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The cilia and flagella of eukaryotes

Group 4 Advanced cell biology learning resource
by

Jessica Oliver

on 19 November 2012

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Transcript of The cilia and flagella of eukaryotes


• Cilia and flagella were first discovered in 1675 by Anton van Leeuwenhoek who was also the first person to observe microorganisms which he called animalcules.

• Cilia and flagella are found in most microorganisms and animals, but not in higher plants. [1]

•Cilia also line the respiratory tract in humans and transport dust and inhaled microorganisms out of the lungs. [1]

•Flagella are found primarily on gametes. For single-celled eukaryotes, cilia and flagella are essential for the locomotion of individual organisms. [1]
•There are usually only one or two long Flagella whereas cilia are short and numerous. Both consist of 9 fused pairs of microtubules which form a ring around a central pair of unfused microtubules. The arrangement of microtubules is known as a 9+2 arrangement. [2]

•Flagella function in locomotion. Cilia's function is to move various materials that may surround a cell.


•Cilia are split into 2 categories, primary and motile.

•The motile cilia are the small and wavy and are found in there hundreds, on specific cells. However primary cilia are usually singular and nearly every mammalian cell has one.

•Unlike motile cilia and flagella were discovered nearly 200 years after primary cilium in 1898. [1] Similarities and Differences of flagella and Cilia Previously two competing views were presented to explain how these motile organelles had evolved:

1.Modified remnants of symbiotic spirochaete-like prokaryotes.
2.Flagella’s and cilia’s arose from a simpler cytoplasmic microtubule-based intracellular transport system. The Autogenous theory.

However although intermediate stages of flagella evolution are no longer present in today’s eukaryotes, due to recent comparisons between phylogenetic (evolution and diversification) relationships of present day eukaryotes by advanced sequence analysis the second hypothesis has been accepted. [3] The basic cylindrical 9 + 2 structural organization of the axoneme was first deduced in 1949 by G. Grigg and Allan Hodge [4] •All of the existing branches of eukaryotes include organisms that posse a 9+2 flagellum (See Figure.1.)

•This suggests that the last common ancestor of all eukaryotic organisms also possessed a 9+2 flagellum.

•Early eukaryotes with a 9+2 flagellum had a great selective advantage, as only posterity of those organisms survived till today.

•Flagellum of the common ancestor was used for gliding motility along surfaces, beating motility to generate fluid flow, and localized distribution of sensory receptors. [3] Ancestry of Evolution Figure 3. Diagram showing possible eukaryotic radiation that gave rise to all existing branches of eukaryotes.
The representative genus that contains species with typical motile 9+2 flagella is named in brackets. [3] Origin and Evolution
of the Flagella Evolutionary Mechanism
• Motile machinery of 9+2 flagellum appears to be highly conserved, however in some present eukaryotic organisms, further changes and alterations to the basic structure were made in order to meet specific needs of those organisms.

• These changes include:

1) Additions:
i) Mastigonemes – projections from the membrane surface, that increase effective hydrodynamic resistance
ii)Accessory structures that increase axoneme firmness

2) Simplifications/Deletions:
i) Loss of the outer row dyneins
ii) Loss of the central apparatus and radial spokes (motile 9+0)
iii) Loss of the central apparatus, radial spokes, and some doublet microtubules (6+0, 3+0)
iv) Loss of all motile components (central pair, spokes, dyneins) leaving only the membrane and the nine doublet microtubules to support IFT and receptor localization.

3) Modifications:
i) Differences in average length, beat frequency, or waveform,
ii) Ability to change beat frequency, beat direction, or waveform in response to signalling cascades The Axoneme: a centric collection of microtubules that extend lengthwise throughout the organelle. It is covered by the plasma membrane of the cell it projects from.
It consists of:
•Nine doublet external microtubules.
•Centric pair of singlet microtubules.
Thus, it has 9 + 2 arrangement structure:

•The nine doublet external microtubules surround the centric pair of microtubules.
All microtubules (external and centric) have similar polarity: plus end at the end of the projection, and minus end at the basis [1]. Figure 6: The picture shows the position where cilium extends from a cell as well as showing a longitudinal and a cross section of cilia/flagella basic structure [8]. •The diameter of the axoneme is approximately 0.25 m, however, the length differs from a few microns up to 2mm, sometimes more.

•The axoneme structure is held and maintained by three cross-link proteins:

1. The centric two microtubules are held together by periodic bridges, and covered by an inner sheath which is a fibrotic structure.
2. The peripheral microtubules; neighbouring microtubules are connected to each other by the protein nexin. Spaced every 86 nm along the axoneme.
3. The peripheral and centric microtubules are attached to each other by radial spokes, which extends from the centric tubules to the A tubule of each peripheral tubule doublets [9]. Figure2: A 3D image of a cross section of cilia/flagella showing the axoneme structure (nine doublet microtubules, radial spokes, dynein arms, pair of singlet Mictotubles Figure 8: A 3D image presents how tubule A (a complete tubule with 13 filaments) and tubule B (incomplete tubule with 11 filaments) diffuse together forming a doublet microtubule structure [10]. Singlet microtubules:
•A hollow rigid shaft about 25nm in diameter and about 4 nm in wall thickness.
•Consist of 13 protofilaments, which are linear rows of globular proteins (α and β), lined up in parallel forming a tubule.
•It has a polarity, where the end that stops with a row of β tubulin will have a plus end, while the opposite end will have a minus end [1].

