Principles and processes in biotechnology.pmdEver since the days of Rene Descartes, the French philosopher,mathematician and biologist of seventeenth century, all human Chapter 11
knowledge especially natural sciences were directed to develop Biotechnology : Principles and technologies which add to the creature comforts of human lives, as also value to human life. The whole approach to Chapter 12
understanding natural phenomena became anthropocentric.
Biotechnology and Its Physics and chemistry gave rise to engineering, technologies and industries which all worked for human comfort and welfare.
The major utility of the biological world is as a source of food.
Biotechnology, the twentieth century off-shoot of modernbiology, changed our daily life as its products broughtqualitative improvement in health and food production. Thebasic principles underlying biotechnological processes and someapplications are highlighted and discussed in this unit.
Herbert Boyer was born in 1936 and brought up in a corner of westernPennsylvania where railroads and mines were the destiny of most youngmen. He completed graduate work at the University of Pittsburgh, in1963, followed by three years of post-graduate studies at Yale.
In 1966, Boyer took over assistant professorship at the University of California at San Francisco. By 1969, he performed studies on a coupleof restriction enzymes of the E. coli bacterium with especially usefulproperties. Boyer observed that these enzymes have the capability ofcutting DNA strands in a particular fashion, which left what has becameknown as ‘sticky ends' on the strands. These clipped ends made pastingtogether pieces of DNA a precise exercise.
This discovery, in turn, led to a rich and rewarding conversation in Hawaii with a Stanford scientist named Stanley Cohen. Cohen hadbeen studying small ringlets of DNA called plasmids and which floatabout freely in the cytoplasm of certain bacterial cells and replicate independently from the coding strand of DNA. Cohen had developed a method of removing these plasmids from the cell and then reinsertingthem in other cells. Combining this process with that of DNA splicingenabled Boyer and Cohen to recombine segments of DNA in desiredconfigurations and insert the DNA in bacterial cells, which could thenact as manufacturing plants for specific proteins. This breakthrough wasthe basis upon which the discipline of biotechnology was founded.
BIOTECHNOLOGY : PRINCIPLESAND PROCESSES 11.1 Principles of Biotechnology 11.2 Tools of Recombinant DNA Biotechnology deals with techniques of using liveorganisms or enzymes from organisms to produce products and processes useful to humans. In this sense, makingcurd, bread or wine, which are all microbe-mediated 11.3 Processes of Recombinant processes, could also be thought as a form of DNA Technology biotechnology. However, it is used in a restricted sensetoday, to refer to such of those processes which usegenetically modified organisms to achieve the same on alarger scale. Further, many other processes/techniques arealso included under biotechnology. For example, in vitrofertilisation leading to a ‘test-tube' baby, synthesising agene and using it, developing a DNA vaccine or correctinga defective gene, are all part of biotechnology.
The European Federation of Biotechnology (EFB) has given a definition of biotechnology that encompasses bothtraditional view and modern molecular biotechnology.
The definition given by EFB is as follows: ‘The integration of natural science and organisms, cells, parts thereof, and molecular analogues for productsand services'.
11.1 PRINCIPLES OF BIOTECHNOLOGY Among many, the two core techniques that enabled birthof modern biotechnology are : Genetic engineering : Techniques to alter thechemistry of genetic material (DNA and RNA), to introduce these into host organisms and thus change thephenotype of the host organism.
Maintenance of sterile (microbial contamination-free) ambiencein chemical engineering processes to enable growth of only thedesired microbe/eukaryotic cell in large quantities for themanufacture of biotechnological products like antibiotics,vaccines, enzymes, etc.
Let us now understand the conceptual development of the principles of genetic engineering.
You probably appreciate the advantages of sexual reproduction over asexual reproduction. The former provides opportunities for variationsand formulation of unique combinations of genetic setup, some of whichmay be beneficial to the organism as well as the population. Asexualreproduction preserves the genetic information, while sexual reproductionpermits variation. Traditional hybridisation procedures used in plant andanimal breeding, very often lead to inclusion and multiplication ofundesirable genes along with the desired genes. The techniques of geneticengineering which include creation of recombinant DNA, use ofgene cloning and gene transfer, overcome this limitation and allows usto isolate and introduce only one or a set of desirable genes withoutintroducing undesirable genes into the target organism.
