control of cell numbers Flashcards
two types of cell cultures
- primary cell cultures. cells are mechanically and enzymatically dissociated from fresh tissue. they are mortal and will die after a number of divisions. (Hayflick Limit)
- immortalized cell lines. abnormal cells that divide indefinitely, good for research not life. created from transforming cells with oncogenes inactivating cell cycle checkpoints or isolated from tumors.
what is the Hayflick Limit?
the number of times a normal human cell population will divide in tissue culture before it reaches replicative cell senescence.human fibroblasts typically divide 25-40 times in culture. the limit is thought to be related to telomere shortening.
what are telomeres and what do they consist of?
the caps of human chromosomes that shorten with each replication event (end replication problem). made up of long TTAGGG repeats at chromosomal ends
what is the end replication problem?
telomeres progressively shorten after each replication as enzymes that duplicate DNA cannot continue their duplication process all the way to the end of a chromosome, thus shortening the chromosome each time. eventually the short telomeres activate a DNA damage response resulting in senescence or apoptosis
what processes counteract telomere shortening?
- telomerase complex has the capacity to elongate telomeres
2. sheltering complex protects telomeres from being recognized as DNA breaks
do all cells have telomerase?
no. human fibroblasts do not express telomerase and stop dividing when telomeres are short
what are HeLa Cells?
the oldest and most commonly used immortalized human cell line. it came from cervical cancer cells taken from Henrietta Lacks.
what is apoptosis? what is it used for?
programmed cell death. necessary for embryonic development and adult physiology. occurs in adult bone marrow and intestinal epithelium. occurs in response to irreparable DNA damage.
example of apoptosis used for development
apoptosis sculpts the digits of a mouse paw by killing the cells between the digits. apoptosis causes a tadpole’s tail to disappear as they become a frog.
what was the first growth factor identified? who discovered?
Nerve growth factor. This allows neurons to grow very large without dividing. Rita Levi-Montalcini
what is necrosis?
a cell breaks open and spills cytoplasm into ECF, causing inflammatory immune response. it is accidental, not intentional like apoptosis
steps of apoptosis
- cell shrinks and condenses
- cytoskeleton collapses
- nuclear envelope dissaembles (caspases)
- Nuclear DNA breaks up into fragments (DNAses)
- cell displays PS on outer layer of PM, triggers phagocytosis by macrophages
- contents are recycled
what are caspases? procaspases?
Caspases: proteases that have a cysteine at a active site and cleave target proteins at aspartate
procaspases: inactive precursors, activated by cleavage by other caspases
steps in activation of caspase cascade
- procaspases are activated by binding adaptor proteins
- adaptor proteins cause aggregation of initiator procaspaces
- aggregation activates initiator procaspases
- initiators activate the full caspase cascade
what is the role of cytochrome C?
it is released from mitochondria and plays a role in formation of Apoptosome, leading to apoptosis. Cytochrome C is released and activates adaptor protein which aggregates initiator procaspases into Apoptosome. aggregation activates procaspases and caspase cascade