The doublet microtubules comprise of:
• “A and B tubules, or subfibers: the A tubule is a complete microtubule with 13 protofilaments, while the B tubule contains 10 protofilaments”.
• Protein tektin provides the support needed to strengthen the connection between tubule A and B, tektin is a helical protein which has the same conformation as intermediate filament proteins, it has a diameter of 2 nm and about 34nm in length.
• The A tubule, of each external doublet microtubules of the axoneme, is connected dynein arms in two sides, in the inner and outer of each A tubule, reaching B tubule of the neighbour doublet [9]. The Basal Body •The basal body has a cylindrical configuration, around 0.4 m in length and 0.2 m in width. It contains nine triple microtubules.
•A triplet microtubule consist of one A tubule: which is a complete 13 protofilament microtubule, and a B and C tibule which are incomplete protofilaments.
•A and B tubules of a basal body continue into the axonemal structure, while the C tubules stops in the transition region between the basal body and the axoneme.
•The centric tubules of the axoneme end in the transition region, as well.
•The axomene is connected to a basal body at the point of attachment to the cell [8]. Figure 5: The image shows the structure of centriole (basal body) in a 3D [7] Function of Flagella Introduction to Cilia and Flagella Difference in Movement Figure 2: Diagram explaining the difference in moventment between the clila and flagellas [2] Theories of Evolution Ancestry of evolution Further Developments The Basal Body The Axonene Structure More on the Axoneme Structure Figure 7: Image showing how the axoneme is held together and maintained. The Microtubules The Cilia and Flagella of Eukaryotes Introduction Structure of Flagella The Axoneme Overview of Structure The short YouTube clip enhance your understanding of the flagella structure. [11] This presentation will explain to you
structure related to function.
A copy of the dialog will be available to you. [1][12][13] Eukaryotic Radiation Axoneme Scaffolding List of Comparisions Learning outcomes: How this Prezi Works: Figure 1: [9]
•The waveform observed in Flagella and the beating patterns observed in cilia are caused by bending of the axoneme.
•This bending is generated by dynein motor proteins situated on the A tubule of each peripheral doublet MT.
•The dynein heads bind to specific sites on the adjacent B tubules using ATP.
•The ATP is then hydrolyzed by the dynein head causing a conformational change in the protein structure to occur. This conformational change causes the dynein heads to move toward the negative basal end in what is called the power stroke.
•This “power stroke” moves one MT doublet relative to another.
•The elastic protein nexin that connects all the doublets to one another resists this movement.
•As a result of this shear force, the doublets begin to bend.
•To generate a waveform only the doublets on one side of the axoneme must be moving. This then switches to the opposite side causing the flagella to bend in the opposite direction.
•This bending is thought to be regulated by the central sheath and radial spokes. [12][13] Summary of Flagella movement •Cilia and flagella were first discovered in 1675 by Anton van Leeuwenhoek who was also the first person to observe microorganisms which he called animalcules.

•Cilia and flagella are found in most microorganisms and animals, but not in higher plants.

•Cilia also line the respiratory tract in humans and transport dust and inhaled microorganisms out of the lungs.

•Flagella are found primarily on gametes. For single-celled eukaryotes, cilia and flagella are essential for the locomotion of individual organisms. [1] History of the Flagella and cilia This learning resource will teach you the necessary information you will need regarding this topic, in a fun interactive way.
Prezi is very easy to navigate:

To go to the next section: Simply click on the forward arrow, as you would when using a PowerPoint
To go back you can use the backward arrow.
You can also scroll along the prezi and or zoom in and out and click to take you to a specific section. This prezi will teach you about the 'Cilia and Flagella of Eukaryotes.' The topics you will cover include:

Origin and evolution

Structure

Function - This section includes an audio PowerPoint movie. Please make sure you have been given a handout of the dialog to aid your learning and understanding. Summary There are only a few differences between the Flagella and Cilia. They share the same structure and very similar functions.
Eukaryotes with the 9+2 flagella all originated from the same common ancestor.
The flagella and eukaryotes both have an axoneme. The axoneme is made of a 9+2 configuration of microtubles.
The axoneme is attached to a basal body.
The cilia and flagellas main function is locomotion.
The cilia also acts to move secretions and objects around. (A) The eukaryote cell that begun the flagella evolution process most probably contained all the necessary building blocks within it. As proposed the flagella arose from a simpler cytoplasmic microtubule-based intracellular transport system [1]: composed of actin and cytoplasmic microtubules. These all congregated in the ‘microtubule-organising centre’ (MOC). The MOC coordinates cell proliferation, migration, and polarity. These are all crucial characteristics of the flagella. [5] (B and B) Microtubules attached to the MOC targeted a specific patch of the cell membrane. The force produced from this unidirectional growth of the microtubules caused the cell to elongate in that section. The elongated section of cell will then specialised and display polarity under the instructions of the MOC. [5] (C) The evolving proto-cilium protrusion, leading to a more specialised shape, may have started to display beneficial characteristics (gliding motility along surfaces, beating motility to generate fluid flow, and localized distribution of sensory receptors [1]) early on that allowed further evolution of it due to natural selection aiding its survival. (C) A transport system soon evolved to supply the developing organelle with the nutrients it requires. [5] (D) The presences of a new specialised organelle, the flagella, arose when the microtubules arranged themselves in nine fold symmetric structure capable of bending. This structure would be known as the axoneme and was anchored to the cell membrane by the basal body. [5] This evolutionary pathway gave rise
to the 9+2 structure that today
characterises the flagella and cilia. Theory of Autogenous (from within) origin: Figure 4: Evolutionary pathway [5] The Flagella and cilia will now both be refereed to as the flagella due to their same evolutionary, structural and functional characteristics.
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