Apoptosis, necrosis, and death Chapter 18 Continuity of life Only from existing cells come new cells. We are all decedents of the first cells on the planet. A cell reproduces by duplicating its contents and then dividing into two This cycle of events is known as the cell cycle
The cell-cycle is regulated by the phosphorylation of special proteins called Cdks (shown in red). However, these proteins must associate with other peptides before they become functional. These other peptides are known as cyclins (shown in green).
The cell just regulates the concentration of different cyclins (hence their name - as they vary in concentration during the cell cycle)18_05_ Cdks.jpg
The cyclin which drives cells into M phase is called M-cyclin. It interacts with a kinase called M-Cdk.
Levels of M-cyclin build steadily.
Then it is rapidly removed from the cell by rapid degradation by the proteasome
Removal of M-cyclin results in the inactivation of M-Cdk and the cell divides.
APC (anaphase promoting complex) decides when to remove M-cyclin from the cell.18_06_M_Cdk.jpg
The key molecular event that marks many proteins for destruction by the proteasome is ubiquitation - a type of modification.
Here the M-cyclin is ubiquitinated and is thus quickly destroyed.18_07_cyclin_degradat.jpg
The Go (G zero) state results in the dismantling of most of the replication machine. Nerve and muscle cells are in this state.
Many other cells come to this important checkpoint each time after they have divided.18_16_G1_checkpoint.jpg
In conclusion, the cell is faced with a number of points in the cell cycle where it has to satisfy certain molecular requirements before it is permitted to continue along the cell cycle.18_17_arrest_checkpt.jpg
b) The tail of the tadpole is absorbed via apoptosis.
Also, in adult multicellular organisms cell death is a regular occurrence. In humans EACH HOUR we lose many many BILLIONS of cells via apoptosis. Most of these are healthy cells which have no defects. WHY?
Development and regulation controls.
i.e. B and T cells are removed that do not pass certain tests.
Apoptosis results in a quick and clean cell death, without damaging its neighbours, or eliciting an immune response. Every cell is equipped with the ‘cell death pathway’. Apoptosis is an intracellular proteolytic pathway. The DNA is broken into small 200 bp units.
The cytoplasm shrinks. The mitochondria release cytochrome c. The outer surface of the plasma membrane gets coated with a different sugar - one that macrophages can sense and phagocytose.18_20_Apoptosis_.jpg