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2. 3.12 Apoptosis, Autophagy, and Proteasomes
Cells that have become obsolete or damaged need to be taken out of system
Autophagy (self-eating) is the process of disposing of nonfunctional organelles and cytoplasmic bits by forming autophagosomes, which can then be degraded by lysosomes
Unneeded proteins can be marked for destruction by ubiquitins
Proteasomes disassemble ubiquitin-tagged proteins, recycling the amino acids and ubiquitin
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3. 3.12 Apoptosis, Autophagy, and Proteasomes
Apoptosis, also known as programmed cell death causes certain cells (examples: cancer cells, infected cells, old cells) to neatly self-destruct
Process begins with mitochondrial membranes leaking chemicals that activate enzymes called caspases
Caspases cause degradation of DNA and cytoskeleton, which leads to cell death
Dead cell shrinks and is phagocytized by macrophages
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4. Developmental Aspects of Cells
All cells of body contain same DNA, but not all cells are identical or carry out same function
Chemical signals in embryo channel cells into specific developmental pathways by turning some genes on and others off
Development of specific and distinctive features in cells is called cell differentiation
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5. Cell Destruction and Modified Rates of Cell Division
Organs are well formed and functional before birth, but we need cell division for growth
Cell division in adults is needed to replace short-lived cells and repair wounds
Hyperplasia is accelerated growth that increases cell numbers when needed
Atrophy is a decrease in size that results from loss of stimulation or use
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6. Cell Aging
The mechanism of aging is a mystery, but there are several theories:
Wear and tear theory: a lifetime of chemical insults and free radicals have cumulative effects
Mitochondrial theory of aging: free radicals in mitochondria diminish energy production
Immune system disorders: autoimmune responses, as well as progressive weakening of immune response
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7. Cell Aging (cont.)
Genetic theory: cessation of mitosis and cell aging are programmed into genes
Telomeres are strings of nucleotides that protect ends of chromosomes (like caps on shoestrings)
Everytime a cell divides, the telomere shortens, so telomeres may act like an hour-glass on how many times a cell can divide
Telomerase is an enzyme that lengthens telomeres
Found in germ cells of embryos but absent in adult cells, except for cancer cells
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