Cell biology theme 3 Flashcards
What is the most basic function of the cell cycle ?
duplicate DNA
separate into genetically identical cells
Besides DNA what else duplicates ?
macromolecules
organelles
doubles in size
what are the 4 phases of the cell cycle
M phase (mitosis and cytokinesis)
G1 phase
S phase these make up interphase
G2 phase
What happens in the S phase ?
DNA replication
What happens in G1 and G2 ?
the cell continues to grow and monitors the external and internal environment
Why do cells need to monitor the external/internal environment ?
ensures the conditions are suitable for reproduction and preparations are complete before the cell commits to S phase and M phase
What are the 2 mechanical process that take place in the M phase ?
separation of the duplicated chromosomes
division of the cytoplasm
Which assemblies carry out the nuclear and cytoplasmic divisions ?
nuclear division is carried out by the mitotic spindle
cytoplasmic division is carried out by the contractile ring
What is the mitotic spindle made of ?
microtubules and motor proteins
it arises from centrosomes
What is the contractile ring made of ?
Actin and myosin microfilaments
arranged in a ring at the equator
What happens during prophase ?
chromosomes condense and the mitotic spindle assembles
What happens in prometaphase ?
nuclear envelope breaks down and spindle attaches to chromosomes
What happens in metaphase ?
chromosomes align at the equator of the spindle
What happens in anaphase ?
paired chromatids separate to form daughter chromosomes , migration to opposite poles
What happens in telophase ?
the nuclear envelope re-assembles and the contractile ring assembles
What happens in cytokinesis ?
the completed nuclear envelope now surrounds the chromosomes
the contractile ring pinches to create 2 daughter cells and cells enter G1.
What are kinetochores ?
microtubules from the spindle grab hold of chromosomes at the kinetochore , kinetochores are protein complexes that form at the centromere of each chromosome.
How do kinetochores recognise the centromeres ?
there is a special DNA sequence at the centromeres , if this is altered the kinetochores fail to assemble and chromosomes dont segregate
What are the 3 phases of interphase ?
G1
S
G2
What happens during interphase ?
allows cell to duplicate contents
monitor the external and internal environment befre committing to S phase
Why do eukaryotic cells need a cell cycle control system ?
this guarantees the events of the cell cycle occur in the correct sequence
How is progression through stages of the cell cycle monitored ?
regulation at cell cycle checkpoints
receive feedback on processes
What are the cell cycle checkpoints ?
G1 to S
G2 to M
M to G
What is checked at the progression from G1 to S ?
the environment is favourable for DNA replication
What do cells require to avoid delayed progress through G1 ?
nutrients are signals from the extracellular environment
What is checked at the progression from G2 to M phase ?
the DNA is undamaged and fully replicated
What is ensured during mitosis ?
chromosomes are fully attached to the spindle
What do cells need to receive to grow ?
signals (esp during G1)
What is the G0 phase ?
a phase where cells arent dividing/preparing to divide because there is a lack of nutrients and growth factors
Which cells are stuck in G0 ?
Neurones and red blood cells - they have reached their mature state and dont need to divide again
What are mitogens ?
signals that stimulate cell division - cells only divide when they receive mitogens
What are growth factors ?
stimulate cell growth when the cells are not dividing
What are survival factors ?
they inhibit apoptosis and therefore promote cell survival
disruption in cell signaling pathways can lead to what ?
cancer
How is the cell cycle controlled ?
activating and deactivating proteins that initiate DNA replication , mitosis and cytokinesis
What controls the activity of these cell cycle initiator proteins
kinases - the activity of kinases follows a cyclic pattern as they are only activated when approproate
what do kinases depend on ?
cyclin
What does cyclin do ?
forms a complex with kinases - cyclic changes in the amount of cyclin drive the assembly and activation of these complexes - they trigger various cell cycle events
different CDK complexes …
trigger different cell cycle events ?
How are mitogens and CDKs linked ?
mitogens promote cell division by stimulating the accumulation of cyclin
how are CDKs switched off ?
by destroying cyclin - this happens via an anaphase promoting complex which tags the cyclin with ubiquitin - the CDK is inactivated.
What is apoptosis ?
the elimination of unwanted cells by programmed cell death - it is a regulated and normal event
How is the number of cells in a community controlled ?
by controlling the rate of cell division
What is necrosis ?
accidental cell death - die to disease , injury or failiure to supply adequate blood
How does necrosis occur ?
cells swell and undergo lysis
the contents is released into tissues
this triggers the inflammatory response which leads to damage.
it is unprogrammed and unregulated
How does apoptosis occur ?
the activation of an inbuilt suicide pathway
cells develop irregular blebs - the cell shrinks and condenses
the cytoskeleton collapses
the nuclear envelope disassembles
why is apoptosis described as clean death ?
the alteration in the cell surface atracts phagocytic cells
they engulf the apoptotic cell before it spills its contents
Which molecules are responsible for apoptosis ?
a family of proteases - called caspases
`What are the inactive form of caspases called and why are they activated ?
procaspases
they are activated in response to signals that induce apoptosis
How can apoptosis be triggerred by caspases ?
apoptosis is triggered by a protease cascade where initiator caspases cleave and activate downstream caspases - this amplifies and broadcasts the response.
