Friday, March 29, 2024
05:36 PM (GMT +5)

Go Back   CSS Forums > CSS Optional subjects > Group V > Zoology

Reply Share Thread: Submit Thread to Facebook Facebook     Submit Thread to Twitter Twitter     Submit Thread to Google+ Google+    
 
LinkBack Thread Tools Search this Thread
  #171  
Old Saturday, October 31, 2009
37th Common
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: CSP Medal: Awarded to those Members of the forum who are serving CSP Officers - Issue reason: Diligent Service Medal: Awarded upon completion of 5 years of dedicated services and contribution to the community. - Issue reason:
 
Join Date: Mar 2006
Posts: 1,514
Thanks: 1,053
Thanked 1,681 Times in 873 Posts
AFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud of
Default Linkage

Every individual organism bears several heritable characters. Which are represented by the innumerable genes present on the chromosomes. During meiosis, the chromosomes move into the gametes as units, all the genes present on any given chromosome will segregate as a group and move together from generation to generation. This tendency of the genes located on the same chromosome, to stay together in hereditary transmission, is known as linkage. The genes located on the same chromosome are called linked genes.

The principle of linkage was discovered by Bateson and Punnet in 1906 in the sweat pea, plant, Lathyrus odoratus. However, linkage, as a concept was put forth by Thomas Hunt Morgan in 1910 based on his experiment on Drosophila melanogaster.

Chromosome Theory of Linkage
Morgan, along with Castle formulated the chromosome theory of linkage. It has the following postulates;

1. Genes are found arranged in a linear manner in the chromosomes.
2. Genes which exhibit linkage are located on the same chromosome.

3. Genes generally tend to stay in parental combination, except in cases of crossing over.
4. The distance between linked genes in a chromosome determines the strength of linkage. Genes located close to each other show stronger linkage than that are located far from each other, since the former are less likely to enter into crossing over.

Linkage Groups
All the genes located on a particular chromosome, form a linkage group. Since the genes present on a particular chromosome have their alleles located on its homologous chromosome, genes on a pair of homologous chromosomes. Hence, the number of linkage groups corresponds to the number of haploid chromosomes found in a species.

Drosophila melanogaster has four linkage groups which can be distinguished into three large and one small linkage groups corresponding to the four pairs of chromosomes. Twenty-three linkage groups are present in humans corresponding to 23 pairs of chromosomes. Pea plant has seven linkage groups, corresponding to the seven pairs of chromosomes.

Kinds of Linkage
Complete Linkage

The genes closely located in the chromosome show complete linkage as they have no chance of separating by crossing over and are always transmitted together to the same gamete and the same offspring. Thus, the parental combination of traits is inherited as such by the young one.

Incomplete Linkage

The genes distantly located in the chromosome show incomplete linkage because they have a chance of separation by crossing over and of going into different gametes and offspring.

Genetic linkage occurs when particular genetic loci or alleles for genes are inherited jointly. Genetic loci on the same chromosome are physically close to one another and tend to stay together during meiosis, and are thus genetically linked. This is called autosomal linkage. Alleles for genes on different chromosomes are usually not linked, due to independent assortment of chromosomes during meiosis.

Because there is some crossing over of DNA when the chromosomes segregate, alleles on the same chromosome can be separated and go to different daughter cells. There is a greater probability of this happening if the alleles are far apart on the chromosome, as it is more likely that a cross-over will occur between them.

The relative distance between two genes can be calculated using the offspring of an organism showing two linked genetic traits, and finding the percentage of the offspring where the two traits do not run together. The higher the percentage of descendants that does not show both traits, the farther apart on the chromosome the two genes are.

Among individuals of an experimental population or species, some phenotypes or traits occur randomly with respect to one another in a manner known as independent assortment. Today scientists understand that independent assortment occurs when the genes affecting the phenotypes are found on different chromosomes or separated by a great enough distance on the same chromosome that recombination occurs at least half of the time.

An exception to independent assortment develops when genes appear near one another on the same chromosome. When genes occur on the same chromosome, they are usually inherited as a single unit. Genes inherited in this way are said to be linked, and are referred to as "linkage groups." For example, in fruit flies the genes affecting eye color and wing length are inherited together because they appear on the same chromosome.

But in many cases, even genes on the same chromosome that are inherited together produce offspring with unexpected allele combinations. This results from a process called crossing over. At the beginning of normal meiosis, a chromosome pair (made up of a chromosome from the mother and a chromosome from the father) intertwine and exchange sections or fragments of chromosome. The pair then breaks apart to form two chromosomes with a new combination of genes that differs from the combination supplied by the parents. Through this process of recombining genes, organisms can produce offspring with new combinations of maternal and paternal traits that may contribute to or enhance survival.

Genetic linkage was first discovered by the British geneticists William Bateson and Reginald Punnett shortly after Mendel's laws were rediscovered.

Linkage mapping

The observations by Thomas Hunt Morgan that the amount of crossing over between linked genes differs led to the idea that crossover frequency might indicate the distance separating genes on the chromosome. Morgan's student Alfred Sturtevant developed the first genetic map, also called a linkage map.

Sturtevant proposed that the greater the distance between linked genes, the greater the chance that non-sister chromatids would cross over in the region between the genes. By working out the number of recombinants it is possible to obtain a measure for the distance between the genes. This distance is called a genetic map unit (m.u.), or a centimorgan and is defined as the distance between genes for which one product of meiosis in 100 is recombinant. A recombinant frequency (RF) of 1 % is equivalent to 1 m.u. A linkage map is created by finding the map distances between a number of traits that are present on the same chromosome, ideally avoiding having significant gaps between traits to avoid the inaccuracies that will occur due to the possibility of multiple recombination events.

