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Lipases

2016-10-17T15:42:22Z

150077062: Created page with "Lipases are a class of enzyme which break down lipids."

Lipases are a class of [[Enzyme|enzyme]] which break down [[Lipid|lipids]].

Spindle fibres

2016-10-17T15:40:15Z

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Spindle fibres consist of [[Microtubules|microtubules]] that are polymeric chains made from the protein [[Tubulin|Tubulin]]. They form during the [[Prometaphase|Prometaphase]] stage of [[Mitosis|Mitosis]] and both the [[Metaphase|Metaphase]] 1 and 2 stages of [[Meiosis|Meiosis]], and they originate from the [[Centrosome|centrosomes]] located at the poles of the dividing [[Cell|cell]], which help organize the [[Microtubules|microtubules]] as [[Tubulin|Tubulin]] is polymerized. During cell replication, these fibres would form and attach to the [[Kinetochore|Kinetochores]] located at the [[Centromeres|centromeres]] of the 2 [[Sister chromatid strands|sister chromatid strands]]. This stabilises the growing end of the polymer and prevents them from depolymerizing. They then retract during the [[Anaphase|Anaphase]] stage of Mitosis or Meiosis, pulling the sister chromatids apart towards the opposite poles of the cell. <ref>This is with reference to the book "Genetics: Analysis of Genes and Genomes" 8th ed. 2012, Hartl, Daniel L. et al, Jones &amp;amp; Barlett Learning, pg 119-131</ref>

=== References ===

<references />

Adrenal cortex

2016-10-17T14:11:19Z

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The [[Adrenal_cortex|Adrenal cortex]] is located on the [[Kidney|kidney]], and produces [[Hormone|hormones]].

It is the outer portion of [[Adrenal gland|adrenal gland]] that produces [[Steroid hormone|steroid hormones]]<ref>Silverthorn, D. (2013) Human Physiology: An Integrated Approach, 6th ed., Boston, Pearson Education pp.386-87</ref>.

The adrenal cortex secretes 4 main [[Hormones|hormones]], these include [[Cortisol|cortisol]], [[Aldosterone|aldosterone]], and 2 weak [[Androgen|androgens]] [[Androstenedione|androstenedione]] and [[Dehydroepiandrostenedione|dehydroepiandrostenedione]] (DHEA). [[Cortisol|Cortisol]] is an important metabolic hormone, [[Aldosterone|aldosterone]] is a hormone involved in salt and water homeostasis, and the androgens are regarded as having little physiological significance when gonadal function is normal. Due to multiple secretions, the adrenal cortex isn't a single endocrine gland<ref>Nussey S, Whitehead S. Endocrinology: An Integrated Approach. Oxford: BIOS Scientific Publishers; 2001. Chapter 4, The adrenal gland. Available from: http://www.ncbi.nlm.nih.gov/books/NBK26/</ref>.

=== References ===

<references />

Lac operon

2015-12-03T22:58:49Z

150077062:

Francois Jacob and Jacques Monad proposed a model for an [[Operon|operon]], which consisted of a [[Regulator gene|regulator gene]], an operator site consisting of a regulatory [[DNA|DNA]] sequence, and one or more structural [[Gene|genes]].

The model displayed how stimuli from the environment can promote/inhibit genetic mechanisms which control metabolic events e.g. the presence/absence of [[Glucose|glucose]] and [[Lactose|lactose]] in the ''lac'' operon <ref>Cellular and Molecular Life Sciences, Matthews KS, Swint-Kruse L, Wilson CJ, Zhan H, The Lactose Repressor System: Paradigms for Regulation, Allosteric Behaviour and Protein Folding.January 2007; 64(1):3-16</ref>. The ''lac'' operon consists of an additional [[Promoter|promoter]], in front of the regulator [[Gene|gene]], the role of which is to ensure the [[RNA Polymerase|RNA Polymerase]] binds to the correct transcription initiator.

