The School of Biomedical Sciences Wiki - User contributions [en]

Gamete

2018-10-17T18:24:35Z

160208744: Added an image, added 2 paragraphs and started on another paragraph, added references

Introduction

A gamete is another word for a reproductive cell. A gamete can either be a [[Sperm]] cell in males, or an [[Ovum]] , also known as an [[Egg cell|Egg cell]], in females. Gametes are [[Haploid]] cells, meaning they only contain 23 [[Chromosomes|chromosomes]], compared to [[Somatic cells|Somatic cells]] , which contain a full set of 46 chromosomes. When a [[Sperm]] cell and [[Ovum]] join during [[Fertilisation]] , they form a [[Zygote]] . The zygote is a combination of the two haploid cells, making it a [[Diploid]] cell, therefore providing it with a full set of 46 chromosomes <ref>Wright DB. Human Physiology and Health. Oxford: Heinemann Educational Publishers. 2000:152</ref>. Following the days after fertilisation, the zygote rapidly divides. Approximately 2 weeks of constant cell division after fertilisation, the zygote is then classed as an [[Embryo]] <ref>Cherry K. The Zygote Phase in Reproduction. 2018 [cited 17/10/18]; Available from: https://www.verywellfamily.com/what-is-a-zygote-2796031</ref>.  

 

Structure and Function of a Sperm

The male gamete's main function is to move towards the ovum, by rotation of the [[Flagella]] , and fuse with the ovum via the [[Acrosome Reaction]].

The [[Spermatozoa]] consists of a head, middle boy, and tail. The head of the spermatozoa contains the hapolid nucleus with 23 chromosomes, which is contained by a cytoplasmic layer containing a range of [[Polysaccharides]]. The head is key in the [[Acrosome Reaction]] as it contains 2 key enzymes; the hyaluronidase and zona acrosin, which are crucial for allowing the sperm to penetrate the ovum  <ref>Chavarría ME, Reyes A, Rosado A. The male factor. II. Spermatozoa. Structure and function. 1997 [cited on 17/10/18]; Available at https://www.ncbi.nlm.nih.gov/pubmed/9432472</ref>. The middle body of the sperm consists of mitochondrial sheath, which is critical for provding the sperm cell with [[ATP]] , used for energy to rotate the flagella for motility <ref>Sutovsky P, Tengowski MW, Navara CS, Zoran SS, Schatten G. Mitochondrial sheath movement and detachment in mammalian, but not nonmammalian, sperm induced by disulfide bond reduction. 1997 [cited 17/10/18]; Available at https://www.ncbi.nlm.nih.gov/pubmed/9110318</ref>. The tail of the sperm is vital for rotating and propelling the sperm, enabling it to travel towards the ovum. The tail is made out of microtubules, which are in a 9+2 arrangement <ref>Jain K. Structure, Functions and Types of Mature Sperms in Animals. 2018 [cited 17/10/18]; Available at http://www.biologydiscussion.com/notes/structure-functions-and-types-of-mature-sperm-in-animals-biology/768</ref>. 

[[Image:Screen_Shot_2018-10-17_at_18.59.21.png]] 

Figure 1 - Structure of a Sperm <ref>Villarreal MR. Sperm. 2006 [cited 17/10/18]; Available at https://en.wikipedia.org/wiki/Sperm</ref>

 

Function of an Ovum

The female ovum, which is significantly larger than the male spermatozoa, also has the same purpose as the spermatozoa; fertilisation. The ovum cannot move like the spermatozoa, since it has no flagella attached. Instead, the ovum contains essential nutrients needed for zygote development into an embryo <ref>Soffar H. The structure and function of the ovum in the female reproductive system. 2015, [cited on 17/10/18]; Available at https://www.online-sciences.com/the-living-organisms/the-structure-and-function-of-the-ovum-in-the-female-reproductive-system/</ref> . 