Linkage mapping is critical for identifying the location of genes that cause genetic diseases. In an ideal population, genetic traits and markers will occur in all possible combinations with the frequencies of combinations determined by the frequencies of the individual genes. For example, if alleles A and a occur with frequency 90% and 10%, and alleles B and b at a different genetic locus occur with frequencies 70% and 30%, the frequency of individuals having the combination AB would be 63%, the product of the frequencies of A and B, regardless of how close together the genes are. However, if a mutation in gene B that causes some disease happened recently in a particular subpopulation, it almost always occurs with a particular allele of gene A if the individual in which the mutation occurred had that variant of gene A and there have not been sufficient generations for recombination to happen between them (presumably due to tight linkage on the genetic map). In this case, called linkage disequilibrium, it is possible to search potential markers in the subpopulation and identify which marker the mutation is close to, thus determining the mutation's location on the map and identifying the gene at which the mutation occurred. Once the gene has been identified, it can be targeted to identify ways to mitigate the disease. Also, the location of a particular gene on a chromosome is called a gene locus.
Linkage map

A linkage map is a genetic map of a species or experimental population that shows the position of its known genes and/or genetic markers relative to each other in terms of recombination frequency, rather than as specific physical distance along each chromosome.

A genetic map is a map based on the frequencies of recombination between markers during crossover of homologous chromosomes. The greater the frequency of recombination (segregation) between two genetic markers, the farther apart they are assumed to be. Conversely, the lower the frequency of recombination between the markers, the smaller the physical distance between them. Historically, the markers originally used were detectable phenotypes (enzyme production, eye color) derived from coding DNA sequences; eventually, confirmed or assumed noncoding DNA sequences such as microsatellites or those generating restriction fragment length polymorphisms (RFLPs) have been used.

Genetic maps help researchers to locate other markers, such as other genes by testing for genetic linkage of the already known markers.

A genetic map is not a physical map (such as a radiation reduced hybrid map) or gene map.
LOD score method for estimating recombination frequency

The LOD score (logarithm (base 10) of odds) is a statistical test often used for linkage analysis in human populations, and also in animal and plant populations. The test was developed by Newton E. Morton. Computerized LOD score analysis is a simple way to analyze complex family pedigrees in order to determine the linkage between Mendelian traits (or between a trait and a marker, or two markers).

. Briefly, it works as follows:

1. Establish a pedigree
2. Make a number of estimates of recombination frequency
3. Calculate a LOD score for each estimate
4. The estimate with the highest LOD score will be considered the best estimate

The LOD score is calculated as follows:


NR denotes the number of non-recombinant offspring, and R denotes the number of recombinant offspring. The reason 0.5 is used in the denominator is that any alleles that are completely unlinked (e.g. alleles on separate chromosomes) have a 50% chance of recombination, due to independent assortment.

Theta is the recombinant fraction, it is equal to R / (NR + R)

In practice, LOD scores are looked up in a table which lists LOD scores for various standard pedigrees and various values of recombination frequency.

By convention, a LOD score greater than 3.0 is considered evidence for linkage. (A score of 3.0 means the Likelihood of observing the given pedigree if the two loci are not linked is less than 1 in 1000). On the other hand, a LOD score less than -2.0 is considered evidence to exclude linkage. Although it is very unlikely that a LOD score of 3 would be obtained from a single pedigree, the mathematical properties of the test allow data from a number of pedigrees to be combined by summing the LOD scores.

Recombination frequency

Recombination frequency (θ) is the frequency that a chromosomal crossover will take place between two loci (or genes) during meiosis. Recombination frequency is a measure of genetic linkage and is used in the creation of a genetic linkage map. A centimorgan (cM) is a unit that describes a recombination frequency of 1%.

During meiosis, chromosomes assort randomly into gametes, such that the segregation of alleles of one gene is independent of alleles of another gene. This is stated in Mendel's Second Law and is known as the law of independent assortment. The law of independent assortment always holds true for genes that are located on different chromosomes, but for genes that are on the same chromosome, it does not always hold true.

As an example of independent assortment, consider the crossing of the pure-bred homozygote parental strain with genotype AABB with a different pure-bred strain with genotype aabb. A and a and B and b represent the alleles of genes A and B. Crossing these homozygous parental strains will result in F1 generation offspring with genotype AaBb. The F1 offspring AaBb produces gametes that are AB, Ab, aB, and ab with equal frequencies (25%) because the alleles of gene A assort independently of the alleles for gene B during meiosis. Note that 2 of the 4 gametes (50 %)—Ab and aB—were not present in the parental generation. These gametes represent recombinant gametes. Recombinant gametes are those gametes that differ from both of the haploid gametes that made up the diploid cell. In this example, the recombination frequency is 50% since 2 of the 4 gametes were recombinant gametes.

The recombination frequency will be 50% when two genes are located on different chromosomes or when they are widely separated on the same chromosome. This is a consequence of independent assortment.

When two genes are close together on the same chromosome, they do not assort independently and are said to be linked. Whereas genes located on different chromosomes assort independently and have a recombination frequency of 50%, linked genes have a recombination frequency that is less than 50%.