The repressor protein is a homotetramer and a product of the ''lac''I gene, and will bind tightly to the operator, under the correct conditions i.e. when [[Glucose|glucose]] is present and [[Lactose|lactose]] absent. When the repressor is bound to the operon, the [[RNA polymerase|RNA polymerase]] is unable to unwind the [[DNA|DNA]] in order to expose the bases and hence is unable to transcribe the structural genes as there is no template for the [[RNA synthesis|RNA synthesis]] to occur. The group of structural genes act as a single transcription unit, coding for a single [[MRNA|mRNA]] [[Molecule|molecule]] termed a [[Polycistronic|polycistronic transcript]] i.e. coding for multiple proteins and transcription is dependent upon the correct environmental conditions as described below:

In the presence of lactose, the lac operon is induced by allolactose, which binds to the lac repressor and a conformational change occurs, which results in a decreased affinity of the ''lac'' repressor for the ''lac'' operator and transcription of the structural genes occurs.

In the absence of glucose, cAMP accumulates (glucose metabolites prevent this build up when glucose is present), and [[CAMP|cAMP]] is able to bind to a cAMP binding site on the ''lac'' operon activating the operon and promoting transcription <ref>Berg Jeremy.M, Tymoczko John.L, Stryer Lubert, 2007, Biochemistry, Sixth Edition, W.H.Freeman, New York, Pages 897-900</ref>.

== What is the Lac operon? ==

The lac operon is a good example of how genes are regulated, in this case through the acts of an activator and/or repressor.The lac [[Operon|operon]] was studied in ''[[E. coli|E. coli]] ''<ref>Hartl, D. L. and Jones, E. W., 2009. Genetics: Analysis of genes and genomes. 7th edition. Sudbury: Jones and Bartlett Publishers pg 383</ref>. It contains 3 [[Gene|genes]] that are needed to produce proteins that are required to break down [[Lactose|lactose]] when it is present in the cell. These 3 [[Gene|genes]] are Lac Z, Lac Y and Lac A. Each code for B- galactosidase, Permease and Transacetylase respectively <ref>http://users.rcn.com (2011) "The operon" – 30th March 2010 – Available from: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LacOperon.html [Accessed 3rd January 2011]</ref>.

Further up the [[Genetic code|genetic code]] from these three genes, upstream, lies the [[Promoter|promoter]] sequence. [[RNA polymerase|RNA polymerase]] needs a region in which it can join the genetic code, the [[Promoter]] sequence, before it can start transcribing. [[RNA polymerase|RNA polymerase]] is required in transcription of the Lac operon <ref>Hartl, D. L. and Jones, E. W., 2009. Genetics: Analysis of genes and genomes. 7th edition. Sudbury: Jones and Bartlett Publishers pg 386</ref>. [[Image:Lac operon - basic.JPG]] 

== When does the Lac operon function? ==

The Lac operon is regulated depending on the absense or presence of certain substances. When [[Lactose|lactose]] is present in the cell and [[Glucose|glucose]] is absent, then the Lac operon is active and the 3 [[Genes|genes]] are transcribed to break down this lactose in the cell.

The lac operon only needs to activate the genes necessary for lactose integration when there is an absence of glucose and a plentiful supply of lactose.

== Negative gene regulation ==

The conditions inside the cell are changing all the time. So what happens when glucose is present and lactose levels are low? The Lac operon is no longer required to make the proteins to break down lactose and so its function is switched off. This is done by the use of a repressor protein <ref>Hartl, D. L. and Jones, E. W., 2009. Genetics: Analysis of genes and genomes. 7th edition. Sudbury: Jones and Bartlett Publishers pg 384</ref>.

Upstream of the promoter sequence there is another gene. This is the Lac I [[Gene|gene]]. The Lac I gene is transcribed to make the repressor protein which binds to 3 different operator sequences present at different parts of the Lac operon. The repressor protein is a tetramer and the binding of this repressor to the operators cause the [[DNA|DNA]] to be looped around.