 

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File:Screen Shot 2018-10-17 at 18.59.21.png

2018-10-17T18:19:35Z

160208744: uploaded a new version of "File:Screen Shot 2018-10-17 at 18.59.21.png"

Structure of a Sperm

Gamete

2018-10-17T18:18:10Z

160208744:

Introduction

A gamete is another word for a reproductive cell. A gamete can either be a [[Sperm]] cell in males, or an [[Ovum]] , also known as an [[Egg cell|Egg cell]], in females. Gametes are [[Haploid]] cells, meaning they only contain 23 [[Chromosomes|chromosomes]], compared to [[Somatic cells|Somatic cells]] , which contain a full set of 46 chromosomes. When a [[Sperm]] cell and [[Ovum]] join during [[Fertilisation]] , they form a [[Zygote]] . The zygote is a combination of the two haploid cells, making it a [[Diploid]] cell, therefore providing it with a full set of 46 chromosomes <ref>Wright DB. Human Physiology and Health. Oxford: Heinemann Educational Publishers. 2000:152</ref>. Following the days after fertilisation, the zygote rapidly divides. Approximately 2 weeks of constant cell division after fertilisation, the zygote is then classed as an [[Embryo]] <ref>Cherry K. The Zygote Phase in Reproduction. 2018 [cited 17/10/18]; Available from: https://www.verywellfamily.com/what-is-a-zygote-2796031</ref>.  

 

Structure and Function of a Sperm

The male gamete's main function is to move towards the ovum, by rotation of the [[Flagella]] , and fuse with the ovum via the [[Acrosome Reaction]].

The [[Spermatozoa]] consists of a head, middle boy, and tail. The head of the spermatozoa contains the hapolid nucleus with 23 chromosomes, which is contained by a cytoplasmic layer containing a range of [[Polysaccharides]]. The head is key in the [[Acrosome Reaction]] as it contains 2 key enzymes; the hyaluronidase and zona acrosin, which are crucial for allowing the sperm to penetrate the ovum  <ref>Chavarría ME, Reyes A, Rosado A. The male factor. II. Spermatozoa. Structure and function. 1997 [cited on 17/10/18]; Available at https://www.ncbi.nlm.nih.gov/pubmed/9432472</ref>. The middle body of the sperm consists of mitochondrial sheath, which is critical for provding the sperm cell with [[ATP]] , used for energy to rotate the flagella for motility <ref>Sutovsky P, Tengowski MW, Navara CS, Zoran SS, Schatten G. Mitochondrial sheath movement and detachment in mammalian, but not nonmammalian, sperm induced by disulfide bond reduction. 1997 [cited 17/10/18]; Available at https://www.ncbi.nlm.nih.gov/pubmed/9110318</ref>. The tail of the sperm is vital for rotating and propelling the sperm, enabling it to travel towards the ovum. The tail is made out of microtubules, which are in a 9+2 arrangement <ref>Jain K. Structure, Functions and Types of Mature Sperms in Animals. 2018 [cited 17/10/18]; Available at http://www.biologydiscussion.com/notes/structure-functions-and-types-of-mature-sperm-in-animals-biology/768</ref>. 

 

Function of an Ovum

The female ovum, which is significantly larger than the male spermatozoa, also has the same purpose as the spermatozoa; fertilisation. The ovum cannot move like the spermatozoa, since it has no flagella attached. Instead, the ovum contains essential nutrients needed for zygote development into an embryo <ref>Soffar H. The structure and function of the ovum in the female reproductive system. 2015, [cited on 17/10/18]; Available at https://www.online-sciences.com/the-living-organisms/the-structure-and-function-of-the-ovum-in-the-female-reproductive-system/</ref> . 