As an example of linkage, consider the classic experiment by William Bateson and Reginald Punnett. They were interested in trait inheritance in the sweet pea and were studying two genes—the gene for flower color (P, purple, and p, red) and the gene affecting the shape of pollen grains (L, long, and l, round). They crossed the pure lines PPLL and ppll and then self-crossed the resulting PpLl lines. According to Mendelian genetics, the expected phenotypes would occur in a 9:3:3:1 ratio of PL:Pl:pL:pl. To their surprise, they observed an increased frequency of PL and pl and a decreased frequency of Pl and pL (see table below).
Bateson and Punnett experiment...... Phenotype and genotype....... Observed Expected from 9:3:3:1 ratio
Purple, long (PpLl)..... 284..... 216
Purple, round (Ppll)..... 21..... 72
Red, long (ppLl).......... 21..... 72
Red, round (ppll) ......... 55 .....24

Their experiment revealed linkage between the P and L alleles and the p and l alleles. The frequency of P occurring together with L and with p occurring together with l is greater than that of the recombinant Pl and pL. The recombination frequency cannot be computed directly from this experiment, but intuitively it is less than 50%.

The progeny in this case received two dominant alleles linked on one chromosome (referred to as coupling or cis arrangement). However, after crossover, some progeny could have received one parental chromosome with a dominant allele for one trait (eg Purple) linked to a recessive allele for a second trait (eg round) with the opposite being true for the other parental chromosome (eg red and Long). This is referred to as repulsion or a trans arrangement. The phenotype here would still be purple and long but a test cross of this individual with the recessive parent would produce progeny with much greater proportion of the two crossover phenotypes. While such a problem may not seem likely from this example, unfavorable repulsion linkages do appear when breeding for disease resistance in some crops.

When two genes are located on the same chromosome, the chance of a crossover producing recombination between the genes is related to the distance between the two genes. Thus, the use of recombination frequencies has been used to develop linkage maps or genetic maps.
Reply With Quote
The Following User Says Thank You to AFRMS For This Useful Post:
Malmeena Khan (Saturday, April 21, 2012)
  #172  
Old Saturday, October 31, 2009
37th Common
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: CSP Medal: Awarded to those Members of the forum who are serving CSP Officers - Issue reason: Diligent Service Medal: Awarded upon completion of 5 years of dedicated services and contribution to the community. - Issue reason:
 
Join Date: Mar 2006
Posts: 1,514
Thanks: 1,053
Thanked 1,681 Times in 873 Posts
AFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud of
Default Linkage

Linkage



Gregor Mendel analyzed the pattern of inheritance of seven pairs of contrasting traits in the domestic pea plant. He did this by cross-breeding dihybrids; that is, plants that were heterozygous for the alleles controlling two different traits.

Example
Producing dihybrids (F1)
He mated a variety that was pure-breeding (hence homozygous) for round (RR), yellow (YY) seeds with one that was pure-breeding for wrinkled (rr), green (yy) seeds. All the offspring (F1) produced from this mating were dihybrids; that is, heterozygous for each pair of alleles (RrYy). Furthermore, all the seeds were round and yellow, showing that the genes for round and yellow are dominant.









Mating the dihybrids to produce an F2 generation
Mendel then crossed these dihybrids. If it is inevitable that round seeds must always be yellow and wrinkled seeds must be green, then he would have expected that this would produce a typical monohybrid cross: 75% round-yellow; 25% wrinkled-green. But, in fact, his mating generated seeds that showed all possible combinations of the color and texture traits.

* 9/16 of the offspring were round-yellow
* 3/16 were round-green
* 3/16 were wrinkled-yellow, and
* 1/16 were wrinkled-green

Finding in every case that each of his seven traits was inherited independently of the others, he formed his "second rule" the Rule of Independent Assortment:

The inheritance of one pair of factors (genes) is independent of the inheritance of the other pair.
Today we know that this rule holds only if two conditions are met:

* the genes are on separate chromosomes or
* the genes are widely separated on the same chromosome.

Mendel was lucky in that every pair of genes he studied met one requirement or the other. The table shows the chromosome assignments of the seven pairs of alleles that Mendel studied. Although all of these genes showed independent assortment, several were, in fact, syntenic with three loci occurring on chromosome 4 and two on chromosome 1. However, the distance separating the syntenic loci was sufficiently great that the genes were inherited as though they were on separate chromosomes.

Trait ....... Phenotype .........Alleles ....... Chromosome
Seed form...... round-wrinkled..... R-r...... 7
Seed color ....... yellow-green...... I-i .....1
Pod color ......... green-yellow ...... Gp-gp ....5
Pod texture....... smooth-wrinkled..... V-v .....4
Flower color ...... purple-white....... A-a..... 1
Flower location ......axial-terminal .....Fa-fa..... 4
Plant height........... tall-dwarf ..........Le-le..... 4

With the rebirth of genetics in the 20th century, it quickly became apparent that Mendel's second rule does not apply to many matings of dihybrids. In many cases, two alleles inherited from one parent show a strong tendency to stay together as do those from the other parent. This phenomenon is called linkage.
An example of linkage
Start with two different strains of corn (maize).

* one that is homozygous for two traits
o yellow kernels (C,C) which are filled with endosperm causing the kernels to be
o smooth (Sh,Sh).
* a second that is homozygous for
o colorless kernels (c,c) that are wrinkled because their endosperm is
o shrunken (sh,sh)

When the pollen of the first strain is dusted on the silks of the second (or vice versa), the kernels produced (F1) are all yellow and smooth. So the alleles for yellow color (C) and smoothness (Sh) are dominant over those for colorlessness (c) and shrunken endosperm (sh).


