HS120: Cell Division & Apoptosis Flashcards
What are the types of cell division?
- Mitosis (in somatic cells)
- Meiosis (in germ cells)
What is mitosis?
mitosis is cellular division that produces 2 identical daughter cells.
What is the no. of chromosomes in a daughter cell after mitosis?
2n (diploid) of chromosomes
What is cytokinesis?
division of cytoplasmic material.
What is karyokinesis?
division of nuclear material.
Why do we need mitosis?
mitosis is needed for:
- growth & development
- repair & maintenance of cells
What are the 4 stages of mitosis?
- Prophase
- Metaphase
- Anaphase
- Telophase
What is prophase?
- the replicated chromatin condenses into discrete thread & become visible microscopically
- each chromosome consists of 2 parallel sister chromatids (d-chromosomes) joined together at the centromere
What do centrosomes do in prophase?
- they divide into 2 regions
- each half of the centrosome contains a pair of centrioles embedded in y-tubulin rings
What is prometaphase?
aka late prophase…
1. phosphorylation of nuclear lamins & inner nuclear membrane: this causes the NPC & nuclear lamina to disassemble & disolve into the cytoplasm as cytoplasmic membrane vesicles.
2. nucleolus disappears
3. chromosomes are arranged randomly throughout the cytoplasm
4. microtubules attach to the kinetochores
5. centrosomes migrate to opposite poles and create a MTOC
What are kinetochores?
large protein complexes at the centromere of each chromosome.
What are the microtubules that attach to the kinetichore?
The mitotic spindle fibres.
What happens after a MTOC (microtubule organizing centre) is formed?
- astral microtubules
- kinetochore microtubules
- polar microtubules
develop to dive rise to the mitotic spindle apparatus
What happens during metaphase?
- chromosomes become maximally condensed & align themselves at the equator of the mitotic spindle
- each chromatid parallels the equator
- kinechore microtubules attach to their respective kinetochore.
What is the equator of the mitotic spindle called?
metaphase plate/ equitorial plate
What happens during anaphase?
- cohesins that hold the chromatids together break down.
- sister chromatids separate & begin to migrate to opposite poles of the cell.
In immunofluorescent imaging, what colour are spindle microtubules?
Green
In immunofluorescent imaging, what colour are chromosomes?
Blue
What happens during telophase?
- it is the teminal phase of mitosis.
- each set of chromosomes has reached its respective pole.
- chomosomes unwind into chromatin
- nucleus & nuclear envelope reconstitute
- mitotic spindle apparatus disappears
- belt-like contractile ring of actin filaments associated with myosins develops in cortical cytoplasm at the cell’s equator.
- a cleavage furrow begins to develop.
What happens at the end of telophase?
cytokinesis: the contractile ring contracts to divide the cytoplasm and its organelles into 2 genetically-identical daughter cells
Where does meiosis occur?
only in sperm and egg cells
What are the 2 crucial results of meiosis?
- to reduce the number of chromosomes from diploid to haploid
- recombination of genes: ensures genetic variability and diversity of the gene pool.
What are the divisions of meiosis:
- Meiosis 1 (reductional division)
- Meiosis ll (equatorial division)
What are the 5 phases within Prophase l?
- Lepotene
- Zygotene
- Pachytene
- Diplotene
- Diakinesis
phase 1 of prophase l
What happens in lepotene?
individual chromosomes (2 chromatids) begin to condense & form long strands in the nucleus.
phase 2 of prophase l
What happens in zygotene?
homologous pairs of chromosomes approximate each other, lining up in register (gene locus to gene locus) & make synapses via synaptonemal complex.
- synaptic chromosomes: tetrads
phase 3 of prophase l
What happens in pachytene?
- chromosomes continue to condense, becoming thicker & shorter.
- chiasmata: crossing over sites that are formed as random exchange of genetic material occurs between homologous chromosomes (same gene locus).
What happens in diplotene?
chromosomes continue to condense & begin to separate, revealing chiasmata.
What happens in diakinesis?
chromosomes condense maximally, nucleolus & nuclear envelope disappears.
So when does the nuclear envelope & nucleolus disappear in mitosis vs meiosis?
- mitosis: prometaphase (late prophase)
- meiosis: diakinesis (prophase l)
What is metaphase l?
- homologous chromosomes align as pairs on the equatorial plate of the spindle apparatus
- kinetochore microtubules become attached to the kinetocheores of the chomosomes
What happens in anaphase l?
homologous chromosomes migrate away from each other, going to opposite poles
What happens in telophase l?
chromosomes reach the opposing poles, nuclei are re-formed & cytokinesis occurs, giving rise to 2 daughter cells.
- each cell possesses 23 chromosomes.
What is meiosis ll?
aka equatorial division…
- very similar to mitosis & is subdivided into prophase ll, metaphase ll, anaphase ll, telophase ll, & cytokinesis.
- 4 daughter cells from orginal germ cell
- there is no S phase prior to meiosis ll.
- all 4 daughter cells are genetically distinct from one another because even if it’s sister chromatids that are being separated in meiosis ll, they still are unidentical after the recombination of genes in pachytene of prophase l.
What is a cell accumulation disorder?
cancer
What are some cell loss disorders?
- AIDS
- Alzheimer’s disease
What is apoptosis?
a rapid, highly regulated cellular activity that shrinks & eliminates defective & unneeded cells.
- aka cell suicide/ programmed cell death
- it is a physiological process
- controlled autodigestion
- cell “dies with dignity” without spilling its contents & damaging neighbors. This is because the cell membrane’s integrity is maintained.
What are causes of apoptosis?
- physiological
- pathological
examples of physiological apoptosis:
- during embryonic development
- follicular atresia in ovaries
What are causes for pathological apoptosis?
DNA damage due to…
- radiation
- chemotherapy
- viral infection
etc.
What is apoptosis regulated by?
caspases/ cysteine proteases: a family of enzymes that degrade regulatory & structural proteins in the nucleus & cytoplasm.
What are the 2 ways caspases are induced to be activated?
- Extrinsic pathway
- Intrinsic pathway
Extrinsic pathway:
aka receptor-mediated apoptosis, since the stimuli inducing apoptosis is coming from outside of the target cell.
1. TNF (tumor necrosis factor) acts on cell membrane “death receptors” to trigger a caspase cascade
2. other activators include: free radicals, oxidants, UV, & other ionizing radiation.
What is the intrinsic pathway?
aka Mitochondria-mediated apoptosis:
- stimuli originates from inside the cell
- internal activators of apoptosis: p53, nutrient-deprivation, & antimetabolites
- Bcl-2 (B-cell lymphoma 2) family consists of anti/pro-apoptotic members that determine life or death.
Which Bcl-2 members are proapoptotic?
- Bak (Bcl-2 antagonist killer)
- Bax (Bcl-2-associated X protein)
these proteins act on mitochondria to regulate the release of cytochrom C
What is cytochrome C?
the most potent apoptosis-inducing agents
- they are released into the cytoplasm to activate caspase cascade.
