Cell Signalling, the Cell Cycle + Cancer Flashcards
When do cells divide?
in response to specific molecular signals (typically from growth factors)
receive signals from other molecules
What do cells divide to produce in mitosis?
2 identical daughter cells
Why do cells stop dividing?
to specialize in structure and function (aka differentiation)
What is differentiation?
a process that produces specialized cells (function and structure)
T or F: once differentiated, all cells stop dividing
false! some continue to divide under certain conditions
Describe apoptosis
programmed cell death - a process that eliminates unnecessary cells during development and removes unhealthy/damaged cells in mature organisms
How must the processes (divide, differentiate, death) occur in relation to one another to ensure a healthy organism?
balanced
What is a result of an unbalanced cell cycle?
an organism may end up with too few or too many cells which can cause problems of varying severity (ex. hair loss vs. growth of warts into tumours)
Describe the eukaryotic cell cycle
a sequence of events that culminate in cell division
What are the cell cycle events regulated by?
various checkpoint proteins
What do checkpoint proteins do?
stimulate or inhibit cell division until conditions are right to proceed to the next phase of the cell cycle
What generally happens to a cell when it specializes?
it stops dividing and ‘exits’ the cell cycle
Describe cancer
uncontrolled division of cells that results from an improperly regulated cell cycle
What is the primary defence against cancer for multicellular organisms?
the detection of DNA damage (mutations) before division (cell cycle regulation)
How do most cancers exist?
because the cells have mutations in genes required for DNA damage detection and arrest of the cell cycle
How many primary cell cycle checkpoints are there in eukaryotes?
3
What are the 3 primary cell cycle checkpoints in eukaryotes?
G1
G2
M
Describe the G1 checkpoint (what is it and when does it occur)
Aka the restriction checkpoint
occurs at the end of G1
checks whether cells are given permission to enter S phase (checks for mutations or whether the environment is favourable)
When does the G1 checkpoint occur?
at the end of G1 before cells enter S phase
What things might prevent a cell from passing the G1 checkpoint and entering S phase?
if it does not contain permissive instructions to do so
if it has mutations
if the environment is not favourable
What happens to a cell if it does not pass the G1 checkpoint?
It will not enter S phase and instead will enter G0 phase
T or F: many cells stay in G0 their entire lives
true (ex. neurons and muscle cells)
Describe the G2 checkpoint
occurs throughout the S phase or at the end of G2 phase
detects damage or errors in DNA as replication occurs
stops the cell cycle to repair damage before cell enters M phase
When does the G2 checkpoint occur?
during S phase and at the end of G2 phase before mitosis
What is the purpose of the G2 checkpoint?
to detect damage or errors in DNA while replication is occurring and to stop the cell cycle for repairs to the DNA and/or trigger apoptosis if the damage is too severe
Describe the M checkpoing
Aka spindle checkpoint
occurs in metaphase, before anaphase and the separation of chromosomes to make sure each centromere is properly attached to a spindle microtubule
prevents large scale mutations resulting in aneuploidy
When does the M checkpoint occur?
At metaphase of the M phase before anaphase pulls apart chromosomes
What is the purpose of the M checkpoint?
to make sure each centromere is properly attached to a spindle fibre so that each cell ends up with the correct number of chromosomes when they divide
prevents aneuploidy
What organism was used in this experiment?
budding yeast (S. cerevisiae)
Why is budding yeast used in this experiment?
because it is a single celled haploid organism which reproduces asexually by mitosis but cytokinesis is not linked to nuclear division so the cells do not divide symmetrically
= cell cycle arrest can be monitored visually with a microscope by observing morphology of the cells
It can reproduce sexually when both alpha and a cells are allowed to fuse to produce the diploid cell and can divide by meiosis
How does budding yeast divide by mitosis?
asymmetrically
the mother cells form small buds at S phase which produce smaller daughter cells
What 4 components does every cell signalling pathway share?
extracellular ligand
cell-surface receptors for the ligand
intracellular enzymes that send the signal when the ligand binds
cellular response (A change in cellular behaviour or metabolism)
What is signal transduction?
the response of cells to signal ligands in their environment
T or F: yeast only undergo asexual reproduction
false, they do sexual reproduction too
What are the 2 different mating types of S. cerevisiae?
