Cell Cycle

From CellBiology


Cell Cycle

The Cell Cycle in relation to cyclin levels


This lecture will introduce the cell cycle, which is the entire life of a cell from birth to death. This overall topic is too large for a single lecture, so will focus on our understanding of the regulation of the cell cycle. Later lectures will cover concepts on cell birth (cell division), differentiation (signaling, development) and death (2 lectures) in more detail.

Lecture Slides: 2017 Lecture PDF


MH - note that content listed below will not match exactly current lecture structure but has been selected as having similar content.

2017 PDF | 2016 | 2013 | 2009 Lecture 15

Textbook - Molecular Cell Biology


  • Understanding of the main stages of the cell cycle
  • Broad understanding of different cell lifespan
  • Understanding of the differences between embryonic and adult cell cycles
  • Understanding of the key regulators of cell cycle
    • cyclins
    • cyclin dependent kinases
  • Broad understanding of abnormal cell cycles
    • How the cell cycle is monitored

Background Reading

Nature - Cell Division Milestones

Recent Nobel Prizes

2001 - Control of the Cell Cycle

  • Leland Hartwell (born 1939), Fred Hutchinson Cancer Research Center, Seattle, USA, is awarded for his discoveries of a specific class of genes that control the cell cycle. One of these genes called "start" was found to have a central role in controlling the first step of each cell cycle. Hartwell also introduced the concept "checkpoint", a valuable aid to understanding the cell cycle.
  • Paul Nurse (born 1949), Imperial Cancer Research Fund, London, identified, cloned and characterized with genetic and molecular methods, one of the key regulators of the cell cycle, CDK (cyclin dependent kinase). He showed that the function of CDK was highly conserved during evolution. CDK drives the cell through the cell cycle by chemical modification (phosphorylation) of other proteins.
  • Timothy Hunt (born 1943), Imperial Cancer Research Fund, London, is awarded for his discovery of cyclins, proteins that regulate the CDK function. He showed that cyclins are degraded periodically at each cell division, a mechanism proved to be of general importance for cell cycle control.

1989 - Cellular Origin of Retroviral Oncogenes

  • Michael Bishop and Harold Varmus used an oncogenic retrovirus to identify the growth-controlling oncogenes in normal cells. In 1976 they published the remarkable conclusion that the oncogene in the virus did not represent a true viral gene but instead was a normal cellular gene, which the virus had acquired during replication in the host cell and thereafter carried along.

Prokaryote Division

Escherichia coli
  • Binary Fission - and seen in eukaryote mitochondria
  • Asexual reproduction - replicates original cell to produce two identical cells
  • Grow in numbers exponentially - adequate nutrients and a fast life cycle
  • single organism can multiply into billions
  • High mutation rate of bacteria

Appear to involve proteins that are homologs of eukaryotic cytoskeleton proteins. Prokaryotic Cytoskeleton Filaments

Cell Lifespan

  • Body Cell Types - about 210 types
  • Lifespan
    • Born
    • Differentiate
    • Function
    • Die or Divide

Cell Lifespan Examples

  • Intestinal epithelial cells 3–5 days
  • Red blood cell 120 days
  • Brain neuron, heart 50 - 100 years
  • Neutrophil
    • in circulation 6-7 hours
    • in tissue 4 days

Epidermis Example

Epidermis cartoon Cornified Epithelium

Lifespan Processes

  • Birth - Mitosis (except germ cells - Meiosis)
  • Growth - Expression of genes and proteins required to grow the cell, its organelles and cytoskeleton
  • Function - Expression of tissue specific genes and proteins
  • Division - DNA during cycle, whole cell in Mitosis
  • Death - Apoptosis (programmed cell death) Necrosis (un-programmed cell death)

Movie - Embryo Mitosis | Movie - Cell Death (Apoptosis)

Apoptosis cell culture.jpg

Cell Cycle Major Phases

  • Mitosis (M phase)

Cell birth(division) small time of cell cycle

  • Interphase

Most cell life Cell growth, function DNA synthesis organelle development

Cell Cycle

  • Time cell comes into existence until that cell divides again
  • Rapidly growing human cells 20-24 hr
  • Liver cells 1-2 year
  • Neurons 1 only
  • Quiescent G0

Cell Cycle- Stages

Rapidly dividing cell (20-24hr)