 

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File:Screen Shot 2018-10-17 at 18.59.21.png

2018-10-17T18:12:14Z

160208744: Structure of a Sperm

Structure of a Sperm

Gamete

2018-10-17T18:11:56Z

160208744:

Introduction

A gamete is another word for a reproductive cell. A gamete can either be a [[Sperm]] cell in males, or an [[Ovum]] , also known as an [[Egg cell|Egg cell]], in females. Gametes are [[Haploid]] cells, meaning they only contain 23 [[Chromosomes|chromosomes]], compared to [[Somatic cells|Somatic cells]] , which contain a full set of 46 chromosomes. When a [[Sperm]] cell and [[Ovum]] join during [[Fertilisation]] , they form a [[Zygote]] . The zygote is a combination of the two haploid cells, making it a [[Diploid]] cell, therefore providing it with a full set of 46 chromosomes <ref>Wright DB. Human Physiology and Health. Oxford: Heinemann Educational Publishers. 2000:152</ref>. Following the days after fertilisation, the zygote rapidly divides. Approximately 2 weeks of constant cell division after fertilisation, the zygote is then classed as an [[Embryo]] <ref>Cherry K. The Zygote Phase in Reproduction. 2018 [cited 17/10/18]; Available from: https://www.verywellfamily.com/what-is-a-zygote-2796031</ref>.  

 

Structure and Function of a Sperm

The male gamete's main function is to move towards the ovum, by rotation of the [[Flagella]] , and fuse with the ovum via the [[Acrosome Reaction]].

The [[Spermatozoa]] consists of a head, middle boy, and tail. The head of the spermatozoa contains the hapolid nucleus with 23 chromosomes, which is contained by a cytoplasmic layer containing a range of [[Polysaccharides]]. The head is key in the [[Acrosome Reaction]] as it contains 2 key enzymes; the hyaluronidase and zona acrosin, which are crucial for allowing the sperm to penetrate the ovum  <ref>Chavarría ME, Reyes A, Rosado A. The male factor. II. Spermatozoa. Structure and function. 1997 [cited on 17/10/18]; Available at https://www.ncbi.nlm.nih.gov/pubmed/9432472</ref>. The middle body of the sperm consists of mitochondrial sheath, which is critical for provding the sperm cell with [[ATP]] , used for energy to rotate the flagella for motility <ref>Sutovsky P, Tengowski MW, Navara CS, Zoran SS, Schatten G. Mitochondrial sheath movement and detachment in mammalian, but not nonmammalian, sperm induced by disulfide bond reduction. 1997 [cited 17/10/18]; Available at https://www.ncbi.nlm.nih.gov/pubmed/9110318</ref>. The tail of the sperm is vital for rotating and propelling the sperm, enabling it to travel towards the ovum. The tail is made out of microtubules, which are in a 9+2 arrangement <ref>Jain K. Structure, Functions and Types of Mature Sperms in Animals. 2018 [cited 17/10/18]; Available at http://www.biologydiscussion.com/notes/structure-functions-and-types-of-mature-sperm-in-animals-biology/768</ref>. 

Structure and Function of an Ovum

The female ovum, which is significantly larger than the male spermatozoa, also has the same purpose as the spermatozoa; fertilisation. The ovum cannot move like the spermatozoa, since it has no flagella attached. Instead, the ovum contains essential nutrients needed for zygote development into an embryo <ref>Soffar H. The structure and function of the ovum in the female reproductive system. 2015, [cited on 17/10/18]; Available at https://www.online-sciences.com/the-living-organisms/the-structure-and-function-of-the-ovum-in-the-female-reproductive-system/</ref> . 

 

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Gamete

2018-10-17T17:39:54Z

160208744: Added 2 new paragraphs, added references

Introduction

A gamete is another word for a reproductive cell. A gamete can either be a [[Sperm]] cell in males, or an [[Ovum]] , also known as an [[Egg cell|Egg cell]], in females. Gametes are [[Haploid]] cells, meaning they only contain 23 [[Chromosomes|chromosomes]], compared to [[Somatic cells|Somatic cells]] , which contain a full set of 46 chromosomes. When a [[Sperm]] cell and [[Ovum]] join during [[Fertilisation]] , they form a [[Zygote]] . The zygote is a combination of the two haploid cells, making it a [[Diploid]] cell, therefore providing it with a full set of 46 chromosomes <ref>Wright DB. Human Physiology and Health. Oxford: Heinemann Educational Publishers. 2000:152</ref>. Following the days after fertilisation, the zygote rapidly divides. Approximately 2 weeks of constant cell division after fertilisation, the zygote is then classed as an [[Embryo]] <ref>Cherry K. The Zygote Phase in Reproduction. 2018 [cited 17/10/18]; Available from: https://www.verywellfamily.com/what-is-a-zygote-2796031</ref>.  

