells in cold T for
various time duration. (Chase)
3. Fix the cells and look for cells
in M-phase under microscope
4. Count % of M-phase cells
with hot chromosome
Radioactive pulse chase experiment
Ø tG( G2 phase) = The time it takes to see the first radioactive
chromosome.
ØtS (S phase) = The width of the radioactive signal.
ØtM (M phase) can be measured with light microscope.
Ø TT (total) ≠ tS + tG + tm è There is another G1 phase.
Ø Length of the G1 phase is the most variable
Ø Cells that stop divide are said to be in Go phase.
Three types of cells
1. Cells that never divide: Nerve cells, muscle cells and red blood
cells lost the ability to divide once it differentiated until its death.
2. Cells that normally do not divide but can be induced to divide:
Liver cells can be induced to divide once surgically remove part of it.
Lymphocytes can be induced to divide by antigens.
3. Cells that normally possess high level of mitotic activity:
Spermatogonia (精細胞), hematopoitic stem cells (造血幹細胞),
epithelia cells (表皮細胞).
Control of Cell Cycle:
Does the cytoplasm of the cells contain regulatory factors that
affect
(a) M +cell
G1 phase
cycle activities (b)
? M + S phase
(c) M + G2 phase
Fuse M-phase Hela cell w/ kangaroo cell in
Control of Cell Cycle:
Does the cytoplasm of the cells contain regulatory factors that
affect
(a) M +cell
G1 phase
cycle activities (b)
? M + S phase
(c) M + G2 phase
Fuse M-phase Hela cell w/ kangaroo cell in
Cell fusing experiment:
Ø G1 + S: Cells were induced to synthesize DNA.
è G1 cells responding to factors in S phase.
Ø G1 + G2: No new round of DNA synthesis
è G2 cells not responding to factor in S
Ø M + any other phases: Chromatins in other phases were
induced to compaction
è Factors in M phase induce chromatin compaction.
è Chromatin in S-phase becomes pulverized due to incomplete
replication and damages..
è Entry into M phase is initiated by a protein kinase called
“maturation-promoting factor” (MPF).
è Activity of MPF is controlled by
varies in sync with cell cycle.
” cyclin” whose concentration
Progression through the cell cycle requires the phosphorylation and
dephosphorylation of certain critical residues od cdc2 kinase
M-phase:
1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase
Ø Cell division is a continuous process. Division
into different phases is for convenient
discussion and may not be as clearcut.
Ø Checkpoints: Surveillance mechanisms that halt progress of
cell cycle if:
(1) the chromosomal DNA is damaged;
(2) cerain critical processes, such as DNA synthesis in S pjase
or chromosome alignment during M phase, are not completed.
Ø Checkpoint control requires three class of proteins:
(1) Sensors that detect abnormalities and emit an appropriate
signal.
(2) Transmitters that send the signals along the proper
pathways within the cell.
(3) Effectors that respond to the signal and inhibit the cell
cycle machinery.
Chromosome Condensation
Chromatid
Centromere
Chromatid
Ø
Kinetichore: Botton-like structure on the surface of centrimere.
1. Site of attachment of microtubules on the
mitotic spindle.
2. Site of several microtubule-based motor proteins.
3. A component of an improtant mitotic checkpoint
Ø Centrosome cycle:
(P.347, Fig. 9.19)
A complex structure that contains two
barrel-shaped centrioles surrounded by
amorphous, electron-dense
pericentriolar material (PCM).
Ø
Ø
Mitotic Spindle:
Dissolution of nuclear envelope and the
participation of cytoplasmic organelles
Telophase:
Meiosis (減數分裂)
Meiosis (減數分裂)
Two divisions:
1. Each chromosome is separated from its homologus.
è Each daughter cell contains one chromosome
(w/ two chromatids)
2. The two chromatids in each chromosome are
separated from eac other.
è Each daughter cell contains only one chromatid.
2. Cross-over occurs often due to close contacts.