  • M phase 1 hr


  • G1 Phase
    • cellular growth 9hr
    • Most variable time
    • Can exit to G0
  • S Phase
    • DNA duplication 9hr
  • G2 Phase
    • growth prepare for mitosis 4 hr
Cell Cycle Phases

Cell Cycle Phases

Cell Cycle Differences

Early Embryonic Cycle

  • no growth occurs
  • each daughter cell is half the size of parent cell
  • cycle time is very short
  • S phases and M phases alternate without any intervening G1 or G2 phases

G0 Phase

  • exits the cycle at G1 (cancer cells do not enter G0)
  • cell can leave the cell cycle (temporarily or permanently)
  • temporarily - quiescent
  • permanently - terminally differentiated
    • cell will never reenter the cell cycle
    • carry out their function until they die
  • not simply the absence of signals for mitosis
  • active repression of the genes needed for mitosis

Stem Cells

Central Nervous System

  • development requires extensive progenitor cell proliferation (Neural stem cells, NSCs) PMID 24859217
  • closely followed by differentiation (neurons and glial cells)
  • gives rise to differential growth and cellular diversity

Cell Cycle Regulation

Cell proliferation is strictly regulated

Unregulated/abnormal proliferation is oncogenesis or Cancer

An overview of the cell-cycle control system

Cell Cycle- External Regulators

  • Cell replacement in different tissues
    • regulated by growth factors
    • can be specific for specific cell types

Growth Factors and Cell Cycle Progress

  • External factors can also regulate progression through cycle
  • Growth factors primarily act on cells in G0 and G1
  • The restriction point is the timepoint in G1 when cells no longer respond to withdrawal of growth factors by returning to G0, but progress to S phase.
    • thought to involve retinoblastoma protein (pRb)

Restriction Point

Growth Factor Model

  • Fibroblasts in culture
    • Serum (Prepared by clotting)- Proliferation
    • Plasma (Prepared by centrifugation, no clotting)- no proliferation
  • Clotting
    • allows platelets to release secretory granules
    • Platelet-derived growth factor (PDGF)
  • Connective tissue cells express PDGF receptors which bind the small PDGF glycoprotein

Other Growth Factors

  • Interleukin-2 (IL-2)
    • Stimulates T lymphocytes
  • Nerve Growth Factor (NGF)
    • Promotes neuronal survival and growth
  • Epidermal Growth Factor (EGF)
  • Vascular Endothelial Growth Factor (VEGF)
  • Insulin-like growth factors (IFG-1, IGF-2)

Cell Cycle- Internal Regulators

1980s - studies in Xenopus eggs and starfish oocytes: purification of M phase-promoting factor (MPF); identification of its components as cyclin B and CDC2 (also called cyclin-dependent kinase, Cell Division Cycle)


Cyclins are synthesized and degraded each cell cycle (hence the name)

Cyclins and Cyclin-Dependent Kinases need to interact for cell cycle progression Cyclins and Cyclin-Dependent Kinases

Cell cycle1.jpg

Cyclin D

Cyclin D Interactions
  • cyclin D1, D2 and D3
  • expression induced by growth factors stimulation
    • serum growth factors to quiescent cells promotes transcription of the cyclin D1 gene
  • Cyclin D1 then binds Cdk4 and Cdk6 in early to mid-G1 phase
    • phosphorylate and inactivate retinoblastoma protein (pRb)
  • also acts as a cofactor for several transcription factors in numerous cell types

Cyclin D1 is a proto-oncogene

  • proto-oncogene - a normal gene that can become an oncogene due to mutations or increased expression.
  • mutations are associated with cancer progression
  • as a regulator of G1 to S-phase transition

Cyclin E

  • required for transition from G1 to S phase
  • Cyclin E binds to G1 phase Cdk2
  • Cyclin E/Cdk2 complex phosphorylates p27Kip1 which then degrades
    • an inhibitor of Cyclin D
  • activation of Cyclin E gene can be blocked by the cdk inhibitor p16 (Cyclin-dependent kinase inhibitor 2A)
    • tumor suppressor protein
  • expression of Cyclin A then increases for progress to S phase

