MBC5 Study Guide – Chapter 18 (Apoptosis)
Apoptosis, or programmed cell death, is a vital process for multicellular organisms. It is
required in certain stages of development. Also, apoptosis is needed to eliminate unwanted
cells, such as those that have exceeded their normal lifespan, or those cells that may have the
potential to cause cancer. In this section, we will consider the molecular mechanisms of
Cells that undergo apoptosis have characteristic morphological and biochemical changes.
The cells shrink and condense, the cytoskeleton collapses, the nuclear envelope
disassembles, and the nuclear chromatin condenses and breaks up into fragments. The cell
surface breaks up into membrane-enclosed fragments called apoptotic bodies. Biochemical
changes cause apoptotic cells to be recognized by macrophages that engulf them before they
can spill their contents. That way the cell is rapidly cleared away, without causing a
damaging inflammatory response.
Apoptosis is to be distinguished from necrosis, which may be caused by an acute insult,
such as trauma or lack of blood supply. Necrotic cells swell and burst, spilling their contents
and eliciting an inflammatory response. The cells look as if they have exploded.
Programmed Cell Death Eliminates Unwanted Cells
Programmed cell death eliminates cells that are no longer needed or because they are
abnormal, nonfunctional or potentially dangerous. It is also a mechanism for regulating cell
number. Cell death helps sculpt hands and feet during embryonic development. The
individual digits separate as the cells between them die, as illustrated for a mouse paw. When
a tadpole changes into a frog, the cells in the tail die, and the tail, which is no longer needed,
Apoptotic Cells Are Biochemically Recognizable
Several assays are available that take advantage of the biochemical changes associated with
apoptosis to detect apoptotic cells:
1 • During apoptosis, an endonuclease cleaves chromosomal DNA into fragments that
can be analyzed by gel electrophoresis.
2 • Cleavage of DNA generates new DNA ends, which can be labeled and detected by
TUNEL (TdT-mediated dUTP nick end labeling) technique. The enzyme terminal
deoxynucleotidyl transferase (TdT) adds chains of labeled dUTP to the 3’-OH ends
of DNA fragments.
3 • Phosphatidylserine in the plasma membrane moves from the inner to the outer
leaflet of the lipid bilayer and can be visualized with a labeled form of the Annexin V
protein. The cell-surface phosphatidylserine serves two purposes: 1) it serves as an
“eat me” signal to macrophages to ingest (phagocytose) them; 2) it blocks the
inflammation associated with phagocytosis.