 

Structure and Function of a Sperm

The male gamete's main function is to move towards the ovum, by rotation of the [[Flagella]] , and fuse with the ovum via the [[Acrosome Reaction]].

The [[Spermatozoa]] consists of a head, middle boy, and tail. The head of the spermatozoa contains the hapolid nucleus with 23 chromosomes, which is contained by a cytoplasmic layer containing a range of [[Polysaccharides]]. The head is key in the [[Acrosome Reaction]] as it contains 2 key enzymes; the hyaluronidase and zona acrosin, which are crucial for allowing the sperm to penetrate the ovum  <ref>Chavarría ME, Reyes A, Rosado A. The male factor. II. Spermatozoa. Structure and function. 1997 [cited on 17/10/18]; Available at https://www.ncbi.nlm.nih.gov/pubmed/9432472</ref>. The middle body of the sperm consists of mitochondrial sheath, which is critical for provding the sperm cell with [[ATP]] , used for energy to rotate the flagella for motility <ref>Sutovsky P, Tengowski MW, Navara CS, Zoran SS, Schatten G. Mitochondrial sheath movement and detachment in mammalian, but not nonmammalian, sperm induced by disulfide bond reduction. 1997 [cited 17/10/18]; Available at https://www.ncbi.nlm.nih.gov/pubmed/9110318</ref>. The tail of the sperm is vital for rotating and propelling the sperm, enabling it to travel towards the ovum. The tail is made out of microtubules, which are in a 9+2 arrangement <ref>Jain K. Structure, Functions and Types of Mature Sperms in Animals. 2018 [cited 17/10/18]; Available at http://www.biologydiscussion.com/notes/structure-functions-and-types-of-mature-sperm-in-animals-biology/768</ref>. 

 

Structure and Function of an Ovum

 

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Protein structure

2017-12-04T12:58:24Z

160208744: Added a reference, added some headings, re-worded some sentences and added some additional information.

[[Proteins|Proteins]] are made up of polymers of [[Amino acids|amino acids]]. The amino acids are joined together by [[Peptide bonds|peptide bonds]] in a [[Condensation reaction|condensation reaction]]. This series of peptide bonds is also known as the [[Polypeptide|polypeptide]] backbone, off which are side chains made up of amino acids. This type of reaction is catalysed by the [[Ribosome|ribosome]] in the [[Cytoplasm|cytoplasm]] and releases a [[Water|water]] molecule. There are four levels of protein structure, which determine the proteins overall structures and functions.

== Primary Structure ==

The [[Primary structure|primary structure]] is the specific linear sequence of [[Amino acids|amino acids]] joined by covalent peptide bonds in a polypeptide chain <ref>http://www.chemguide.co.uk/organicprops/aminoacids/proteinstruct.html</ref>. The primary structure is determined by DNA base sequence via triplet base coding. The [[R_Group|R group]] of an amino acid determines the function and properties of the protein, which is reflected in the proteins tertiary structure. 

== Secondary Structure  ==

The [[Secondary structure|secondary structures]] of proteins is formed by the way the protein coils, the main ones are the [[Alpha-helix|alpha helix]] and the [[Beta pleated sheet|beta pleated sheet]]<ref>Alberts et al. (2008). The Biology of the Cell. 5th ed. New York: Garland Science. 154</ref>. The secondary protein sturcture is stabilised by hydrogen bonds which are 1/10 the strength of covalent bonds <ref>Khan Academy. Hydrogen Bonds in Water. 2015 [cited 04/12/17]; Available from: https://www.khanacademy.org/science/biology/water-acids-and-bases/hydrogen-bonding-in-water/a/hydrogen-bonding-in-water</ref>. These hydrogen bonds are provided by the peptide bonds. For example, the bond is formed between the carbonyl oxygen on amino acid residue 'N' and the amide nitrogen on amino acid 'N + 4'.