To simplify the analysis, mate the dihybrid with a homozygous recessive strain (ccshsh). Such a mating is called a test cross because it exposes the genotype of all the gametes of the strain being evaluated.

According to Mendel's second rule, the genes determining color of the endosperm should be inherited independently of the genes determining texture. The F1 should thus produce gametes in approximately equal numbers.

* CSh, as inherited from one parent.
* csh, as inherited from the other parent
* Csh, a recombinant
* cSh, the other recombinant.

All the gametes produced by the doubly homozygous recessives would be csh.

If the inheritance of these genes observes Mendel's second rule; i.e., shows independent assortment, union of these gametes should produce approximately equal numbers of the four phenotypes. But as the chart shows, there is instead a strong tendency for the parental alleles to stay together. It occurs because the two loci are relatively close together on the same chromosome. Only 3.6% of the gametes contain a recombinant chromosome.









During prophase I of meiosis, pairs of duplicated homologous chromosomes unite in synapsis and then nonsister chromatids exchange segments during crossing over. It is crossing over that produces the recombinant gametes. In this case, whenever a crossover occurs between the locus for kernel color and that for kernel texture, the original combination of alleles (CSh and csh) is broken up and a chromosome containing Csh and one containing cSh will be produced.
Chromosome Maps

The percentage of recombinants formed by F1 individuals can range from a fraction of 1% up to the 50% always seen with gene loci on separate chromosomes (independent assortment). The higher the percentage of recombinants for a pair of traits, the greater the distance separating the two loci. In fact, the percent of recombinants is arbitrarily chosen as the distance in centimorgans (cM), named for the pioneering geneticist Thomas Hunt Morgan. In our case, the c and sh loci are said to be 2.8 cM apart.
The procedure can be continued with another locus on the same chromosome.




Example

Test crossing a corn plant that is dihybrid for the C,c alleles and the alleles for bronze color (Bz, bz) produces 4.6% recombinants. So these two loci are 4.6 cM apart. However, is the bz locus on the same side of c as sh or is it on the other side?

The answer can be found by test crossing the dihybrid Shsh, Bzbz. If the percentage of recombinants is less than 4.6%, then bz must be on the same side of locus c as locus sh. If greater than 4.6%, it must be on the other side.










Mapping by linkage analysis is best done with loci that are relatively close together; that is, within a few centimorgans of each other. Why? Because as the distance between two loci increases, the probability of a second crossover occurring between them also increases.
Reply With Quote
  #173  
Old Friday, November 13, 2009
37th Common
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: CSP Medal: Awarded to those Members of the forum who are serving CSP Officers - Issue reason: Diligent Service Medal: Awarded upon completion of 5 years of dedicated services and contribution to the community. - Issue reason:
 
Join Date: Mar 2006
Posts: 1,514
Thanks: 1,053
Thanked 1,681 Times in 873 Posts
AFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud of
Default Enzymes

How Do Enzymes Function?

Enzymes are "biological catalysts." "Biological" means the substance in question is produced or is derived from some living organism. "Catalyst" denotes a substance that has the ability to increase the rate of a chemical reaction, and is not changed
or destroyed by the chemical reaction that it accelerates.

Generally speaking, catalysts are specific in nature as to the type of reaction they can catalyze. Enzymes, as a subclass of catalysts, are very specific in nature. Each enzyme can act to catalyze only very select chemical reactions and only with very select substances. An enzyme has been described as a "key" which can "unlock" complex compounds. An enzyme, as the key, must have a certain structure or multi-dimensional shape that matches a specific section of the "substrate" (a substrate is
the compound or substance which undergoes the change). Once these two components come together, certain chemical bonds within the substrate molecule change much as a lock is released, and just like the key in this illustration, the enzyme is free to execute its duty once again.

Many chemical reactions do proceed but at such a slow rate that their progress would seem to be imperceptible at normally encountered environmental temperature. Consider for example, the oxidation of glucose or other sugars to useable energy by
animals and plants. For a living organism to derive heat and other energy from sugar, the sugar must be oxidized (combined with oxygen) or metabolically "burned"

However, in a living system, the oxidation of sugar must meet an additional condition; that oxidation of sugar must proceed essentially at normal body temperature. Obviously, sugar surrounded by sufficient oxygen would not oxidize very rapidly at this temperature. In conjunction with a series of enzymes created by the living organism, however, this reaction does proceed quite rapidly at temperatures up to 100°F (38°C). Therefore, enzymes allow the living organism to make use of the potential energy contained in sugar and other food substances.