[[Image:Lac operon - with lac I and operator seq.JPG|690x391px|Lac operon - with lac I and operator seq.JPG]] [[Image:Lac operon - with lac I and operator seq nd repressor.JPG|690x391px|Lac operon - with lac I and operator seq nd repressor.JPG]]

Once the repressor protein is bound, it stops the [[RNA polymerase|RNA polymerase]] enzyme from transcribing the genes. Effectively, it acts as a block <ref>Sadava (2011) “ The Lac Operon” – 2008 – Available from: http://www.sumanasinc.com/webcontent/animations/content/lacoperon.html [Accessed 3rd January 2011]</ref>.

When the glucose levels are depleted and the lactose levels rare high, the repressor has to be removed in order to transcribe the required genes. This is done by an inducer molecule. This [[Molecule|molecule]] comes from lactose and is Allolactose. Allolactose is synthesised from lactose by the enzyme B-galatidase, which is also a translational product of the lac gene. The allolacose binds to the [[Repressor protein|repressor protein]] and causes a conformational change in the repressor which causes it to dissociate from the DNA.. [[RNA polymerase|RNA polymerase]] can work as it is not blocked and the Lac Z, Lac Y and Lac A genes are transcribed <ref>BioCoach Activity (2011) "The lac inducer: Allolactose" – Available from: http://www.phschool.com/science/biology_place/biocoach/lacoperon/inducer.html [Accessed 3rd January 2011]</ref>.

[[Image:Lac operon - allolactose.JPG]]

== Positive Gene regulation ==

Sometimes promoters are not strong enough to initiate transcription on their own and so require another molecule or complex to help. In the Lac operon, this is done by the CRP – [[CAMP|cAMP]] complex. This is because glucose acts as an inhibitor of the enzyme andenynyl Cyclase which is responsible for converting ATP into cAMP (cyclic adenosine mono phosphate) When glucose levels in the cell are low, the levels of cAMP build up <ref>Mulligan, M. E. (2002) “The lac operon: positive regulation” – Available from: http://www.mun.ca/biochem/courses/3107/Topics/Lac_positive_control.html [Accessed 3rd January 2011]</ref>. This then combines with a CRP protein (sometimes also referred to as the CAP protein) and forms a complex called the CRP-cAMP complex. This complex then join to a sequence of nucleotides downstream from the LacI promoter known as the CRP binding site. The binding increases the affinity of  [[RNA polymerase|RNA polymerase]] for the lac promoter sequence and hence it binds and as a result transcription can take place. <ref>Hartl, D. L. and Jones, E. W., 2009. Genetics: Analysis of genes and genomes. 7th edition. Sudbury: Jones and Bartlett Publishers pg 389</ref>. When the levels of [[Glucose|glucose]] increase again, the amount of [[CAMP|cAMP]] synthesised is reduced and so the complex levels decrease. This therefore inhibits the Lac operon from working because the affinity of the RNA pol II for the lac gene promoter without the activator bound is quite low. 

== References ==

<references />

Lac operon

2015-12-03T22:50:39Z

150077062:

Francois Jacob and Jacques Monad proposed a model for an [[Operon|operon]], which consisted of a [[Regulator gene|regulator gene]], an operator site consisting of a regulatory [[DNA|DNA]] sequence, and one or more structural [[Gene|genes]].

The model displayed how stimuli from the environment can promote/inhibit genetic mechanisms which control metabolic events e.g. the presence/absence of [[Glucose|glucose]] and [[Lactose|lactose]] in the ''lac'' operon <ref>Cellular and Molecular Life Sciences, Matthews KS, Swint-Kruse L, Wilson CJ, Zhan H, The Lactose Repressor System: Paradigms for Regulation, Allosteric Behaviour and Protein Folding.January 2007; 64(1):3-16</ref>. The ''lac'' operon consists of an additional [[Promoter|promoter]], in front of the regulator [[Gene|gene]], the role of which is to ensure the [[RNA Polymerase|RNA Polymerase]] binds to the correct transcription initiator.

