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

‘sticky’ ends

2016-10-20T11:33:26Z

150189114:

A [[Restriction enzyme|restriction enzyme]] can cut [[DNA|DNA]] at a specific sequence of [[Nucleotides|nucleotides]] usually 4, 6 or 8 [[Nucleotides|nucleotides]] long. This may result in [[Blunt ends|symmetrical cleavage]] leading to [[Blunt ends|blunt ends]] or [['sticky' ends|assymetrical cleavage]] causing '[['sticky' ends|sticky' ends]]. A 'sticky' end is produced when the [[Restriction enzyme|restriction enzyme]] cuts at one end of the sequence, between two bases on the same strand, then cuts on the opposite end of the [[Complementary strand|complementary strand]]. This will produce two ends of [[DNA|DNA]] that will have some [[Nucleotides|nucleotides]] without any complementary bases. A [[Restriction enzyme|restriction enzyme]] will only cut at a specific sequence and it recognises [[Palindromic sequence|palindromic sequence]] that is, sequences that are the same whether they are read forwards or backwards (For example words like Hannah and Race car are palindromes). These 'sticky' ends allow the insertion of 'foreign' DNA into the host [[Genome|genome]]. By cutting the plasmid with the same restriction [[Enzyme|enzyme]], the same 'sticky ends' are produced. For example, complementary bases of the plasmid can pair with those of the host DNA and form hydrogen bonds which anneal the two strands together. However, there will still be nicks in the [[Phosphodiester bond|phosphodiester bonds which]] form the rigid phosphate backbone of DNA. In this scenario DNA ligase can be added which will form the phosphodiester bonds between the [[Recombinant_DNA_Technology|recombinant strands]]. The [[Genes|genes]] carried on the plasmid will now be incorporated into the hosts genome. These steps are commonly used in the lab <ref>http://www.scienceaid.co.uk/biology/genetics/engineering.html</ref>.

For example:

…..GAATTC….. …..CTTAAG….. After using restiction enzymes to cut at specific sites:

…..G    AATTC….. …..CTTAA    G….. 

Also see [[Blunt end|"blunt" ends]].

=== References ===

<references />

‘sticky’ ends

2016-10-20T11:32:44Z

150189114:

A [[Restriction enzyme|restriction enzyme]] can cut [[DNA|DNA]] at a specific sequence of [[Nucleotides|nucleotides]] usually 4, 6 or 8 [[Nucleotides|nucleotides]] long. This may result in [[Blunt ends|symmetrical cleavage]] leading to [[Blunt ends|blunt ends]] or [['sticky' ends|assymetrical cleavage]] causing '[['sticky' ends|sticky' ends]]. A 'sticky' end is produced when the [[Restriction enzyme|restriction enzyme]] cuts at one end of the sequence, between two bases on the same strand, then cuts on the opposite end of the [[Complementary strand|complementary strand]]. This will produce two ends of [[DNA|DNA]] that will have some [[Nucleotides|nucleotides]] without any complementary bases. A [[Restriction enzyme|restriction enzyme]] will only cut at a specific sequence and it recognises [[Palindromic sequence|palindromic sequence]] that is, sequences that are the same whether they are read forwards or backwards (For example words like Hannah and Race car are palindromes). These 'sticky' ends allow the insertion of 'foreign' DNA into the host [[Genome|genome]]. By cutting the plasmid with the same restriction [[Enzyme|enzyme]], the same 'sticky ends' are produced. For example, complementary bases of the plasmid can pair with those of the host DNA and form hydrogen bonds which anneal the two strands together. However, there will still be nicks in the [[Phosphodiester bond|phosphodiester bonds which]] form the rigid phosphate backbone of DNA. In this scenario DNA ligase can be added which will form the phosphodiester bonds between the recombinant strands. The [[Genes|genes]] carried on the plasmid will now be incorporated into the hosts genome. These steps are commonly used in the lab <ref>http://www.scienceaid.co.uk/biology/genetics/engineering.html</ref>.

For example:

…..GAATTC….. …..CTTAAG….. After using restiction enzymes to cut at specific sites:

…..G    AATTC….. …..CTTAA    G….. 

Also see [[Blunt end|"blunt" ends]].

=== References ===

<references />

Action potential

2015-12-03T21:35:45Z

150189114: addition of chemical symbols

An action potential is a message in the form of an electrical impulse caused by a rapid change in a cell's [[Membrane|membrane]] potential. 

When a stimulus reaches the threshold at the [[Axon hillock|axon hillock]], an action potential is generated.

An action potential relies on many [[Protein|protein]] channels.  In a neurone, the [[Potassium leak channel|Potassium leak channel]] and [[Sodium pump|Sodium-Potassium pump]] maintain the resting potential. The [[Sodium voltage-gated ion channels|voltage gated sodium channel]] and the [[Voltage gated potassium channels|voltage gated potassium channel]] are involved in the progression of an action potential along the membrane.

