cell cycle Flashcards
what are 2 main phases of cell cycle?
- interphase and mitotic phase
what are the phases of interphase?
G1, (G0), S, G2,
what are the phases of mitotic phase?
prophase
(prometaphase)
metaphase
anaphase
telophase
(cytokinesis)
what are cyclins? and examples in cell cycle?
a family of proteins whose concentration increases and decreases throughout cell cycle
- cyclin A (S phase)
- cyclin B (G2 phase)
- cyclin E (G1 phase)
what do cyclins do in cell cycle?
they activate cyclin dependant kinases (CDK’s) which act in conjunction with with cyclins
what happens with cyclins & CDK’s after completion of cell cycle?
they are degraded - the residues pf used up molecules taken up by cytoplasmic care taker protein = ubiquitin
what happens in G1 phase?
gap in cell cycle before S phase (first phase of interphase)
- period where cells respond to growth factors directing initiation of new cycle
(phase that needs most molecular machinery is required to complete another cycle)
what is G1 phase controlled by?
cyclin E (for entry) and CDK’s
what is Go pahse?
cells enter G0 phase when:
cells remain capacity for proliferation and in process of differentiation (still have potential for being involved) but no longer dividing
- described as quiescent (like dormant)
how can cells leave G0 phase?
by cells in G0 being stimulated by growth factors
what blocks the cell cycle in G1?
proteins encoded by certain tumour suppressor genes like retinoblastoma protein (regulates transition form G1 to S phase)
how long does each phase of cell cycle take to complete?
G1 = varies from 2- more than 100 hours
S = 7/8 hours
G2= 2-4 hours
prophase = 1.5 hours
metaphase = 20 mins
anaphase & telophase = don’t know?
what is S phase?
DNA replication phase = new DNA synthesised and relation of genes to DNA established
what controls S phase?
cyclin A and CDK’s
how can DNA replication/DNA synthesis not work in S phase?
certain anti-cancer drugs can exert their cytotoxic effect by intercalating into narrow groove of DNA double helix
(DNA has 2 chains coiled around helical axis and paired in antiparallel menner, hydrophilic deoxy ribose- phosphate on outside and hydrophobic on inside - arrangement makes major (wide) groove and minor (narrow) groove)
how can DNA be damaged during replication?
= deoxynucleotide (amino acid monomer) incorporation error
= spontaneous deamination (removal of amino group) of bases
= x-radiation causing nicks to DNA
= UV radiation
= various chemicals
what is G2 phase?
second growth phase
when the cell prepares for division; this period ends with the breakdown of the nuclear membrane and the onset of chromosome condensation
what is G2 phase controlled by?
cyclin B and CDK’s
when are chromosome first individually visable?
in prophase
(in interphase they’re just long, narrow structures)
what happens in prophase?
= individual chromosomes visualised as thin threads and are tightly coiled along entire length
= each chromosome splits longitudinally into 2 chromatids joined at centromere
= 2 paired centriole (that were outside nuclear envelope in interphase) separate by elongation of continuous microtubules of achromatic spindle & go to opposite poles of nucleus
what happens in prometaphase?
nuclear membrane & nucelolus disappears
- chromosomes entangles in meshwork of continuous microtubules
what happens in metaphase?
= centromere region of each chromosome presents bilateral disc called kinetochores
= from kinetechores, set of chromosomal microtubules are organised and extend bilaterallly towards opposite centrioles
= growth of chromosomal microtubules allow centromere of individual chromosomes with their paired chromatid to occupy equatorial plane of spindle (they line up along equator)
what is function of colchicine?
arrests cell cycle at metaphase
= prevents formation of microtubule of spindle
why is colchicine used in cell culture and what result is found?
it’s important tool in karyotyping
- when used in metaphase, chromosomes assemble around centriole
underpins rationale for cytotoxic drugs used in cancer therapy (something to use to stop replication)
what happens in anaphase?
= centromeres split longitudinally and chromatids seperate to form 2 new chromosomes
= spindle fibres contract and exert centrifugal force in centromere region and newly formed chromosome pulled apart to opposite poles
what is non- disjunction?
when in anaphase, the chromosomes fail to migrate properly - tends to lead to one daughter cells having extra chromosome and one having less
what are isochromosomes?
centromere splits transversely instead of longitudinally in anaphase
= forms 2 daughter chromosomes of unequal length
what happens in telophase?
= recognisation period
= daughter chromosome enveloped by new nuclear membrane & nucleolus appears
= chromosome becomes uncoiled and individual identity lost
what happens in cytokinesis?
cytoplasm divides and cleavage furrow develops (the pinching shape of plasma membrane) around the equator region of achromatic spindle
= contraction of fibrillar component of cytoplasm help in hour glass constriction of cleavage
(actin-myosin at cleavage furrow)