Enzymes or biological catalysts allow reactions that are necessary to sustain life proceed relatively quickly at the normal environmental temperatures. Enzymes often
increase the rate of a chemical reaction between 10 and 20 million times what the speed of reaction would be when left uncatalyzed (at a given temperature).
Nutrients locked in certain organics are complex macromolecules, or in hard-to-digest matrices may be released or predigested by a high degree of heat or concentrated acid treatment. In an alternative manner, specific enzymes can promote the pre-digestion of certain complex nutrients and facilitate the release of highly digestible nutrients in organics during processing without the need of excessive heat or rigorous chemical treatment.
Reply With Quote
  #174  
Old Friday, November 13, 2009
37th Common
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: CSP Medal: Awarded to those Members of the forum who are serving CSP Officers - Issue reason: Diligent Service Medal: Awarded upon completion of 5 years of dedicated services and contribution to the community. - Issue reason:
 
Join Date: Mar 2006
Posts: 1,514
Thanks: 1,053
Thanked 1,681 Times in 873 Posts
AFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud of
Default Enzymes

Enzymes are very efficient catalysts for biochemical reactions. They speed up reactions by providing an alternative reaction pathway of lower activation energy.
Like all catalysts, enzymes take part in the reaction - that is how they provide an alternative reaction pathway. But they do not undergo permanent changes and so remain unchanged at the end of the reaction. They can only alter the rate of reaction, not the position of the equilibrium.
Most chemical catalysts catalyse a wide range of reactions. They are not usually very selective. In contrast enzymes are usually highly selective, catalysing specific reactions only. This specificity is due to the shapes of the enzyme molecules.
Many enzymes consist of a protein and a non-protein (called the cofactor). The proteins in enzymes are usually globular. The intra- and intermolecular bonds that hold proteins in their secondary and tertiary structures are disrupted by changes in temperature and pH. This affects shapes and so the catalytic activity of an enzyme is pH and temperature sensitive.
Cofactors may be:
  • organic groups that are permanently bound to the enzyme (prosthetic groups)
  • cations - positively charged metal ions (activators), which temporarily bind to the active site of the enzyme, giving an intense positive charge to the enzyme's protein
  • organic molecules, usually vitamins or made from vitamins (coenzymes), which are not permanently bound to the enzyme molecule, but combine with the enzyme-substrate complex temporarily.
How Enzymes Work
For two molecules to react they must collide with one another. They must collide in the right direction (orientation) and with sufficient energy. Sufficient energy means that between them they have enough energy to overcome the energy barrier to reaction. This is called the activation energy.
Enzymes have an active site. This is part of the molecule that has just the right shape and functional groups to bind to one of the reacting molecules. The reacting molecule that binds to the enzyme is called the substrate.
An enzyme-catalysed reaction takes a different 'route'. The enzyme and substrate form a reaction intermediate. Its formation has a lower activation energy than the reaction between reactants without a catalyst.
A simplified picture
Route A reactant 1 + reactant 2 ----------- product
Route B reactant 1 + enzyme ------------intermediate
intermediate + reactant 2 ---------------- product + enzyme
So the enzyme is used to form a reaction intermediate, but when this reacts with another reactant the enzyme reforms.


Lock and Key Hypothesis
This is the simplest model to represent how an enzyme works. The substrate simply fits into the active site to form a reaction intermediate.



Induced fit hypothesis

In this model the enzyme molecule changes shape as the substrate molecules gets close. The change in shape is 'induced' by the approaching substrate molecule. This more sophisticated model relies on the fact that molecules are flexible because single covalent bonds are free to rotate.



Factors affecting catalytic activity of enzymes

Temperature




As the temperature rises, reacting molecules have more and more kinetic energy. This increases the chances of a successful collision and so the rate increases. There is a certain temperature at which an enzyme's catalytic activity is at its greatest (see graph). This optimal temperature is usually around human body temperature (37.5 oC) for the enzymes in human cells.
Above this temperature the enzyme structure begins to break down (denature) since at higher temperatures intra- and intermolecular bonds are broken as the enzyme molecules gain even more kinetic energy.


ph




Each enzyme works within quite a small pH range. There is a pH at which its activity is greatest (the optimal pH). This is because changes in pH can make and break intra- and intermolecular bonds, changing the shape of the enzyme and, therefore, its effectiveness.


he rate of an enzyme-catalysed reaction depends on the concentrations of enzyme and substrate. As the concentration of either is increased the rate of reaction increases (see graphs). For a given enzyme concentration, the rate of reaction increases with increasing substrate concentration up to a point, above which any further increase in substrate concentration produces no significant change in reaction rate. This is because the active sites of the enzyme molecules at any given moment are virtually saturated with substrate. The enzyme/substrate complex has to dissociate before the active sites are free to accommodate more substrate.
Provided that the substrate concentration is high and that temperature and pH are kept constant, the rate of reaction is proportional to the enzyme concentration.


Inhibition of enzyme activity

Some substances reduce or even stop the catalytic activity of enzymes in biochemical reactions. They block or distort the active site. These chemicals are called inhibitors, because they inhibit reaction.
Inhibitors that occupy the active site and prevent a substrate molecule from binding to the enzyme are said to be active site-directed (or competitive, as they 'compete' with the substrate for the active site).
Inhibitors that attach to other parts of the enzyme molecule, perhaps distorting its shape, are said to be non-active site-directed (or non competitive).
Immobilized enzymes

Enzymes are widely used commercially, for example in the detergent, food and brewing industries. Protease enzymes are used in 'biological' washing powders to speed up the breakdown of proteins in stains like blood and egg. Pectinase is used to produce and clarify fruit juices. Problems using enzymes commercially include:
  • they are water soluble which makes them hard to recover
  • some products can inhibit the enzyme activity (feedback inhibition)
Enzymes can be immobilized by fixing them to a solid surface. This has a number of commercial advantages:
  • the enzyme is easily removed
  • the enzyme can be packed into columns and used over a long period
  • speedy separation of products reduces feedback inhibition
  • thermal stability is increased allowing higher temperatures to be used
  • higher operating temperatures increase rate of reaction
There are four principal methods of immobilization currently in use:
  • covalent bonding to a solid support
  • adsorption onto an insoluble substance
  • entrapment within a gel
  • encapsulation behind a selectively permeable membrane


Reply With Quote
  #175  
Old Friday, November 13, 2009
37th Common
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: CSP Medal: Awarded to those Members of the forum who are serving CSP Officers - Issue reason: Diligent Service Medal: Awarded upon completion of 5 years of dedicated services and contribution to the community. - Issue reason:
 
Join Date: Mar 2006
Posts: 1,514
Thanks: 1,053
Thanked 1,681 Times in 873 Posts
AFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud of
Default

Power point presentation on Enzymes.