Cdk/cyclin complexes

Cyclin A

  • accumulates from early S phase
    • role not fully understood, required for S phase progress
  • Cyclin A binds Cdk2
  • disappears ahead of cyclin B during mitosis
  • can bind to both Cdc2 and Cdk2

Cyclin B

  • accumulates from S phase
  • Cyclin B forms a complex with Cdc2
    • complex is kept inactive by phosphorylation of Cdc2
    • abruptly activated by Cdc25 during mitosis
  • cyclin B is destroyed at mitosis exit by ubiquitin-mediated mechanism (catalyzed by the APC/C)

Anaphase-Promoting Complex (APC, cyclosome, APC/C)

  • degrades the mitotic (B) cyclins
  • triggers the events leading to destruction of the cohesins
  • allowing the sister chromatids to separate


Links: MBC Figure 13-2. Current model for regulation of the eukaryotic cell cycle | control of proteolysis by SCF and APC during the cell cycle

Cyclin-dependent Kinases

Cell cycle - Cdk cyclins
  • Inactive until bound to a specific cyclin
  • kinase - means an enzyme which phosphorylates a target protein(s)
  • Drive M to S Phase
    • cdk1 and cdk2
  • Cdk1 activated at G2 to M
    • unphosphorylated Cdk1 increases as a function of cell size in the G2 phase (mechanism that regulates cell size in animal cells) PMID 23316440
  • Cdk2 activated at G1 to S
  • nuclear proteins and proteins involved in regulating metabolic processes
    • phosphorylated (inactivated) by CDK1 or CDK2 in mitotic cells. PMID 20068231

Movie CDK/Cyclin: Cell-Cycle Control

Cell Cyclin Changes

Interphase and M Phase

  • Division controlled by synthesis/degradation cyclin B
  • regulatory subunit of Cdc2 protein kinase
  • interphase cyclin B synthesis leads to formation of active cyclin B–Cdc2 complex
  • induces entry into mitosis

Rapid degradation of cyclin B leads to inactivation Cdc2 kinase

  • Allow cell to exit mitosis and return to interphase next cell cycle

  • cyclin B-CDC2 acts as M phase-promoting factor (MPF)
    • activate other proteins through phosphorylation
  • cyclin A down-regulation induces a G2 phase arrest through a checkpoint-independent inactivation of cyclin B-CDC2 by inhibitory phosphorylation.
    • cyclin A cannot form MPF independent of cyclin B

Regulator Checkpoints

These regulators are often described as "tumor suppressor proteins" due to their ability to block tumor (cancer) growth. Conversely, mutations in these genes often lead to tumor growth.

Checkpoints in the cell-cycle control system

How DNA damage arrests the cell cycle in G1

Multiple checkpoints control cell cycle progression


p53 pathways
  • (TP53) A multifunctional protein Mr 53 kDa regulating cell cycle and apoptosis
  • As a cell cycle regulator it recognizes and binds damaged DNA
    • single-stranded DNA, insertion/deletion mismatches, and free DNA ends
  • Acts as a transcription factor activating p21 transcription
    • p21 protein then inhibits G1 cyclin-dependent kinases p21

p53-induced cell-cycle arrest in response to DNA damage

unstressed cells

  • p53 levels are low through a continuous degradation
  • Mdm2 (HDM2 in humans)
    • binds p53
    • blocks action
    • transports p53 from nucleus to the cytosol
  • gene has been shown to be damaged in cells by mutagens
  • mutations found with

Effect of mutation of the p53 tumorsuppressor gene on G1 DNA-damage checkpoint control

Science magazine designated p53 the 'Molecule of the Year' for 1993


pRb Interactions
  • retinoblastoma protein regulating cell cycle
  • As a cell cycle regulator it recognizes damaged DNA
    • restricts cell ability to replicate DNA by preventing its progression from the G1 to S
  • binds and inhibits transcription factors of the E2F family
    • active pRb is hypophosphorylated
    • inactive pRb is phosphorylated
  • mutations in this gene lead to the disease retinoblastoma OMIM 180200


Restriction point control

  • also called Cyclin-dependent kinase inhibitor 2A
  • blocks activation of Cyclin E gene
  • mutations found with
    • pancreatic adenocarcinoma OMIM 260350
    • esophageal and gastric cancer cell lines


MH - There are many detailed resources on oncogenesis (cancer) this is only a brief mention in regard to cell cycle. See also the tumor suppressor section above. An important issue is the concept of "multiple hits".
  • some viruses can infect human cells and lead to oncogenesis (cervical cancer, liver cancer, and certain lymphomas, leukemias, and sarcomas)
    • for example - human papillomavirus (HPV)
Cancer requires multiple mutations

Leukaemia deregulated cell cycle check point proteins.jpg

Cell cycle phases showing some of the check point proteins that can be deregulated in leukaemia.

