Molecular & Cell Biology

S. Rahgozar,PhD

University of Isfahan

Faculty of Science

3. Cell Cycle

3. 1. The Eukaryotic Cell Cycle and Regulation

1389-90

Phases of the cell cycle

Mitosis

Interphase

Nuclear division ending with cytokinesis

Typically 24h for a proliferating human cell in culture

o G1 Phase: The cell is

metabolically active and

continuously grows without

DNA replication

o S Phase: DNA replication

o G2 Phase: Cell growth

continues and proteins are

synthesized.

1 h

4 h

8 h

11h

Embryonic cell cycle

Start

Quiescent stage in cell cycle;G0

30m 30m 30m

Nerve cell/fibroblast

cell cycle

Determination of cellular DNA content

Regulation of cell cycle

Extracellular signals

Saccharomyces cerevisiae

Regulation of animal cell cycles

Growth factors

Vertebrate oocytes can remain in G2 for

long periods of time until their

progression to M phase is triggered by

hormonal stimulation.

Hormones

The restriction point

can be placed in

different regions

DNA damage checkpoints

Spindle assembly checkpoints

Monitor

o The integrity of DNA

o The incorporation of correct bases

Monitor

o The alignment of chromosomes on the mitotic spindle

Cell cycle checkpoints

Intracellular factors

Check point for

once per cycle

DNA replications

ORC: Origin Recognition Complex proteins

The MCM proteins are controlled

by “liscencing factors” to allow

replication to initiate.

Origin recognition complex

Green: rich in As and Ts

Blue: Protein that facilitates ORC binding

o A new pre-RC cannot form at the origin

until the cell progress to G1 phase.

o an Origin of Replication can be used only

if a prereplicative complex forms there in

G1 phase.

o Pre-RC disassemble through

phosphorylation, and DNA replication

starts.

Mechanism of DNA replication initiation

Each origin of replication is activated only once per cell cycle

Regulators of

cell cycle progression

MPF (Mitosis Promoting Factor)

Further experiments

showed that MPF is not

restricted to oocytes and

appeared to act as a general

regulator of the transition

from G2 to M .

cdc (cell division cycle) mutant genes

o Studies on genetic analysis of Saccharomyces cerevisiae and the discovery of

cdc28.

o Studies on genetic analysis of Schizosaccharomyces pombe showed a functionally

homologous gene; cdc2 which arrested the fission yeast both in G1 and G2 to M.

k1( the protein kinase encoded by cdc2 & cdc28)

A conserved cell regulator in all eukaryotes

Cyclins

In 3 years later, Joan Ruderman and colleagues supported the importance of cyclins

as the cell cycle regulators, by showing that microinjection of cyclin A into frog

oocytes is sufficient to trigger the G2 to M transition.

cdc28 Cd

Accumulation and degradation of cyclins in sea urchin embryos

These proteins were suggested that they might function to induce mitosis,

with their periodic accumulation and destruction controlling entry and exit from

M phase.

Structure of MPF (the conserved regulator of the cell cycle)

Cyclin B is a regulatory unit required for the catalytic activity of Cdk1.

MPF activity is controlled by the periodic accumulation and degradation

of cyclin B during cell cycle progression.

MPF regulation in mammalian cells

Ubiquitin dependent Cell exits M, undergoes Cytokinesis and returns to G1

Happens in G2

cdc25

MPF regulation in mammalian cells

Ubiquitin dependent Cell exits M, undergoes Cytokinesis and returns to G1

Happens in G2

cdc25

Complexes of cyclins and

cyclin-dependent kinases