From CellBiology

Conceptual Overview

The Group Five Project, The Cell Cycle, gives a detailed overview of the cell cycle for a typical mammalian cell. It covers topics ranging from the major components of the cycle, through to what can ensue when the cycle is disturbed. In the Group Project, I focused chiefly on Gap 1 phase (G1), touching upon its functions and components, and its relationship to the other phases of the cycle. My Individual Project looks at G1 again, however this time I am focusing on one particular protein, Cyclin D, and discussing its role in G1.

Cyclin D: Role, Structure and Functions

Figure 1. Cyclin 3D structure

The mammalian cell cycle is regulated by the sequential formation, activation and inactivation of a series of cell cycle regulatory molecules, that include the cyclins and the cyclin-dependent kinases (Cdks). Different cyclins bind specifically to different Cdks to form distinct complexes at specific phases of the cell cycle, thereby driving the cell from one phase to another (Blow & Li, 2001; Bryant & Francis, 2008). Specifically, the progression through G1 into Synthesis (S phase) is achieved by complexes formed by Cyclin D and Cdk4 and Cdk6 (Bryant & Frnacis, 2008; Kaldis, 2006).

Structure / Function (Cyclin D)

The cyclins are a family of proteins. They are helix bundles (see Fig. 1), and share a common core of about 100 amino acids (blue/green) that make up the five helices that contact the Cdk. Outside this region (red) there can be considerable variation, both in size and sequence. Cyclins can help determine which substrates are targets for the cyclin-Cdk combination (in particular for yeast, where there is only one Cdk for the whole cell cycle, and only the cyclins vary for different stages) (Blow & LI, 2001; Kipreos & Lui, 2000).

Most cyclins show a dramatic fluctuation in their expression during the cell cycle (see Fig. 2), for example the expression of cyclins D levels rise in G1 and remain elevated until mitosis. Cyclin D is most prominent in the G1/S phase transition. Cyclin D comprises of three closely related proteins, cyclin D1, D2 and D3. They are encoded by separate genes but show substantial amino acid similarities and are expressed in a highly overlapping fashion (see Fig. 2) (Heath, 2001; Kaldis, 2006).

Figure 2. Cyclin expression throughout the cell cycle

Structure / Function (Cyclin-Cdk Complex)

Cyclin-dependent kinases are a family of serine/theronine kinases that bind to, and are activated by, specific cyclins. Cdks 4, 5 and 6 complex mainly with the cyclin D family and function in the progression through the G1 phase of the cycle (Pines 1995). Cdks have an N-terminal Beta-sheet that forms the ATP and substrate binding site, followed by the PSTAIRE helix (PSTAIRE is an acronym for the single letter codes of the amino acids in the helix) to which cyclins bind. Hence PSTAIRE is a sequence signature to identify Cdks. This is followed by the Thr 160 loop, which blocks access to the substrate site in the absence of bound cyclin. When cyclin binds, the Thr 160 loop moves into contact with the cyclin, and this realigns Glu 51 correctly for its role in catalysis. Presence of cyclin activates CDK by a factor of 10,000. Finally, the C-terminal end of CDK forms another helix bundle (Bártová, Koča, Kříž, & Otyepka, 2004; De Bondt, 1993; Mateyak, 1999).

Figure 3. Cyclin-CDK complex

Cyclins have no enzymatic activity themselves; they have to bind to cell cycle kinases before the kinases can become enzymatically active. (Alberts, Bray, Hopkin, Johnson, Lewis, Raff, Roberts & Walter, 2004). Thereafter those kinases are known as cyclin-dependent kinases (see Fig. 3).


The regulators and biochemical pathways that involve these complexes in the G1/G0/S phase transition are beyond the scope of this project, however sufficient information is provided in the Links section for further reading. In addition, a number of simplified flowcharts have been provided summarizing the basic functional pattern of CCdk4/6.


Included in this section are links to a number of key publications in the cell cycle control research, and links to animations showing the role of cyclin D and Cdk4/6 in the cell cycle.

  • Mixing nuclei of mammalian cells together in the same cytoplasm to determine whether they could influence one another. Rao, P. N., & Johnson, R., T. 1970. Mammalian cell fusion: studies on the regulation of DNA synthesis and mitosis. Nature 225: 159-64 Link
  • Hartwell, L. H., Culotti, J., Pringle, J. R., & Reid, B., J. 1974. Genetic control of the cell division cycle in yeast. Science 183: 46-51. Link
  • It was proposed that CDKs act as a cell cycle engine. Murray, G. & Hunt, T. 1993. The cell cycle. Freeman, New York
  • Different CDKs control the onset of S phase and M phase. Van den Heuvel, S., & Harlow, E. 1993. Distinct roles for cyclin-dependent kinases in cell cycle control. Science 262: 2050-4 Link
  • Flowchart of Cyclin D regulation of G1 phase of cell cycle Link
  • Schematic diagram of G1-S phase transition of the cell cycle. Link
  • Schematic diagrams of the control of the cell cycle Link; Link
  • Cell cycle milestones: the 1980's saw the discovery of cyclins and Cdks, and cell cycle check-points. Link


