Transcript The cell cycle

Chapter 4
生物醫學暨環境生物學系
助理教授 張學偉
The cell cycle
Mitotic cycle time (Tc)
= cell cycle time
= time between successive divisions
Autoradiography 自動放射顯影
Bromodeoxyuridine
(BrdUrd)
Two advantages in BrdUrd over autoradiography
1. Not involve radioactive material
2. Shorten the time for result
1. The relative length of different cell cycle phase vary in different cell types
2. Human cells of Tc > Hamster cells of Tc
3. S> G2 > M
Synchronously Dividing cell cultures
同步化
Definition:
All of the cells occupy the same phase of the cell cycle
at a given time.
Techniques for synchrony:
1. Mitotic harvest [shake off]
2. Drug [e.g.,hydroxyurea]
<1> S phase cells are killed
<2> Block at the end of G1
Most population in mitosis
Irradiated at various times afterward,
Corresponding to different cell cycle phase
Cell cycle phase & radiosensitivity
shoulder
Early
G1
later
G1
The pattern cannot be distinguished in hamster cells because G1 is too short.
1. Pattern similar
2. Important difference
Summary of radiosensitivity
with cell age in the mitotic cycle
1. Cells are most sensitive at or close to mitosis
2. Resistance is usually greatest in later S phase
3. Resistance at early G1 & sensitivity at later G1
4. G2 phase is usually sensitive, perhaps as M phase
Molecular checkpoint genes
細胞流速分析儀
Flow cytometer
Cell number
Flow cytometer & separation of cell cycle phase
G0
/G1
M
2N
4N
DNA content
The effect of oxygen
at various phases of the cell cycle
The ratio of hypoxic to aerated doses
needed to achieve the same
biological effect is called OER.
Survival
Oxygen enhancement ratio (OER) ch6
OER
S (2.3-2.4) < G1 & G2
OER varies significantly through the cycle.
hypoxic
aerated
Dose (Gy)
The age-response function for a tissue in vivo
Variation of sensitivity with cell age for neutrons
CPM = count per min
1. The variation of thymidine
uptake with time.
2. First wave
= DNA synthesis period
3. Correlated to
max resistance to x-ray
The in vivo pattern of response as a function
of cell age is similar to g-ray and neutrons,
with max resistance occurring in
later S phase for in vitro culture cells.
Mechanism for the age-response function
1. DNA amount or form
2. Naturally occuring sulfhydryl compounds
The possible implications of the age-response
function in radiotherapy
Dose at Asynchronous cell population
 S phase remained  synchronize  cell cycle move
 Dose later at sensitive phase  sensitizing
 called
Sensitization due to reassortment