Click here.

Regards.
Reply With Quote
  #176  
Old Tuesday, December 01, 2009
37th Common
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: CSP Medal: Awarded to those Members of the forum who are serving CSP Officers - Issue reason: Diligent Service Medal: Awarded upon completion of 5 years of dedicated services and contribution to the community. - Issue reason:
 
Join Date: Mar 2006
Posts: 1,514
Thanks: 1,053
Thanked 1,681 Times in 873 Posts
AFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud of
Default The Synapse

The Synapse

The junction across which a nerve impulse passes from an axon terminal to a neuron, muscle cell, or gland cell.

A synapse, or synaptic cleft, is the gap that separates adjacent neurons or a neuron and a muscle. Transmission of an impulse across a synapse, from presynaptic cell to postsynaptic cell, may be electrical or chemical. In electrical synapses, the action potential travels along the membranes of gap junctions, small tubes of cytoplasm along the membranes of gap junctions, small tubes of cytoplasm that allow the transfer of ions between adjacent cells. In chemical synapses, action potentials are transferred across the synapse by the diffusion of chemicals, as follows:
·Calcium (Ca2+) gates open. When an action potential reaches the end of an axon, the depolarization of the membrane causes gated channels to open that allow Ca2+ to enter.
·Synaptic vesicles release neurotransmitter. The influx of Ca2+ into the terminal end of the axon causes synaptic vesicles to merge with the presynaptic membrane, releasing a neurotransmitter into the synaptic cleft.
·Neurotransmitter binds with postsynaptic receptors. The neurotransmitter diffuses across the synaptic cleft and binds with specialized protein receptors on the postsynaptic membrane. Different proteins are receptors for different neurotransmitters.
·The postsynaptic membrane is excited or inhibited. Depending upon the kind of neurotransmitter and the kind of membrane receptor, there are two possible outcomes for the postsynaptic membrane, both of which are graded potentials.
oIf positive ion gates open (which allow more Na+ and Ca2+ to enter than K+ to exit), the membrane becomes depolarized, which results in an excitatory postsynaptic potential (EPSP). If the threshold potential is exceeded, an action potential is generated.
oIf K+ or chlorine ion (Cl−) gates open (allowing K+ to exit or Cl− to enter), the membrane becomes more polarized (hyperpolarized), which results in an inhibitory postsynaptic potential (IPSP). As a result, it becomes more difficult to generate an action potential on this membrane.
·The neurotransmitter is degraded and recycled. After the neurotransmitter binds to the postsynaptic membrane receptors, it is either transported back to and reabsorbed by the secreting neuron, or it is broken down by enzymes in the synaptic cleft. For example, the common neurotransmitter acetylcholine is broken down by cholinesterase. Reabsorbed and degraded neurotransmitters are recycled by the presynaptic cell.
Here are some of the common neurotransmitters and the kinds of activity they generate:
·Acetylcholine (ACh) is commonly secreted at neuromuscular junctions, the gaps between motor neurons and muscle cells, where it stimulates muscles to contract (by opening gated positive ion channels). At other kinds of junctions, it typically produces an inhibitory postsynaptic potential.
·Epinephrine, norepinephrine (NE), dopamine, and serotonin are derived from amino acids and are secreted mostly between neurons of the CNS.
·Gamma aminobutyric acid (GABA) is usually an inhibitory neurotransmitter (opening gated Cl− channels) among neurons in the brain.











Reply With Quote
  #177  
Old Tuesday, December 01, 2009
37th Common
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: CSP Medal: Awarded to those Members of the forum who are serving CSP Officers - Issue reason: Diligent Service Medal: Awarded upon completion of 5 years of dedicated services and contribution to the community. - Issue reason:
 
Join Date: Mar 2006
Posts: 1,514
Thanks: 1,053
Thanked 1,681 Times in 873 Posts
AFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud of
Default Transmission of Nerve Impulses

Transmission of Nerve Impulses

The transmission of a nerve impulse along a neuron from one end to the other occurs as a result of chemical changes across the membrane of the neuron. The membrane of an unstimulated neuron is polarized—that is, there is a difference in electrical charge between the outside and inside of the membrane. The inside is negative with respect to the outside



Polarization is established by maintaining an excess of sodium ions (Na+) on the outside and an excess of potassium ions (K+) on the inside. A certain amount of Na+ and K+ is always leaking across the membrane through leakage channels, but Na+/K+ pumps in the membrane actively restore the ions to the appropriate side.

Other ions, such as large, negatively charged proteins and nucleic acids, reside within the cell. It is these large, negatively charged ions that contribute to the overall negative charge on the inside of the cell membrane as compared to the outside.

