Cell Cycle Flashcards
1
Q
Overview of cell cycle
A
- growth and reproduction
- chromosome number duplicates and gene expression occurs
- sister chromatids are separated during mitosis - M phase
- cell divides forming 2 genetically identical daughter cells
- during interphase, cells grow and duplicate their genome
- s phase = duplication
2
Q
Chromatin organisation - during interphase
A
- mix of DNA and associated proteins (histones)
- allows for DNA compaction and involved in regulation of DNA activity
- nucleosomes - basic structural units of chromatin
3
Q
Histone organisation during interphase
A
- 4 types, H2A, H2B, H3 and H4
- each core histone possesses 2 domains, amino terminal tail and histone fold
- form dimers with eachother
- 8 histones comprise 1 nucleosome - octamer structure
- have tails which allow other proteins to bind
- H1 sits outside nucleosome, interacts with DNA and determines overall nucleosome structure
- individual nucleosomes are connected by linker histones and linker DNA, can change the path that DNA exits the nucleosome
- SMC - structural maintenance of chromosomes - 2 complexes:
- cohesin - organise chromatin during interphase
- condensin - important during mitosis
- chromatin organised into loops during interphase
- loop domains are organised by cohesin complexes and CTCF dimer complex (DNA binding protein- stops looping)
- loops are important for regulation of gene expression and chromatin compaction
- after mitosis, chromosomes decondense in specific regions of the nucleus - chromosomal territories
4
Q
Genome organisation during mitosis
A
- 2 chromatids
- joined by centromere
- Telomeres - each mitotic chromosome = 2 DNA molecules = sister chomatids
- each DNA molecule in a mitotic chromosome is 10000 fold shorter than its extended length
- in early mitosis, condensins replace cohesins
- condensins are loaded onto chromatin and cohesins are removed
- forms loops - randomly, no regulation - very different than during interphase
5
Q
G1 checkpoint
A
- is the environment favourable?
- sufficient cell growth?
- damaged DNA?
- if all passed, move to S phase
6
Q
G2 checkpoint
A
- is all DNA replicated?
- is there any damaged DNA?
- if passed, enter mitosis
7
Q
M checkpoint
A
- are all chromosomes attached to spindle?
- metaphase to anaphase transition
- if incorrect, initiation of sister chromatid separation can be blocked
8
Q
Cyclin dependent kinases (Cdks)
A
- when active, trigger specific cell cycle events - Cdk + cyclin = active Cdk
- cyclin levels change throughout the cell cycle
- Cdk activity also fluctuates - concentrations remain stable throughout
- phosphorylation of proteins drives transition through the cell cycle - each Cdk/cyclin complex phosphorylates a different set of substrate proteins
- cyclins also direct the activated Cdk to its target protein
- accessibility of substrates changes throughout the cycle
9
Q
Role of G1 cyclins
A
- G1 cyclins = bind + activate Cdks that stimulate entry into new cell at start, concentration depends on rate of cell growth/on promoting signals (not phase of cycle)
10
Q
Role of G1/S cyclins
A
- activate Cdks that stimulate progression through start, results in commitment to cell cycle entry, concentration depends peaks in late G1
11
Q
Role of S cyclins
A
- activate Cdks necessary for DNA synthesis, conc increases and remains high during S phase, G2 and early mitosis, contribut to some early mitotic events
12
Q
Role of M cyclins
A
- activate Cdks necessary for entry to mitosis, conc rises at approach to mitosis + peaks in metaphase
13
Q
Cdk activating kinases - mechanism 1
A
- before cyclin binds, active site in Cdk is blocked by t-loop
- when cyclin binds, t-loop unfolds = partially activated Cdk
- phosphorylation of Cdk by CAK further activates the Cdk by changing the shape of the t-loop
14
Q
Cdk activating kinases - mechanism 2 (regulatory pathway)
A
- active cyclin/Cdk complex can be inactivated by Wee1 or Myt 1
- dephosphorylation by phosphatase Cdc25 leads to reactivation (reversible reaction)
15
Q
Cdk inhibitor proteins - mechanism 3
A
- p27 protein
- binds to whole complex, causing structural changes, inhibits complex
- usually in G1 or in response to inhibitory signals from environment or damaged DNA
16
Q
Transition through G1 in favourable conditions
A
- E2F - transcription factor
- inhibited by protein Rb
- Cdk/cyclin complex phosphorylates Rb and releases it from E2F
- E2F can then transcribe genes important for S phase