a cells and alpha cells
When will yeast reproduce asexually?
in the absence of EITHER a cells OR alpha cells
they only require one
When will yeast reproduce sexually?
if both a cells and alpha cells are present
they will fuse into a diploid cell for meiosis
what does sexual mating of yeast cells require of the cell cycle? How do yeast cells make this happen?
the cell cycle must be stopped in order for the alpha and a cells to fuse and form a diploid cell
alpha cells release alpha-factor signal ligands to arrest the cell cycle before DNA synthesis (S phase)
How do alpha factors work?
alpha cells constantly release this signal ligand (small peptide) that binds to the alpha-factor receptors on the yeast cell surfaces which triggers the arrest of the cell cycle before DNA synthesis (S phase)
they also trigger the transcription of genes required for cell fusion
What kind of cell surface receptor are alpha-factor receptors?
G-protein couple receptors (GPCR)
When is the cell cycle stopped if an alpha factor binds to the alpha factor receptor?
At the G1 checkpoint in G1 phase (before S phase)
What are the 3 cell morphology types we saw in lab?
schmoo
dumbbell
buds
At what stage of the cell cycle and what checkpoints would you see a schmoo?
checkpoint: G1
at the end of G1 phase before S phase
ex. this is when the alpha factor has bonded to the alpha factor receptors on yeast cell surfaces in order to allow the a cells and alpha cells to fuse
Describe schmoo morphology
large and oblong
At what stage of the cell cycle and what checkpoints would you see dumbbells?
checkpoint: M phase
Describe dumbbell morphology
2 cells have formed and they are even sized roughly spherical and touching
At what stage of the cell cycle and what checkpoints would you see buds of variable sizes?
during S phase or at the end of G2 phase
checkpoint: G2 phase
Describe the variable bud morphology
cells have large buds of varying sizes with uncondensed chromatin
What was the purpose of this lab?
to treat yeast with different chemicals to observe the cell morphology at different stages in the cell cycle
in this lab, what causes cells to arrest at the G1 checkpoint?
the signal transduction with the alpha factor - when it binds to the alpha factor receptors on the cell surface
in this lab, what causes cells to arrest at the G2 checkpoint?
chemically-induced DNA damage
In this lab, What causes cells to arrest at the M checkpoint?
chemically-induced damage to the spindle microtubules
What was the yeast mutant used in this lab?
MAD1
Which 3 chemical treatments did we use in lab?
alpha factor
hydroxyurea
nocodazole
What was hydroxyurea used for in this lab?
To induce cell arrest at the S phase by slowing DNA polymerase function
What checkpoint did hydroxyurea stop the cell cycle at and what did this result in?
Stopped at the G2 checkpoint and made errors during DNA replication
resulted in budding
What was nocodazole used for in this lab?
inducing damage to the spindle microtubules
What checkpoint did nocodazole stop the cell cycle at and what did this result in?
stopped the cell cycle at M checkpoint because it damaged spindle microtubules
results in dumbbells
What was alpha factor used for in this experiment?
it synchronizes the opposite mating type (a cells) in yeast for sexual reproduction
the binding of the alpha factor to the alpha factor surface receptors on the cells causes the cell cycle to stop before DNA replication (before the S phase) in order for cell fusion of the tw o cell types
What checkpoint did alpha factors stop the cell cycle at and what did this result in?
at the G1 phase
results in schmoos
What are regulators?
proteins that can stimulate or inhibit cell progression through the cell cycle
What are protooncogenes?
genes that code for regulatory proteins that stimulate cell progression through the cell cycle
What are tumour suppressor genes?
genes that encode regulatory proteins that inhibit cell progression through the cell cycle
What are the most important cell cycle regulators?
cyclin-dependent kinases (CDKs)
What is the function of kinases?
to phosphorylate things (add a phosphate to things)
T or F: CDKs are always present in the cell but not always active
true
When are CDKs active?
when they are bound to cyclin proteins
Why does the activity of CDKs depend on cyclin?
to be active, CDKs must be bound to cyclin
the concentration of cyclin inside the cell fluctuates depending on the phase of the cell cycle
How do CDK-cyclin regulate the progression of cells through the cell cycle?