Is the caspase cascade reversible or not ?
no- it is irreversible the decision to die is highly regulated
How can you activate a caspase ?
remove a peptide from the procaspase
What is the function of activated caspases ?
cleave other caspases in a cascade - activate them
cleave nuclear lamins - structural proteins in the nucleus - nuclear fragmentation
Activate DNase which turns DNA into fragments
cleave the cytoskeleton loosing contact with the ECM
in an apoptotic cell how will the DNA fragments appear ?
as a ladder as the DNA is now in fragments
Why is apoptosis needed in development ?
to remove tissue and to model organs into the correct shape. to allow the formation of complex tissues
Why else is apoptosis needed ?
to control cell numbers (eg. removal of autoreactive T cells)
removal of infected cells (eg. cytotoxic T cells can kill virally infected cells)
Removal of stressed or damaged cells (DNA)
What signal types can induce apoptosis ?
hormonal signals
cytokines
direct signals from a contracting cell
How does the death receptor Fas work ?
a cytokine signal from the TNF family causes a death receptor be expressed
this is activated by binding of a ligand
triggers the death inducing complex- caspase cascade
Which signals suppress apoptosis ?
survival factors
mitogens
growth factors
How can survival factors control the number of nerve cells ?
survival factors are secreted by target cells
they are picked up by nerve cells
survival factors suppress the suicide program
there is an over production of nerve cells so some cells wont receive the survival factor - apoptosis of these cells occurs
this ensures that the number of nerve cells matches the number of target cells
How do mitogens suppress apoptosis ?
they bind to receptors that trigger intracellular pathways that release molecular brakes blocking transition from G1 to S.
how do growth factors suppress apoptosis ?
they activate intracellular pathways that lead to an accumulation of certain proteins required for the cell to grow.
What are integrins and how do they promote cell survival?
fibronectin , a glycoprotein attaches to collagen (at the membrane of fibroblasts) via integrin which is attached to actin in the cell cortex
this inhibits apoptosis as there is a contact with the ECM
which molecules regulate activation of caspases ?
members of the Bcl2 family - some promote caspase activation others inactivate caspases leading to apoptosis being inhibited
What determines whether a cell dies or survives ?
the balance of activities of different members of the Bcl2 family
How can mitochondrial injury lead to apoptosis ?
damaged mitochondria leak cytochrome C (from between the inner and outer membrane) into the cytosol
Cytochrome C activates caspases - this is the action of BAX and BAD proteins (members of Bcl2)
this leads to a caspase cascade
some Bcl2 members can also inhibit apoptosis
damaged DNA leads to what being activated ?
p53- a protein
what does activation of p53 do ?
it inhibits CDKs
Excessive damage to DNA causes what ?
a large amount of p53 being activated
this overcomes a threshold
overcoming the threshold means apoptosis is activated as Bcl2 members are activated.
What are diploid cells ?
they have 2 versions of each chromosome
one from each parent
only the sex chromosomes differ
Where does meiosis only occur ?
in the ovaries and the testes
What is the process of meiosis ?
DNA replication
duplicated chromosomes attach to the corresponding homolog
2 sets of cell division
diploid cells turn into haploid gametes
the assignment of each homolog to a haploid cell is …
random - each gamete receives a mixture of maternal and paternal chromosomes creating diversity
what do the 2 separate cell divisions entail ?
the first division separates the chromosomes at anaphase
the second division separates the sister chromatids at anaphase
What is recombination ?
the exchange of fragments of maternal and paternal homologous chromatids
when does recombination occur ?
during prophase of meiosis 1
How does recombination lead to genetic variation ?
produces individuals with novel assortments of alleles
can provide an evolutionary advantage
Are chromosomal abnormalities acquired or inherited ?
they are acquired
What are most chromosomal abnormalities to do with?
structure and number - they produce abnormal embryos that are not viable
What is aneuploidy ?
the loss or gain of one or more chromosomes
What is an example of aneuploidy ?
trisomy 21 - where an individual has an additional chromosome 21 - 3 in total leading to Downs Syndrome
What is polyploidy ?
the addition of one or more complete haploid complements (46,69,92 etc)
What is an example of polyploidy ?
triploidy- where individuals have 3 sets of chromosomes (69) leads to spontaneous miscarriages.
What is chromosome non-disjunction ?
the failure of homologous chromosomes/sister chromatids to separate properly during cell division. leads to gametes with an unusual number of chromosomes
Why does non-disjunction usually occur ?
inappropriate spindle formation
What is translocation ?
a chromosomal abnormality caused by rearrangements of parts between non homologous chromosomes- they are usually identified by karyotype analysis.
What is the 9:22 translocation ?
exchange of genetic material between chromosomes 9 and 22 - this results in the formation of the philadelphia chromosome - a chromosome 22 derivative
What are structural abnormalities in chromosomes ?
includes chromosome breakage and reunion in a new configuration.
What are the p and q regions of the chromosome ?
refers to the parts of the chromosome separated by the centromere - the p is the short region and q is the long region.
what is the 15q12 chromosome partial deletion ?
the long (q) arm is deleted on one copy of the chromosome. it increases the risk of birth defects and developmental delay.
what is a karyotype ?
the full complement of chromosomes in the cells of an individual
how do we create a karyotype ?
the metaphase spread
all cells are given phtochemagglutinin which inhibits the formation of the spindle
cells are stuck in metaphase and are condensed
What do we use karyotypes for ?
observing number and shape
cytogenetics