The action potential progression can be separated into several steps; 

#Voltage channels are closed and the Potassium (K+) leak channel and the sodium (Na+) pump maintain the resting membrane potential of -70 mV. The Sodium/Potassium Pump (ATPase) is responsible for maintaining the membrane potential at -70mv, the protein actively pumps three sodium ions out of the cell and pumps two potassium ions into the cell.
#The [[Neurone|neurone]] becomes stimulated. The [[Voltage gated sodium channels|voltage gated sodium channels]] begin to open and the membrane potential begins to slowly depolarises and sodium enters the cell down its concentration gradient. All the voltage gated Sodium channels open when the membrane potential reaches around -55 mV and there's a large influx of Sodium, causing a sharp rise in voltage. As the potential nears +30mV, the rate of depolarisation slows down as the voltage gated Sodium channels become saturated and inactivate, preventing further sodium ions from entering the cell.
#[[Voltage gated potassium channels|Voltage gated potassium channels]] open, and [[Potassium|potassium]] leaves the cell down its concentration gradient. The depolarisation of the cell stops and repolarisation can occur through these voltage gated Potssium channnels.
#Voltage gated sodium channels are completely deactivated and potassium floods out through the [[Voltage gated potassium channels|voltage gated potassium channels]],
#Voltage gated potassium channels are slow to close, and therefore [[Hyperpolarisation|hyperpolarisation]] occurs. This is where the membrane potential drops below the resting potential of -70 mV as potassium continues to leave.
#Once the [[Voltage gated potassium channels|voltage gated potassium channels]] close, the resting state can be re-established through the Potassium leak channel and Sodium pump. 

The action potential travels along the neurone's [[Axon|axon]] via current loops in order to reach the [[Axon terminal|axon terminal]].

An action potential is a transient, electrical signal, which is caused by a rapid change in [[Resting membrane potential|resting membrane potential ]](-70 mV). This occurs when the [[Threshold potential|threshold potential]] (-55 mV) is reached, this causes a rapid opening in the voltage gated sodium channels leading to a influx of sodium into the cell.  The [[Threshold potential|threshold potential]] also causes a slow opening of voltage gated potassium channels leading to the eflux of potassium out of the cell. This causes the cell to [[Depolarisation|depolarise]], meaning the inside of the cell is now positive compared to the outside.

The action potenial starts in the axon hilock as there is a high density of voltage gated sodium channels here, it is also where [[Graded potentials|graded potentials]] need to reach the threshold potential to cause a action potential. If the graded potential do not reach the [[Supratheshold|supratheshold level]], then an Action Potenitial is not triggered and the graded potenital is known as [[Subthreshold|subthreshold]]<ref>http://www.ncbi.nlm.nih.gov/books/NBK11104/</ref>. Above threshold, increase in the strength of a stimulus will not increase the size of the corresponding action potential. The strength of a stimulus, or the size of a graded potential, is indicated by frequency of action potentials travelling along a neurone.

The action potential travels via current loops. In myelinated axons its jumps from [[Node of ranvier|node of ranvier]] to node of ranvier, this is known as [[Saltatory conduction|saltatory conduction]].

The higher the density of the [[Myelin Sheath|myelin sheaths]] and higher the membrane resistance of the myelinated axon, the faster the axon potential can travel.

The point at which the membrane of an axon is depolarised causes a local circuit to be set up between the depolarised region and the region either side of it. This causes the resting at regions either side to become depolarised also. In this way the action potential sweeps along the axon.

The [[Refractory Period|refractory period]] prevents the action potential from travelling backwards. There are two types of refractory periods, the absolute refractory period and the relative refractory period. The absolute refractory period is when the membrane cannot generate another action potential no matter how large the stimulus is, this is because the voltage- gated sodium ion channels are inactivited. The relative refactory period is when the membrane can produce another action potential if the stimulus is larger than normal, this is because some of the voltage-gated soduim ion channels have recoveredand the voltage-gated potassium ion channels are still open. The relative refractory period is the period of hyperpolarisation after an action potential <ref>The McGill Physiology Virtual Lab, Refractory Period. Available at: http://www.medicine.mcgill.ca/physio/vlab/cap/refract.htm (Last accessed 9.11.13)</ref>.

Action potentials in neurons are also known as "nerve impulses" or "spikes" <ref>Silverthorn, D. (2012). Human physiology. 5th ed. San francisco Claifornia: Pearson Education, p.261.</ref><ref>http://www.ncbi.nlm.nih.gov/books/NBK11104/</ref>.

=== References ===

<references />