  • Cell cycle: A sequence of events by which a cell duplicates it contents and divides into two.
  • Cyclin: A class of proteins that fluctuate in concentration at specific points during the cell cycle and that regulate the cycle by binding to a kinase.
  • Cyclin-dependent kinases (Cdks): A family of kinases that, once activated by cyclin, regulate the cell cycle by adding phosphate groups to a variety of protein substrates that control processes in the cycle. Abbreviated Cdk.
  • G1 Phase: The period of time in the cell cycle before DNA replication starts and during which the cell contains only one copy of its genome.
  • G2 Phase: The period of time in the cell cycle after DNA replication has been completed, but before cell division.
  • Mammal: A warm blooded vertebrate with hair or fur female mammals produce mile to feed their young.
  • Protein: A molecule composed of amino acids linked together in a particular order specified by a gene’s DNA sequence. Proteins perform a wide variety of functions including serving as enzymes, structural components or signaling molecules.
  • S phase: The phase in the cell cycle during which DNA synthesis occurs.


  1. Alao, J. P. 2007. The regulation of cyclin D1 degradation: roles in cancer development and the potential for therapeutic invention. Molecular Cancer 6:24 Link
  2. Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2003). Essentials in Cell Biology, 2nd ed. Garland Science Textbooks.
  3. Bártová, I., Koča, J., Kříž, Z., & Otyepka, M. 2004. Activation and inhibition of cyclin-dependent kinase-2 by phosphorylation; a molecular dynamics study reveals the functional importance of the glycine-rich loop. Protein Science June; 13(6): 1449–1457. Link
  4. Blow, J., & Li, A. 2001. The Origin of CDK Regulation. Nature Cell Biology, 3, 182-184. Link
  5. Bryant, J. A., & Francis, D. 2008. The Eukaryotic Cell Cycle. Taylor & Francis Droup.
  6. Chang. K, C., Choi. H, C., Jeon, E. M., Kang. J, Y., & Lee. K, Y. 2009. Hemin Inhibits Hypertensive Rat Vascular Smooth Muscle Cell Proliferation through Regulation of Cyclin D and P21. Archives of Pharmacal Research Vol 32, No 3, 375-382 Link
  7. De Bondt, H.L., Jancarik. J., Jonest, H. D., Kim. S-H., & Morgan, D. O. 1993. Crystal structure of cyclindependent kinase 2. Nature 363:595–602. Link
  8. Heath, J. K., Principles of Cell Proliferation. 2001. Blackwell Science.
  9. Kaldis, P. 2006. Cell Cycle Regulation. Springer-Verlag, Berlin.
  10. Kipreos. E, T., & Lui. J. 2000. Evolution of Cyclin-Dependent Kinases (CDKs) and CDK-Activating Kinases (CAKs): Differential Conservation of CAKs in Yeast and Metazoa. Molecular biology and Evolution 17 (7): 1061 Link
  11. Mateyak, M.K. et. al. 1999. c-Myc regulates cyclin D-Cdk4 and -Cdk6 activity but affects cell cycle progression at multiple independent points. Mol. Cell Biol. 19, 4672-4683. Link


  1. Chemistry, Structures & 3D Molecules. (2008). Cyclin 3D structure, Retrieved May 1, 2009 from http://www.3dchem.com/imagesofmolecules/1aq1.jpg
  2. Wikipedia. (2008). Cyclin D, Retrieved April 29, 2009 from http://en.wikipedia.org/wiki/File:Cyclinexpression_waehrend_Zellzyklus.png
  3. Wikimedia Commons. (2008). Cyclin-Cdk complex, Retrieved May 3, 2009 from http://upload.wikimedia.org/wikipedia/commons/7/76/Cyclin-Cdk.JPG

Lecture Feedback

  • Lecture 4 - Nucleus:

I was fascinated by the nucleus' framework, i.e., its 'inside and outside' lamins. I also found the chromosome territories interesting.

  • Lecture 5 - Exocytosis:

The movement and transport of vesicles inside and outside the cell.

  • Lecture 7 - Mitochondria:

Glycolysis. Muscle contraction (especially cardiac and skeletal). Cell division.

  • Lecture 8 - Cell Junctions:

Cell Adhesion Molecules (CAMs) are proteins located on the cell surface involved with the binding with other cells or with the extracellular matrix (ECM) in the process called cell adhesion. L-CAM is a liver cell adhesion molecule. Ng-CAM is neuron-glia adhesion molecule. I-CAM is an intracellular adhesion molecule.

  • Lecture 10 - Intermediate Filaments:

Desomsomes are found in simple and stratified squamous epithelium.

  • Lab 6 -
  • Lecture 14 - Confocal Microscopy:

Laser and Spining Disc.

  • Lecture 15 - Cell Cycle:

S, as in the S-phase, stands for Synthesis!