because the concentration of different CDK-cyclins varies and fluctuates throughout the cell and at different stages of the cell cycle
If CDK-cycling complexes are activated, what happens to the cell cycle?
it is stimulated
If CDK-cycling complexes are inactivated, what happens to the cell cycle?
the cell cycle is inhibited
How do cells progress from the G1 phase into the S phase?
growth factors stimulate signals inside cells that cause G1-phase cyclin concentrations to rise
the cyclins bind to CDKs which phosphorylate each other and drive the cell into S phase
How are cells inhibited from passing from the G1 phase into S phase?
if DNA is damaged, protein p53 inhibits the G1 CDK-cyclin complex which stalls the cycle for DNA to be repaired or to trigger apoptosis
Rb also prevents cells from entering S phase when there’s no growth factor signals (no kinases to phosphorylate them)
What does Rb stand for?
retinoblastoma protein
How would an Rb function if there were growth factors?
the growth factors would signal for the production of CDK-cyclin complexes which would phosphorylate the Rb and prevent it from inhibiting progression
What type of regulatory genes are the ones that code for CDK and cyclin?
protooncogenes
What type of regulatory genes are the ones that code for p53 and Rb?
tumour suppressor genes
How might cancer arise at the G1 checkpoint?
if there’s mutations in the tumour suppressor genes that code for p53 or Rb, the inhibition of the cell cycle may occur which may lead to cells with damaged or mutated DNA being replicated
What kind of mutations are ones on the tumour suppressor genes that code for p53 and Rb?
recessive (must occur on both alleles for cell cycle to be affected)
What proteins are involved in stimulating the rise in S-phase cyclins?
growth factors
What protein would be activated if a DNA strand breaks during replication?
ATM protein will halt the cell cycle and stimulate other proteins that can repair the DNA one of which is BRCA1
Which stage of the cell cycle would ATM be involved in?
S phase
there would be an arrest at the G2 checkpoint
What type of genes are the genes that code for ATM and BRCA1?
tumour suppressor genes
What happens when there’s mutations in the genes that code for the ATM and BRCA1 proteins?
higher risk of cancers such as
ATM = leukemia and lymphoma cancers
BRCA1 = breast and ovarian cancer
What does ATM stand for?
ataxia telangiectasia mutated
What does BRCA1 stand for?
Breast Cancer 1
What proteins stimulate progression from the M checkpoint?
M-phase cyclins + CDKs activate a complex called the Anaphase-Promoting Complex/Cyclosome (APC/C)
What does APC/C stand for?
Anaphase Promoting Complex/Cyclosome
What activates the APC/C?
M-phase cyclins + CDKs
What happens when the APC/C is activated?
the attachment of spindle microtubules to the centromeres of chromosomes
What does APC/C do if there is an incorrect attachment of spindle microtubules to chromosomes?
active APC/C stimulates the destruction of proteins that hold the 2 copies of each chromosome/chromatid together at the centromere so that the chromosomes/chromatids can separate properly during anaphase
What are the inhibiting proteins of the M phase?
MAD = Mitotic Arrest Deficient proteins
What does MAD stand for?
Mitotic Arrest Deficient proteins
How do MAD proteins function in inhibiting the cell from progressing from metaphase to anaphase?
if chromosomes/chromatids are not properly attached to the mitotic spindle, MAD will inhibit APC/C and prevent anaphase
What is the goal of MAD?
to prevent aneuploidy by stopping anaphase from happening in cells where chromosomes / chromatids are incorrectly attached to mitotic spindles
What are mutated versions of proto-oncogenes called?
oncogenes
What do oncogenes do?
they increase stimulation of regulatory proteins and allow for uncontrolled cell division that can lead to cancer
What do tumour suppressor gene mutations cause?
a loss of inhibition in regulating the cell cycle which can lead to uncontrolled cell division and cancer
What do mutations in proto-oncogenes cause?
a gain of function
Are mutations of proto-oncogenes dominant or recessive? what does this mean?
dominant
a mutation in only one allele will produce a protein that puts the cell cycle into overstimulation
Are mutations of tumour suppressor genes dominant or recessive? What does this mean?
recessive
both alleles must be mutated for the cell cycle to be affected