== Tertiary Structure  ==

The protein then continues to fold from the secondary structure to form a three-dimensional structure. This is known as the [[Tertiary structure|tertiary structure]]. There are many bonds which maintain the tertiary structure including: ionic bonds (between NH3+ and COO-), hydrogen bonds, hydrophobic interactions (i.e. some amino acids have hydrophobic 'R' groups which position themselves furthest away from water) and disulphide bridges. Disulphide bridges are formed in the tertiary structure. They are formed between amino acids which contain a thiol group (SH). The H+ is lost very easily in an oxidation reaction with another SH group and the two join with a disulphide bond. Proteins that have to work outside of the cell use disulphide bonds to increase their stability.

== Quaternary Structure ==

If two or more tertiary structures form a single structure then it is a [[Quaternary structure|quaternary structure]]. An example of a quaternary structure are [[Haemoglobin|haemoglobin]] molecules, which are made up of four [[Globin|globin]] [[Molecules|molecules]] which contain 2 alpha and 2 beta subunits. These are also known as [[Red blood cells|red blood cells]] and are found in [[Blood|blood]] <ref>Alberts et al., (2008) Molecular Biology of the Cell, 5th Edition, Garland Science, Chapter 3, Page 136</ref>. 

Proteins can come in all different shapes and sizes <ref>Alberts et al.(2008) Molecular Biology of the Cell, 5th Edition, Garland Science Chapter 3 Page 144</ref> due to the fact that there is any possible sequence of [[Amino acid|amino acids]] and that a protein can be made of an alpha helix, a beta pleated sheet or both. The amino acids which tend to be conserved in proteins are those which make up the [[Enzyme active site|active site]], as this is the part of the protein which has most functional significace <ref>Thomas E. Creighton(1993) Proteins, 2nd edition, USA: W.H. Freeman and Company.</ref>.

== Polypeptide Backbone ==

The polypeptide backbone is made up of rigid [[Peptide bonds|peptide bonds and]] some flexible links which allow protein molecules to fold. The backbone also consists of a repeated sequence of three atoms of each residue in the chain-the amide N, the alpha carbon and the carbonyl carbon, the highest distance between corresponding atoms of adjacent [[Amino acid residues|residues]] is 3.80A when the peptide bond is trans but, when the chain is fully extended, the residues are staggered, so the maximium linear dimension of a polypeptide with n residues is n x 3.63A. Energetically, the trans form is highly favoured probably because of the fewer repulsion btween non-bonded atoms. The intrinsic stability of the [[Isomer|cis isomer is]] comparable to that of the [[Isomer|trans isomer]].

=== References ===

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Microtubule

2017-12-04T11:51:44Z

160208744: Added an image and reference

Microtubules are a component of the cells [[Cytoskeleton|cytoskeleton]], which is responsible for vital [[Cell|cell]] functions, such as cell structure, shape, and transport.

=== Structure of Microtubules ===

Microtubules are hollow cylinder-shaped polymers that are made up of [[Globular protein|globular protein]] subunits called [[Tubulin|tubulin]]. Each tubulin subunit, which are also known as [[Heterodimer|heterodimers]], consist of one alpha-tubulin and one beta tubulin<ref>Cooper GM. The Cell: A Molecular Approach. 2nd Ed. London: ASM Press. 2000.</ref>. Alpha-tubulin always attaches to beta-tubulin, with beta-tubulin always attaching to alpha-tubulin. Microtubules have both a positive end, where beta-tubulin is exposed and a negative end, where alpha tubulin is exposed<ref>http://study.com/academy/lesson/microtubules-definition-functions-structure.html</ref>. This means that microtubules are polar structures because of the positive and negative ends.

Microtubules are 25 nm in length and 14 nm in width, making them the largest cytoskeletal filaments found in [[Eukaryotic cells|eukaryotic cells]]<ref>Cooper GM. The Cell: A Molecular Approach. 2nd Ed. London: ASM Press. 2000.</ref>.