The repressor protein is a homotetramer and a product of the ''lac''I gene, and will bind tightly to the operator, under the correct conditions i.e. when [[Glucose|glucose]] is present and [[Lactose|lactose]] absent. When the repressor is bound to the operon, the [[RNA polymerase|RNA polymerase]] is unable to unwind the [[DNA|DNA]] in order to expose the bases and hence is unable to transcribe the structural genes as there is no template for the [[RNA synthesis|RNA synthesis]] to occur. The group of structural genes act as a single transcription unit, coding for a single [[MRNA|mRNA]] [[Molecule|molecule]] termed a [[Polycistronic|polycistronic transcript]] i.e. coding for multiple proteins and transcription is dependent upon the correct environmental conditions as described below:

In the presence of lactose, the lac operon is induced by allolactose, which binds to the lac repressor and a conformational change occurs, which results in a decreased affinity of the ''lac'' repressor for the ''lac'' operator and transcription of the structural genes occurs.

In the absence of glucose, cAMP accumulates (glucose metabolites prevent this build up when glucose is present), and [[CAMP|cAMP]] is able to bind to a cAMP binding site on the ''lac'' operon activating the operon and promoting transcription <ref>Berg Jeremy.M, Tymoczko John.L, Stryer Lubert, 2007, Biochemistry, Sixth Edition, W.H.Freeman, New York, Pages 897-900</ref>.

== What is the Lac operon? ==

The lac operon is a good example of how genes are regulated. It is an example of how genes can be regulated through the acts of an activator and/or repressor.The lac [[Operon|operon]] was studied in ''[[E. coli|E. coli]] ''<ref>Hartl, D. L. and Jones, E. W., 2009. Genetics: Analysis of genes and genomes. 7th edition. Sudbury: Jones and Bartlett Publishers pg 383</ref>. It contains 3 [[Gene|genes]] that are needed to produce proteins that are required to break down [[Lactose|lactose]] when it is present in the cell. These 3 [[Gene|genes]] are Lac Z, Lac Y and Lac A. Each code for B- galactosidase, Permease and Transacetylase respectively <ref>http://users.rcn.com (2011) "The operon" – 30th March 2010 – Available from: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LacOperon.html [Accessed 3rd January 2011]</ref>.

Further up the [[Genetic code|genetic code]] from these three genes, upstream, lies the [[Promoter|promoter]] sequence. [[RNA polymerase|RNA polymerase]] needs a region in which it can join the genetic code, the [[Promoter]] sequence, before it can start transcribing. [[RNA polymerase|RNA polymerase]] is required in transcription of the Lac operon <ref>Hartl, D. L. and Jones, E. W., 2009. Genetics: Analysis of genes and genomes. 7th edition. Sudbury: Jones and Bartlett Publishers pg 386</ref>. [[Image:Lac operon - basic.JPG]] 

== When does the Lac operon function? ==

The Lac operon is regulated depending on the absense or presence of certain substances. When [[Lactose|lactose]] is present in the cell and [[Glucose|glucose]] is absent, then the Lac operon is active and the 3 [[Genes|genes]] are transcribed to break down this lactose in the cell.

The lac operon only needs to activate the genes necessary for lactose integration when there is an absence of glucose and a plentiful supply of lactose.

== Negative gene regulation ==

The conditions inside the cell are changing all the time. So what happens when glucose is present and lactose levels are low? The Lac operon is no longer required to make the proteins to break down lactose and so its function is switched off. This is done by the use of a repressor protein <ref>Hartl, D. L. and Jones, E. W., 2009. Genetics: Analysis of genes and genomes. 7th edition. Sudbury: Jones and Bartlett Publishers pg 384</ref>.

Upstream of the promoter sequence there is another gene. This is the Lac I [[Gene|gene]]. The Lac I gene is transcribed to make the repressor protein which binds to 3 different operator sequences present at different parts of the Lac operon. The repressor protein is a tetramer and the binding of this repressor to the operators cause the [[DNA|DNA]] to be looped around.