In addition to crossing the membrane through leakage channels, ions may also cross through gated channels. Gated channels open in response to neurotransmitters, changes in membrane potential, or other stimuli.
The following events characterize the transmission of a nerve impulse
·Resting potential. The resting potential describes the unstimulated, polarized state of a neuron (at about −70 millivolts).

·Graded potential. A graded potential is a change in the resting potential of the plasma membrane in the response to a stimulus. A graded potential occurs when the stimulus causes Na+ or K+ gated channels to open. If Na+ channels open, positive sodium ions enter, and the membrane depolarizes (becomes more positive). If the stimulus opens K+ channels, then positive potassium ions exit across the membrane and the membrane hyperpolarizes (becomes more negative). A graded potential is a local event that does not travel far from its origin. Graded potentials occur in cell bodies and dendrites. Light, heat, mechanical pressure, and chemicals, such as neurotransmitters, are examples of stimuli that may generate a graded potential (depending upon the neuron).

·Action potential. Unlike a graded potential, an action potential is capable of traveling long distances. If a depolarizing graded potential is sufficiently large, Na+ channels in the trigger zone open. In response, Na+ on the outside of the membrane becomes depolarized (as in a graded potential). If the stimulus is strong enough—that is, if it is above a certain threshold level—additional Na+ gates open, increasing the flow of Na+ even more, causing an action potential, or complete depolarization (from −70 to about +30 millivolts). This, in turn, stimulates neighboring Na+ gates, farther down the axon, to open. In this manner, the action potential travels down the length of the axon as opened Na+ gates stimulate neighboring Na+ gates to open. The action potential is an all-or-nothing event: When the stimulus fails to produce depolarization that exceeds the threshold value, no action potential results, but when threshold potential is exceeded, complete depolarization occurs.

·Repolarization. In response to the inflow of Na+, K+ channels open, this time allowing K+ on the inside to rush out of the cell. The movement of K+ out of the cell causes repolarization by restoring the original membrane polarization. Unlike the resting potential, however, in repolarization the K+ are on the outside and the Na+ are on the inside. Soon after the K+ gates open, the Na+ gates close.



·Hyper polarization. By the time the K+ channels close, more K+ have moved out of the cell than is actually necessary to establish the original polarized potential. Thus, the membrane becomes hyperpolarized (about −80 millivolts).

·Refractory period. With the passage of the action potential, the cell membrane is in an unusual state of affairs. The membrane is polarized, but the Na+ and K+ are on the wrong sides of the membrane. During this refractory period, the axon will not respond to a new stimulus. To reestablish the original distribution of these ions, the Na+ and K+ are returned to their resting potential location by Na+/K+ pumps in the cell membrane. Once these ions are completely returned to their resting potential location, the neuron is ready for another stimulus.









Reply With Quote
  #178  
Old Tuesday, December 01, 2009
37th Common
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: CSP Medal: Awarded to those Members of the forum who are serving CSP Officers - Issue reason: Diligent Service Medal: Awarded upon completion of 5 years of dedicated services and contribution to the community. - Issue reason:
 
Join Date: Mar 2006
Posts: 1,514
Thanks: 1,053
Thanked 1,681 Times in 873 Posts
AFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud of
Default

Synaptic Transmission

http://www.youtube.com/v/z3F5dfmQ3hk




Action Potential of a Nerve Impulse




http://www.youtube.com/v/m79HiApDJ2I
Reply With Quote
The Following User Says Thank You to AFRMS For This Useful Post:
mahamkhan93 (Sunday, August 28, 2011)
  #179  
Old Wednesday, December 02, 2009
37th Common
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: CSP Medal: Awarded to those Members of the forum who are serving CSP Officers - Issue reason: Diligent Service Medal: Awarded upon completion of 5 years of dedicated services and contribution to the community. - Issue reason:
 
Join Date: Mar 2006
Posts: 1,514
Thanks: 1,053
Thanked 1,681 Times in 873 Posts
AFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud of
Default Control of Respiration

Control of Respiration

Respiration is controlled by these areas of the brain that stimulate the contraction of the diaphragm and the intercostal muscles. These areas, collectively called respiratory centers, are summarized here:


·The medullary inspiratory center, located in the medullar oblongata, generates rhythmic nerve impulses that stimulate contraction of the inspiratory muscles (diaphragm and external intercostal muscles). Normally, expiration occurs when these muscles relax, but when breathing is rapid, the inspiratory center facilitates expiration by stimulating the expiratory muscles (internal intercostal muscles and abdominal muscles).
·The pheumotaxic area, located in the pons, inhibits the inspiratory center, limiting the contraction of the inspiratory muscles, and preventing the lungs from overinflating.
·The apneustic area, also located in the pons, stimulates the inspiratory center, prolonging the contraction of inspiratory muscles.
The respiratory centers are influenced by stimuli received from the following three groups of sensory neurons:
·Central chemoreceptors (nerves of the central nervous system), located in the medulla oblongata, monitor the chemistry of cerebrospinal fluid. When CO2 from the plasma enters the cerebrospinal fluid, it forms HCO3- and H+, and the pH of the fluid drops (becomes more acidic). In response to the decrease in pH, the central chemoreceptors stimulate the respiratory center to increase the inspiratory rate.
·Peripheral chemoreceptors (nerves of the peripheral nervous system), located in aortic bodies in the wall of the aortic arch and in carotid bodies in the walls of the carotid arteries, monitor the chemistry of the blood. An increase in pH or pCO2, or decrease in pO2, causes these receptors to stimulate the respiratory center.
·Stretch receptors in the walls of bronchi and bronchioles are activated when the lungs expand to their physical limit. These receptors signal the respiratory center to discontinue stimulation of the inspiratory muscles, allowing expiration to begin. This response is called the inflation (Hering-Breur) reflex.