=== Function of Microtubules ===

Microtubules have different functions. They are most commonly found as [[Cilia|cilia]] and [[Flagella|flagella ]]. Cilia, which are short, hair-like structures,are found in places such as the [[Trachea|trachea]] and [[Fallopian tube|fallopian tube]]. The cilia are attach[[Image:Screen Shot 2017-12-04 at 11.40.37.png|thumb|left]]ed to the [[Basal body|basal body]] and stick outwards of the cell surface, where they beat back and forth. This assists the removal of [[Mucus|mucus]] in the [[Trachea|trachea]], and allows for an [[Egg cell|egg cell]] to move through the [[Fallopian tube|fallopian tube]]<ref>Nigg EA. Centrioles, Centrosomes, and Cilia in Health and Disease. 2009. [cited 20 November 2016]; Available from: http://www.sciencedirect.com/science/article/pii/S0092867409013622</ref>. In flagella, microtubules form the long tail, which is necessary for [[Sperm cell|sperm cells to]] propel themselves towards egg cells for [[Fertilisation|fertilisation]].

Both cilia and flagella have the [[9+2 arrangement|9+2 arrangement ]], which is observed in cross sections when the [[Axoneme|axoneme]] is cut. As shown in Figure 1, the 9+2 arrangement consists of a central singlet microtubule inside the cilia/flagella, and 9 surrounding doublet microtubules<ref>Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J. Molecular Cell Biology. 4th Ed. New York: W H Freeman and Company. 2000.</ref>. [[Dynein|Dyneins]] attached to the inside and outside of the 9 doublet microtubules enable microtubules of the 9+2 arrangement to bend, which is why cilia can beat back and forth and why flagella, accompanied with [[ATP|ATP]], can propel the sperm cell. Without the 9+2 arrangement, microtubules can only slide vertically on each other and cannot bend like cilia and flagella<ref>Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J. Molecular Cell Biology. 4th Ed. New York: W H Freeman and Company. 2000.</ref>.

=== ===

Figure 1 - The 9+2 arrangement of microtubules

in cilia and flagella <ref>Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J. Molecular Cell Biology. 4th Ed. New York: W H Freeman and Company. 2000.</ref>

=== ===

=== References ===

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File:Screen Shot 2017-12-04 at 11.40.37.png

2017-12-04T11:45:15Z

160208744: The 9+2 arrangement of microtubules, found in cilia and sperm.

The 9+2 arrangement of microtubules, found in cilia and sperm.

Graded potential

2017-12-04T11:31:46Z

160208744: Added some references and cleaned up some sentences

Graded Potentials occur in [[Dendrites|dendrites]], cell bodies or [[Axon|axon]] terminals<ref>http://www.bio.davidson.edu/courses/genomics/chip/chip.html</ref>. Graded potential refers to the postsynaptic electrical impulse. These potentials are known as ‘graded’ because their size or amplitude is directly proportional to the strength of the triggering event. For example, a large stimulus leads to the generation of a strong graded response and a small stimulus leads to the generation of a weak graded response. A graded potential will lose its strength and consequently die out within 1-2 mm from the origin <ref>Valberg A. Light Vision Color. Wiltshire: John Wiley and Sons Ltd. 2007. 138.</ref>. Summation of graded potentials may generate an [[Action potential|action potential]].

A graded potential may be depolarising or [[Hyperpolarisation|hyperpolarising]]. A depolarising local response will cause the membrane potential to be less negative, bringing it closer to the threshold value to generate an action potential <ref>Brown AG. Nerve Cells and Nervous Systems: An Introduction to Neuroscience. 2nd Ed. London: Springer-Verlag. 2012. 27.</ref>. This response increases the excitability of the cell. Meanwhile, a hyperpolarising local response has a vice versa effect towards the membrane potential value. It will make the inside of the membrane become more negative, reducing its excitability<ref>Sukkar M.Y, El-Munshid H.A, Ardawi M.S.M., (2000), Concise Human Physiology, 2nd edition. Page 44-45</ref>.