Stem Cells

Example of Cell cycle regulation in hematopoietic stem cells (HSCs). PMID 22123859

Additional Information

The following recent research is not examinable.

Intrinsic and extrinsic mechanisms regulating satellite cell function

Development. 2015 May 1;142(9):1572-1581. Review

Dumont NA1, Wang YX2, Rudnicki MA3.

Abstract Muscle stem cells, termed satellite cells, are crucial for skeletal muscle growth and regeneration. In healthy adult muscle, satellite cells are quiescent but poised for activation. During muscle regeneration, activated satellite cells transiently re-enter the cell cycle to proliferate and subsequently exit the cell cycle to differentiate or self-renew. Recent studies have demonstrated that satellite cells are heterogeneous and that subpopulations of satellite stem cells are able to perform asymmetric divisions to generate myogenic progenitors or symmetric divisions to expand the satellite cell pool. Thus, a complex balance between extrinsic cues and intrinsic regulatory mechanisms is needed to tightly control satellite cell cycle progression and cell fate determination. Defects in satellite cell regulation or in their niche, as observed in degenerative conditions such as aging, can impair muscle regeneration. Here, we review recent discoveries of the intrinsic and extrinsic factors that regulate satellite cell behaviour in regenerating and degenerating muscles. © 2015. Published by The Company of Biologists Ltd. KEYWORDS: Aging; Asymmetric division; Cell cycle regulation; Muscle stem cell; Myogenesis; Quiescence; Regeneration; Satellite cell; Self-renewal; Skeletal muscle PMID 25922523


Regulation of the cell cycle in satellite cells.


Cdks, cyclins and CKIs: roles beyond cell cycle regulation

Development. 2013 Aug;140(15):3079-93. doi: 10.1242/dev.091744.

Lim S1, Kaldis P.


Cyclin-dependent kinases (Cdks) are serine/threonine kinases and their catalytic activities are modulated by interactions with cyclins and Cdk inhibitors (CKIs). Close cooperation between this trio is necessary for ensuring orderly progression through the cell cycle. In addition to their well-established function in cell cycle control, it is becoming increasingly apparent that mammalian Cdks, cyclins and CKIs play indispensable roles in processes such as transcription, epigenetic regulation, metabolism, stem cell self-renewal, neuronal functions and spermatogenesis. Even more remarkably, they can accomplish some of these tasks individually, without the need for Cdk/cyclin complex formation or kinase activity. In this Review, we discuss the latest revelations about Cdks, cyclins and CKIs with the goal of showcasing their functional diversity beyond cell cycle regulation and their impact on development and disease in mammals.

PMID 23861057 Development

Connecting the nucleolus to the cell cycle and human disease

FASEB J. 2014 Apr 30. [Epub ahead of print]

Tsai RY1, Pederson T.


Long known as the center of ribosome synthesis, the nucleolus is connected to cell cycle regulation in more subtle ways. One is a surveillance system that reacts promptly when rRNA synthesis or processing is impaired, halting cell cycle progression. Conversely, the nucleolus also acts as a first-responder to growth-related stress signals. Here we review emerging concepts on how these "infraribosomal" links between the nucleolus and cell cycle progression operate in both forward and reverse gears. We offer perspectives on how new cancer therapeutic designs that target this infraribosomal mode of cell growth control may shape future clinical progress.

PMID 24790035

p53 and ribosome biogenesis stress: The essentials

FEBS Lett. 2014 Apr 18. pii: S0014-5793(14)00300-7. doi: 10.1016/j.febslet.2014.04.014. [Epub ahead of print]

Golomb L1, Volarevic S2, Oren M3.


