LT9-10 The Cell Cycle

The Cell Cycle

What is the Cell (Division) Cycle?

 All organisms, whether unicellular/multicellular need to reproduce = carrying out an
orderly sequence of events in which it duplicates its contents and then divides in
two – cycle of duplication and division
 Unicellular organisms -> new organism produced
Multicellular organism -> Growth, differentiation
 Fundamentally – the cell cycle is essential for replicating and passing on its genetic
information to next generation of cells
 Considerations of producing two genetically identical cells:
- DNA in each chromosome must be the same
- Replicated chromosome accurately segregated
- Duplicate macromolecules and organelles
- Double in size before they divide – to maintain size
 Cell cycle durations vary = yeast cells (1.5 hours), human liver cells (1 year)

Typical Eukaryotic Cell Cycle

Interphase
Transcribes genes, synthesise proteins, grow in mass
G1 + G2 Phase  Cell monitors internal and external
environments to ensure that conditions
are suitable to progress to S/M Phase
 Both G phases provide additional time
for the cell to grow, duplicate
cytoplasmic organelles – double in size
S Phase Synthesis of DNA
Two copies of each chromosome retain
tightly bound together
Mitosis (+ Cytokinesis not part of mitosis)
M Phase Produce two daughter cells
First sign – progressive condensation of its
Variations of the cell cyclechromosome, which makes DNA less likely
Early embryonic stage, where there
to get are no(easier
entangled gap phases: cleavage
to segregate into 2divisions subdivide a giant egg cell into
daughter cells)


Experimental Systems

 Organisms with inherently synchronous embryonic cell division can be induced by
hormones to undergo meiotic maturation (drives oocyte from interphase –arrested
state to metaphase-arrested state awaiting fertilisation)

, Frog (Xenopus laevis), sea urchin (Arbacia punctulata), clams (Spisula
salidissima)
 Synchronous division allows one to study the behaviour of populations of cells
rather than individual cell
 Sizeable populations of mollusc/amphibian oocytes can be collected for biochemical
analyses
 Large size also amenable to injection of proteins/drugs and the effect of treatment
can be studied
 Cytoplasmic extracts can be prepared from oocytes and stored for future use –
extracts retain ability to assemble nuclear envelopes around DNA, form mitotic
spindles – cell cycle events can be recapitulated in vitro
 Xenopus oocyte especially convenient test system for detecting an event that
drives cells into M phase due to large size, completed DNA replication, arrested at
state in meiotic cell cycle equivalent to G2 phase of mitotic cell cycle
 Budding yeast (Saccharomyces cerevisiae) and fission yeast (Saccharomyces
pombe)
 Easy to grow and manipulate – fast growing division cycle of 1-4 hours
 Nuclear envelope of yeasts does not break down during mitosis – otherwise similar
to other eukaryotic cell cycles
 Both yeast species can be grown as haploid
 Conditional loss of function mutants allows isolation (permissive growth condition)
(Hartwell et al, 1970) isolated
temperature-sensitive mutants = cell
division cycle (cdc) – cdc screens highly
productive in identifying genes regulating diverse cell cycle
events (arrest with uniform bud size
indicates that mutant cells are defective
in progression through specific stage of cell cycle)
 Drosophila melanogaster
 Inherently synchronous – nuclear division cycles within common cytoplasm
 Mammalian tissue culture cells
 Ideal to study mammalian cell cycle in tissue culture using normal primary cells
(those obtained directly from organism with no genetic alteration) =these stop
dividing 25-40 divisions
 Immortalised cell lines derived from normal/tumour cells widely used for cell cycle
analyses – genetic alterations that allow them to divide indefinitely when supplied
with appropriate media and growth-promoting agents
 Various discoveries (LOOk at pg.679 need sources)

, Cell cycle is coordinated

 Cell cycle needs to be highly coordinated – various
checkpoints that help to ensure accuracy and timing of
events
 Concept of checkpoints: monitor proper completion of
distinct cell cycle processes and control transition from
one stage to another
 Central cell cycle control receives constant feedback
throughout the cycle – as long as event still in progress,
signal is sent to central control system that prevents
initiation of next event

Cyclical Activation of Protein kinases (Cyclin-dependent protein kinases)

 Activation of protein kinases at appropriate times are key components of the
control system, activity mainly controlled by the presence of cyclins
 Cyclins: concentration vary in cyclical fashion during cell cycle – drives the cyclic
assembly and activation of cyclin-cdk complexes; activation of these complexes
triggers various cell-cycle events (eg. entry into S/M phase)
 Cdk is a complex of 2 polypeptides – one that binds to ATP and the other containing
an active site – only possesses kinase activity when bound to cyclin
 During period where each kinase is active – phosphorylates large number of proteins
to activate/progress a stage in cell cycle or to inhibit/prevent repetition of a stage
Eg. Cdk that initiates mitosis phosphorylates lamin proteins so nuclear lamina can
break down, phosphorylates other proteins which regulate mitotic spindle assembly

Discovery of cyclins and cdks

 Cdks first identified through
identification of cdc mutants in yeast
Sequencing of S.pombe cdc2+ gene was
homologous to CDC28 gene in S,cerevisiae
(both proteins were orthologs and had a
shared function)
Through complementation, the human homolog CDC2 was found
- Cell-cycle control genes have been highly conserved

Yeast with defective copy encoding its only Cdk fails to divide, the mutant will divide
normally if copy of appropriate human gene is artificially introduced (source?)

 Maturation Promoting Factor (MPF – now called M-cdk)