17
Q
Transition through G1 if DNA is damaged (start checkpoint/G1 arrest
A
- ATM/ATR + Chk1/Chk2 pathways signal that DNA is damaged
- triggers Mdm2 to release from P53
- P53 is phosphorylated - active (acts as a transcription factor)
- P53 binds to regulatory region of P21
- P21 transcribed and translated
- P21 is a Cdk inhibitor protein
18
Q
Regulated proteolysis
A
- ubiquitin pathway used to degrade proteins - controlled manner
- 3 ubiquitin ligases are required: E1, E2 and E3
- ubiquitin covalently attached to lysine, targeted for degradation
- polyubiquitin chain produced
- ubiquitinated protein broken down by proteasomes
19
Q
What are Proteasomes
A
- large protein complexes
- proteolytic activity
- responsible for degrading proteins marked by polyubiquitin modification
20
Q
Prophase
A
- chromosomes condense
- mitotic spindle assembles (produced by centrosomes)
21
Q
Prometaphase
A
- nuclear envelope breaks down (only in some organisms) = open mitosis
- chromosomes attach to spindle microtubules via kinetochores and undergo active movement
22
Q
Metaphase
A
- chromosomes aligned at equator of spindle (metaphase plate)
- kinetochore microtubules attach sister chromatids to opposite poles of the spindle
23
Q
Features of kinethochores
A
- multiprotein complexes responsible for attachment of chromosomes to microtubules of mitotic spindle, assembled on centromeric chromatin
- assemble during early mitosis
- microtubules attach to outer kinetochore
- each chromatid of a chromosome has its own kinetochore
- 20-30 microtubules per kinetochore
- correct attachment of all kinetochores is required for anaphase to begin
24
Q
Cohesion of sister chromatids
A
- cohesins bind to sister chromatids, keeping them together
- some of the cohesin is removed before anaphase
- remaining cohesin is localised at the centromeric region
25
How is cohesin removed? APC/C INACTIVE
- APC/C remains inactive until all kinetochores are properly attached to microtubules
- inhibited APC/C stops progression of cells in metaphase, allowing more time for correct kinetochore attachment
26
How is cohesin removed? APC/C ACTIVE
1)
- cyclin B + Cdk bound
- APC/C ubiquitilates cyclin B
- degrading it
- leaves Cdk inactive
- allows mitotic exit
2) metaphase to anaphase transition
- enzyme separase, inhibited by securin
- APC/C ubiquitilates securin, degrading it
- separase now active
- separase cleaves cohesin ring
- triggering anaphase
27
Anaphase
- kinetochore microtubules shorten
- pulling apart sister chromatids to form 2 daughter chromosomes
28
Telophase
- 2 sets of daughter chromosomes assemble at opposite poles + decondense
- nuclear envelope reassembles around each set forming 2 nuclei
- central spindle is formed
29
Cytokinesis
- contractile ring ( actin and myosin filaments) forms cleavage furrow
- forms 2 genetically identical daughter cells each with 1 nucleus
30
Centrosomes (Microtubule organising centres MTOCs)
- contains centrioles - short cylindrical arrays of microtubules and pericentriole material
- gamma tubulin - nucleates microtubules at centrosomes (polymerisation role)
- centrosomes duplication mirrors DNA replication - during S phase
- more than 2 centrosomes in a cell causes genomic instability - usually found in cancer cells
31
Mitotic spindle assembly - animal cells
- migrate around nuclear envelope to opposite poles of the cell - start to produce microtubules
- microtubules attach to sister chromatids during prometaphase - not fully correct attachment
- metaphase - spindle assembly checkpoint - all joined together
- microtubules have inherent polarity, slowly depolymerise at (-) ends and rapidly polymerise or depolymerise at (+) ends
32
Microtubules of mitotic spindle
- (+) ends of the microtubules project away from the spindle pole
- (-) ends are anchored at the spindle pole
- kinetochore microtubules connect the spindle poles with the kinetic horse of sister chromatids
- interpolar microtubules from the 2 poles interlock at the spindle equator
- astral microtubules radiate out from the 2 poles into the cytoplasm and contact the cell cortex
- plant cells do not contain centrosomes but they have fully functional mitotic spindles
33
Inhibitors of microtubule dynamicity
- e.g. colchicine - binds at beta tubulin interface and blocks polymerisation of microtubules, leading to cell shortening
- unattached kinetochores trigger spindle assembly response
34
How are cancer cells made?
