Inner World of the Cell

Molecular Tracks :

Microtubule, Actin, DNA and mRNA as Molecular Tracks

Molecular Tracks :



Microtubule - In Figure 05, the web of microtubules are emanated from the MTOC near the nucleus. It is a hollow cylinder about 24 nm (2.4x10-6 cm) in diameter (see insert in Figure 05) and can grow as long as 25 micrometres (2.5x10-3 cm) spanning over the entire cellular space providing support for the cytoskeleton. It binds to the kinesin, which transports various cargoes (vesicles) along the track (Figure 06a and Figure 09). The spindles during cell division are made of microtubules.

Figure 06 Molecular Machines [view large image]

Actin (Microfilament) - It has a globules (G-actin) and a polymeric (F-actin) form, which are made from many G-actin monomers. It has a helical structure measuring about 7 nm in diameter with the helix repeating every 37 nm (see insert in Figure 05).

Actin participates in many important cellular processes, including muscle contraction, cell motility, cell division, vesicle/organelle movement, cell signalling, and the establishment/maintenance of cell shape. The following provides a very brief description for each of the actin structure in Figure 05 :


• Podosome - providing cellular motility and invasion.
• Lamellipodia - for quick repair of wounds.
• Filopodia - the leading edge of lamellipodia
• F-actin filament - the thin filaments in muscle fibers.
• Microvilli - for cellular absorption, secretion, adhesion, and mechano-transduction.
• Ruffle - structural change in response to signal transduction.
• Cortactin - for rearrangement of the actin cytoskeleton by external stimuli.
• Stress Fiber - for cell contraction of non-muscle cells.


• DNA and mRNA as Molecular Tracks - In DNA replication (Figure 12) or RNA transcription (Figure 07), the

  		topoisomerase would untwist the two strands at the middle, the helicase follows behind to split the helix, finally it is the polymerase, which either perform the DNA replication or produce a RNA transcript. All these processes are executed along the DNA, thus it can be considered as a track for these molecular motors to move. In the translation from genetic codes to protein,
  Figure 07 RNA Transcription [view large image]	Figure 08 mRNA Translation [view large image]	the ribosome slides along the mRNA (Figure 08), which provides the track. See more detail in DNA, and RNA.

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