Reply With Quote
The Following User Says Thank You to AFRMS For This Useful Post:
mahamkhan93 (Sunday, August 28, 2011)
  #180  
Old Wednesday, December 02, 2009
37th Common
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: CSP Medal: Awarded to those Members of the forum who are serving CSP Officers - Issue reason: Diligent Service Medal: Awarded upon completion of 5 years of dedicated services and contribution to the community. - Issue reason:
 
Join Date: Mar 2006
Posts: 1,514
Thanks: 1,053
Thanked 1,681 Times in 873 Posts
AFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud ofAFRMS has much to be proud of
Default Regulation of Digestion

Regulation of Digestion

The activities of the digestive system are regulated by both hormones and neural reflexes. Four important hormones and their effects upon target cells follow.

·Gastrin is produced by enteroendocrine cells of the stomach mucosa. Effects include


oStimulation of gastric juice (especially HCl) secretion by gastric glands.
oStimulation of smooth muscle contraction in the stomach, small intestine, and large intestine, which increases gastric and intestinal motility.
oRelaxation of the pyloric sphincter, which promotes gastric emptying into the small intestine.


·Secretin is produced by the enteroendocrine cells of the duodenal mucosa. Effects include


oStimulation of bicarbonate secretion by the pancreas, which neutralizes the acidity of chyme when released into the duodenum.
oStimulation of bile production by the liver.
oInhibition of gastric juice secretions and gastric motility, which, in turn, slows digestion in the stomach and retards gastric emptying.


·Cholecystokinin (CCK) is produced by enteroendocrine cells of the duodenal mucosa. Effects include


oStimulation of bile release by the gallbladder.
oStimulation of pancreatic juice secretion.
oRelaxation of the hepatopancreatic ampulla, which allows flow of bile and pancreatic juices into the duodenum.


·Gastric inhibitory peptide (GIP) is produced by enteroendocrine cells of the duodenal mucosa and causes the inhibition of gastric juice secretion and gastric motility, which, in turn, slows digestion in the stomach and retards gastric emptying.


The second regulatory agent of the digestive system is the nervous system. Stimuli that influence digestive activities may originate in the head, the stomach, or the small intestine. Based on these sites, there are three phases of digestive regulation:
·The cephalic phase comprises those stimuli that originate from the head: sight, smell, taste, or thoughts of food, as well as emotional states. In response, the following reflexes are initiated:


oNeural response. Stimuli that arouse digestion are relayed to the hypothalamus, which, in turn, initiates nerve impulses in the parasympathetic vagus nerve. These impulses innervate nerve networks of the GI tract (enteric nervous system), which promote contraction of smooth muscle (which causes peristalsis) and secretion of gastric juice. Stimuli that repress digestion (emotions of fear or anxiety, for example) innervate sympathetic fibers that suppress muscle contraction and secretion.
oGeneral effects. The stomach prepares for the digestion of proteins.


·The gastric phase describes those stimuli that originate from the stomach. These stimuli include distention of the stomach (which activates stretch receptors), low acidity (high pH), and the presence of peptides. In response, the following reflexes are initiated:


oNeural response. Gastric juice secretion and smooth muscle contraction are promoted.
oHormonal response. Gastrin production is promoted.
oGeneral effects. The stomach and small intestine prepare for the digestion of chyme, and gastric emptying is promoted.


·The intestinal phase describes stimuli originating in the small intestine. These include distention of the duodenum, high acidity (low pH), and the presence of chyme (especially fatty acids and carbohydrates). In response, the following reflexes are initiated:


oNeural response. Gastric secretion and gastric motility are inhibited (enterogastric reflex). Intestinal secretions, smooth muscle contraction, and bile and pancreatic juice production are promoted.
oHormonal response. Production of secretin, CCK, and GIP is promoted.
oGeneral effects. Stomach emptying is retarded to allow adequate time for digestion (especially fats) in the small intestine. Intestinal digestion and motility are promoted.


Reply With Quote
The Following User Says Thank You to AFRMS For This Useful Post:
mahamkhan93 (Sunday, August 28, 2011)
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Very Important : How to Prepare Study Notes Shaa-Baaz Tips and Experience Sharing 5 Sunday, May 21, 2017 08:30 PM
Effective Study Skills Sureshlasi Tips and Experience Sharing 1 Friday, November 16, 2007 09:28 AM
Regarding Notes Anonymous84 Tips and Experience Sharing 1 Wednesday, August 15, 2007 06:56 PM


CSS Forum on Facebook Follow CSS Forum on Twitter

Disclaimer: All messages made available as part of this discussion group (including any bulletin boards and chat rooms) and any opinions, advice, statements or other information contained in any messages posted or transmitted by any third party are the responsibility of the author of that message and not of CSSForum.com.pk (unless CSSForum.com.pk is specifically identified as the author of the message). The fact that a particular message is posted on or transmitted using this web site does not mean that CSSForum has endorsed that message in any way or verified the accuracy, completeness or usefulness of any message. We encourage visitors to the forum to report any objectionable message in site feedback. This forum is not monitored 24/7.

Sponsors: ArgusVision   vBulletin, Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.