- colonal evolution = division in an uncontrolled, autonomous way
- tumour develops through repeated rounds of mutation + proliferation - gives clone of fully malignant cancer cells
- normal cell division + apoptosis = homeostasis (normal)
- increased cell division + normal apoptosis = tumour
- normal cell division + decreased apoptosis = tumour
35
External factors regulating cell division
- mitogens - stimulate cell division (trigger G1/S - Cdk activity)
- growth factors - stimulate cell growth (synthesis of proteins)
- survival factors - promote cell survival (suppress apoptosis)
- all, via receptors, induce signalling pathways/cascades affecting cell cycle progression (induce G1/S gene regulators, prevent passing start checkpoint = Ras pathway)
36
Mutations leading to cancer development
E.g. mutant receptor (protein kinases) could be constantly active - doesn’t require signals
E.g. amplified receptor - excessive receptor activity
- mutation of ANY component of a signalling pathway may lead to cancer development
37
Feature of cancer cells
- bypass normal proliferation controls
- altered control of growth
- can colonise other tissues
- derive from a single abnormal cell
- contain + accumulate somatic mutations
- require multiple mutations to form
- ability to survive stress and DNA damage
- genetically unstable
38
Oncogenes
- a gene whose protein product promotes cancer, generally because of mutations in a normal gene (proteo-oncogene) have resulted in a protein that is overactive/overproduced
- dominant, require 1 mutation to have an effect
- proteo-oncogenes regulate cell growth and division
39
Tumour suppressor genes
- encodes a protein that restrains cell proliferation
- if mutated, can cause tumours
- recessive, requires mutation in 2 alleles of a gene to eliminate the tumour suppressor gene - promotes cell transformation
40
Meiosis
- nuclear division leading to haploid cells
- produces 2 gametes
- similar control systems to mitosis
41
Meiosis 1 - Prophase 1
- long process during meiosis
- 2 closely aligned duplicated homologs called a bivalent
- homologs joined by a protein complex called a synaptonemal complex (SC)
42
Synaptonemal complex
- each homologs organised around a protein axial core
- the synaptonemal complex forms when these homolog axes are linked by rod shaped transverse filaments
- the axial core of each homolog interacts with cohesin complexes that hold sister chromatids together
43
Homolog synapsis and desynapsis during prophase 1
- in a single bivalent: at leptotene, the 2 sister chromatids coalesce and their chromatid loops extend out from a common axial core
- assembly of the synaptonemal complex begins in early zygotene and is complete in pachytene
- the complex disassembles during diplotene
- followed by diakinesis
44
Crossing over
- thin connection between homologs called a chiasmata
45
Products of meiosis 1 & 2
Meiosis 1: haploid cells with chromosomes containing 2 chromatids
Meiosis 2: haploid cells with chromosomes containing 1 chromatid
46
Meiosis vs mitosis
- homologs separate rather than sister chromatids
- sister kinetochores in a homolog must be stably attached to the same spindle pole
- chiasmata hold homologs together allowing their bi-orientation at the equator
- centromeric cohesin stays on during anaphase 1, this allows sister chromatid pairs to correctly bi-orient during meiosis 2