The depolarising event may occur due to the inwards movement of positively-charged ions, such as [[Sodium|sodium]] ions (Na+) to the cell, or the outwards movement of negatively-charged ions from the cell. The hyperpolarising local potential can arise due to either the negative [[Anions|anions]], such as [[Chloride|chloride]] ions (Cl-) entering the cell or the positive cations exiting the cell.

A depolarising graded potential is known as an [[Excitatory postsynaptic potential|excitatory postsynaptic potential (EPSP)]].

A hyperpolarising graded potential is known as an [[Inhibitory postsynaptic potential|inhibitory postsynaptic potential (IPSP)]].

If graded potentials reach the axon hillock and depolarise the membrane to the threshold voltage (-55 mV), an action potential is initiated.

=== Reference ===

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Microtubule

2017-11-30T16:31:42Z

160208744: I have added the information on the structure and the functions of microtubules.

Microtubules are a component of the cells [[Cytoskeleton|cytoskeleton]], which is responsible for vital cell functions, such as cell structure, shape, and transport.

== Structure of Microtubules  ==

Microtubules are hollow cylinder-shaped polymers that are made up of globular protein subunits called [[Tubulin|tubulin]]. Each tubulin subunit, which are also known as [[Heterodimer|heterodimers]], consist of one alpha tubulin and one beta tubulin <ref>Cooper GM. The Cell: A Molecular Approach. 2nd Ed. London: ASM Press. 2000.</ref>. Alpha tubulin always attaches to beta tubulin, with beta tubulin always attaching to alpha tubulin. Microtubules have both a positive end, where beta tubulin is exposed, and a negative end, where alpha tubulin is exposed <ref>http://study.com/academy/lesson/microtubules-definition-functions-structure.html</ref>. This means that microtubules are polar structures because of the positive and negative ends.

Microtubules are 25nm in length and 14nm in width, making them the largest cytoskeletal filaments found in eukaryotic cells <ref>Cooper GM. The Cell: A Molecular Approach. 2nd Ed. London: ASM Press. 2000.</ref>.

 

== Function of Microtubules ==

Microtubules have different functions. They are most commonly found as [[Cilia|cilia]] and [[Flagella|flagella ]]. Cilia, which are short, hair-like structures, are found in places such as the [[Trachea|trachea]] and [[Fallopian tube|fallopian tube]]. The cilia are attached to the [[Basal body|basal body]] and stick outwards of the cell surface, where they beat back and forth. This assists the removal of [[Mucus|mucus]] in the trachea, and allows for an [[Egg cell|egg cell]] to move through the fallopian tube <ref>Nigg EA. Centrioles, Centrosomes, and Cilia in Health and Disease. 2009. [cited 20 November 2016]; Available from: http://www.sciencedirect.com/science/article/pii/S0092867409013622</ref>. In flagella, microtubules form the long tail, which is necessary for [[Sperm cell|sperm cells to]] propell themselves towards egg cells for [[Fertilisation|fertilisation]]. 

Both cilia and flagella have the [[9+2 arrangement|9+2 arrangement ]], which is observed in cross sections when the [[Axoneme|axoneme]] is cut. The 9+2 arrangement consists of a central singlet microtubule inside the cilia/flagella, and 9 surrounding doublet microtubules <ref>Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J. Molecular Cell Biology. 4th Ed. New York: W H Freeman and Company. 2000.</ref>. [[Dynein|Dyneins]] attached to the inside and outside of the 9 doublet microtubules enable microtubules of the 9+2 arrangement to bend, which is why cilia can beat back and forth and why flagella, accompanied with [[ATP|ATP]], can propell the sperm cell. Without the 9+2 arrangement, microtubules can only slide vertically on each other and cannot bend like cilia and flagella <ref>Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J. Molecular Cell Biology. 4th Ed. New York: W H Freeman and Company. 2000.</ref>. 

 

=== References ===

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