Cell proliferation and cell growth are two tightly linked processes, as the proliferation program cannot be executed without proper accumulation of cell mass, otherwise endangering the fate of the two daughter cells. It is therefore not surprising that ribosome biogenesis, a key element in cell growth, is regulated by many cell cycle regulators. This regulation is exerted transcriptionally and post-transcriptionally, in conjunction with numerous intrinsic and extrinsic signals. Those signals eventually converge at the nucleolus, the cellular compartment that is not only responsible for executing the ribosome biogenesis program, but also serves as a regulatory hub, responsible for integrating and transmitting multiple stress signals to the omnipotent cell fate gatekeeper, p53. In this review we discuss when, how and why p53 is activated upon ribosomal biogenesis stress, and how perturbation of this critical regulatory interplay may impact human disease. Copyright © 2014. Published by Elsevier B.V.

PMID 24747423



Essential Cell Biology

  • Essential Cell Biology Chapter 17

Molecular Biology of the Cell

Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter New York and London: Garland Science; c2002

Molecular Cell Biology

Lodish, Harvey; Berk, Arnold; Zipursky, S. Lawrence; Matsudaira, Paul; Baltimore, David; Darnell, James E. New York: W. H. Freeman & Co.; c1999

The Cell- A Molecular Approach

Cooper, Geoffrey M. Sunderland (MA): Sinauer Associates, Inc.; c2000

Search Online Textbooks



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  • Entrez is the integrated, text-based search and retrieval system used at NCBI for the major databases, including PubMed, Nucleotide and Protein Sequences, Protein Structures, Complete Genomes, Taxonomy, and others Entrez

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  • The great divide: coordinating cell cycle events during bacterial growth and division. Haeusser DP, Levin PA. Curr Opin Microbiol. 2008 Apr;11(2):94-9. Epub 2008 Apr 7. Review. PMID 18396093 | PMC
  • Cell cycle studies based upon quantitative image analysis. Stacey DW, Hitomi M. Cytometry A. 2008 Apr;73(4):270-8. Review PMID 18163464
  • Analysis of cell cycle phases and progression in cultured mammalian cells. Schorl C, Sedivy JM. Methods. 2007 Feb;41(2):143-50. Review. PMID 17189856
  • Cell cycle regulation of DNA replication. Sclafani RA, Holzen TM. Annu Rev Genet. 2007;41:237-80. Review. PMID 17630848


External Links


  • Fzy and Fzr cooperate to destroy cyclin B in flies Raff et al. examine two regulators of fly cyclin B destruction: Fizzy (Fzy)/Cdc20 and Fzy-related (Fzr)/Cdh1. Fzy/Cdc20 is concentrated at kinetochores and centrosomes early in mitosis, whereas Fzr/Cdh1 is concentrated at centrosomes throughout the cell cycle. In syncytial embryos, only Fzy/Cdc20 is present, and only the spindle-associated cyclin B is degraded at the end of mitosis. A mutant form of cyclin B that cannot be targeted for destruction by Fzy/Cdc20 is no longer degraded on spindles of syncytial embryos, but still targeted by Fzr/Cdh1 in cellularized embryos, albeit more slowly than normal. This suggest that Fzy/Cdc20 is responsible for catalyzing the first phase of cyclin B destruction that occurs on the mitotic spindle, whereas Fzr/Cdh1 is responsible for catalyzing the second phase of cyclin B destruction that occurs throughout the cell.
  • Jove - Cell Cycle - Analysis of Cell Cycle Position in Mammalian Cells

2017 Course Content


Lectures: Cell Biology Introduction | Cells Eukaryotes and Prokaryotes | Cell Membranes and Compartments | Cell Nucleus | Cell Export - Exocytosis | Cell Import - Endocytosis | Cytoskeleton Introduction | Cytoskeleton - Microfilaments | Cytoskeleton - Microtubules | Cytoskeleton - Intermediate Filaments | Cell Mitochondria | Cell Junctions | Extracellular Matrix 1 | Extracellular Matrix 2 | Cell Cycle | Cell Division | Cell Death 1 | Cell Death 2 | Signal 1 | Signal 2 | Stem Cells 1 | Stem Cells 2 | Development | 2017 Revision

2017 Laboratories: Introduction to Lab | Fixation and Staining |

2017 Projects: Group 1 - Delta | Group 2 - Duct | Group 3 - Beta | Group 4 - Alpha

Dr Mark Hill 2015, UNSW Cell Biology - UNSW CRICOS Provider Code No. 00098G