[[Image:Lac operon - with lac I and operator seq.JPG|690x391px|Lac operon - with lac I and operator seq.JPG]] [[Image:Lac operon - with lac I and operator seq nd repressor.JPG|690x391px|Lac operon - with lac I and operator seq nd repressor.JPG]]

Once the repressor protein is bound, it stops the [[RNA polymerase|RNA polymerase]] enzyme from transcribing the genes. Effectively, it acts as a block <ref>Sadava (2011) “ The Lac Operon” – 2008 – Available from: http://www.sumanasinc.com/webcontent/animations/content/lacoperon.html [Accessed 3rd January 2011]</ref>.

When the glucose levels are depleted and the lactose levels rare high, the repressor has to be removed in order to transcribe the required genes. This is done by an inducer molecule. This [[Molecule|molecule]] comes from lactose and is Allolactose. Allolactose is synthesised from lactose by the enzyme B-galatidase, which is also a translational product of the lac gene. The allolacose binds to the [[Repressor protein|repressor protein]] and causes a conformational change in the repressor which causes it to dissociate from the DNA.. [[RNA polymerase|RNA polymerase]] can work as it is not blocked and the Lac Z, Lac Y and Lac A genes are transcribed <ref>BioCoach Activity (2011) "The lac inducer: Allolactose" – Available from: http://www.phschool.com/science/biology_place/biocoach/lacoperon/inducer.html [Accessed 3rd January 2011]</ref>.

[[Image:Lac operon - allolactose.JPG]]

== Positive Gene regulation ==

Sometimes promoters are not strong enough to initiate transcription on their own and so require another molecule or complex to help. In the Lac operon, this is done by the CRP – [[CAMP|cAMP]] complex. This is because glucose acts as an inhibitor of the enzyme andenynyl Cyclase which is responsible for converting ATP into cAMP (cyclic adenosine mono phosphate) When glucose levels in the cell are low, the levels of cAMP build up <ref>Mulligan, M. E. (2002) “The lac operon: positive regulation” – Available from: http://www.mun.ca/biochem/courses/3107/Topics/Lac_positive_control.html [Accessed 3rd January 2011]</ref>. This then combines with a CRP protein (sometimes also referred to as the CAP protein) and forms a complex called the CRP-cAMP complex. This complex then join to a sequence of nucleotides downstream from the LacI promoter known as the CRP binding site. The binding increases the affinity of  [[RNA polymerase|RNA polymerase]] for the lac promoter sequence and hence it binds and as a result transcription can take place. <ref>Hartl, D. L. and Jones, E. W., 2009. Genetics: Analysis of genes and genomes. 7th edition. Sudbury: Jones and Bartlett Publishers pg 389</ref>. When the levels of [[Glucose|glucose]] increase again, the amount of [[CAMP|cAMP]] synthesised is reduced and so the complex levels decrease. This therefore inhibits the Lac operon from working because the affinity of the RNA pol II for the lac gene promoter without the activator bound is quite low. 

== References ==

<references />

Promoter

2015-11-17T09:33:45Z

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A promoter site is a region of [[DNA|DNA]] that [[RNA polymerase|RNA polymerase]] binds to in order to carry out [[Transcription|transcription]]. It is located upstream of the target sequence <ref>Biochemistry, 6th Edition, Jeremy M. Berg, John L. Tymoczko, Lubert Stryer, W. H. Freeman and Company, New York, 2008</ref>. A promoter sequence determines where transcription of a [[Gene|gene]] will begin and also dictates the efficiency with which [[Transcription|transcription]] takes place - cells use the degree of efficiency regulate the amount of a particular [[Protein|protein]] produced.

When analysed, the promoter regions of different genes are found to be very similar. If we compare these sequences from different [[Genes|genes]], the most common bases for each position in the promoter region provide sequences known as '[[Consensus sequence|consensus sequences]]'. These often determine the strength of the promoter regions. The more closely the sequence of a promoter matches the consensus sequence, the stronger it is. A weak promoter will not recruit [[RNA polymerase|RNA polymerase]] as efficiently as a strong promoter.

A common feature of promoter regions in [[Eukaryote|eukaryotes]] is the [[Tata box|TATA box]].

=== References ===

<references />