Do you know the likely fate of a piece of DNA, which is somehow transferred into an alien organism? Most likely, this piece of DNA wouldnot be able to multiply itself in the progeny cells of the organism. But,when it gets integrated into the genome of the recipient, it may multiplyand be inherited along with the host DNA. This is because the alien pieceof DNA has become part of a chromosome, which has the ability toreplicate. In a chromosome there is a specific DNA sequence called theorigin of replication, which is responsible for initiating replication.
Therefore, for the multiplication of any alien piece of DNA in an organismit needs to be a part of a chromosome(s) which has a specific sequenceknown as ‘origin of replication'. Thus, an alien DNA is linked with theorigin of replication, so that, this alien piece of DNA can replicate andmultiply itself in the host organism. This can also be called as cloning ormaking multiple identical copies of any template DNA.
Let us now focus on the first instance of the construction of an artificial recombinant DNA molecule. The construction of the first recombinantDNA emerged from the possibility of linking a gene encoding antibiotic resistance with a native plasmid (autonomously replicating circularextra-chromosomal DNA) of Salmonella typhimurium. Stanley Cohen andHerbert Boyer accomplished this in 1972 by isolating the antibioticresistance gene by cutting out a piece of DNA from a plasmid which wasresponsible for conferring antibiotic resistance. The cutting of DNA atspecific locations became possible with the discovery of the so-called BIOTECHNOLOGY : PRINCIPLES AND PROCESSES
‘molecular scissors'– restriction enzymes. The cut piece of DNA wasthen linked with the plasmid DNA. These plasmid DNA act as vectors totransfer the piece of DNA attached to it. You probably know that mosquitoacts as an insect vector to transfer the malarial parasite into human body.
In the same way, a plasmid can be used as vector to deliver an alien pieceof DNA into the host organism. The linking of antibiotic resistance genewith the plasmid vector became possible with the enzyme DNA ligase,which acts on cut DNA molecules and joins their ends. This makes a newcombination of circular autonomously replicating DNA created in vitroand is known as recombinant DNA. When this DNA is transferred intoEscherichia coli, a bacterium closely related to Salmonella, it couldreplicate using the new host's DNA polymerase enzyme and make multiplecopies. The ability to multiply copies of antibiotic resistance gene inE. coli was called cloning of antibiotic resistance gene in E. coli.
You can hence infer that there are three basic steps in genetically modifying an organism — (i) identification of DNA with desirable genes; (ii) introduction of the identified DNA into the host; (iii) maintenance of introduced DNA in the host and transfer of the DNA to its progeny.
11.2 TOOLS OF RECOMBINANT DNA TECHNOLOGY Now we know from the foregoing discussion that genetic engineering orrecombinant DNA technology can be accomplished only if we have thekey tools, i.e., restriction enzymes, polymerase enzymes, ligases, vectorsand the host organism. Let us try to understand some of these in detail.
11.2.1 Restriction Enzymes In the year 1963, the two enzymes responsible for restricting the growthof bacteriophage in Escherichia coli were isolated. One of these addedmethyl groups to DNA, while the other cut DNA. The later was calledrestriction endonuclease. The first restriction endonuclease–Hind II, whose functioning depended on a specific DNA nucleotide sequence was isolated andcharacterised five years later. It was found that Hind II always cut DNAmolecules at a particular point by recognising a specific sequence ofsix base pairs. This specific base sequence is known as therecognition sequence for Hind II. Besides Hind II, today we know more than 900 restriction enzymes that have been isolated from over 230 strainsof bacteria each of which recognise different recognition sequences.
The convention for naming these enzymes is the first letter of the name comes from the genus and the second two letters come from the species ofthe prokaryotic cell from which they were isolated, e.g., EcoRI comes fromEscherichia coli RY 13. In EcoRI, the letter ‘R' is derived from the name of strain. Roman numbers following the names indicate the order in whichthe enzymes were isolated from that strain of bacteria.
Restriction enzymes belong to a larger class of enzymes called nucleases. These are of two kinds; exonucleases and endonucleases.
Exonucleases remove nucleotides from the ends of the DNA whereas,endonucleases make cuts at specific positions within the DNA.
Each restriction endonuclease functions by ‘inspecting' the length of a DNA sequence. Once it finds its specific recognition sequence, itwill bind to the DNA and cut each of the two strands of the doublehelix at specific points in their sugar -phosphate backbones(Figure 11.1). Each restriction endonuclease recognises a specificpalindromic nucleotide sequences in the DNA.
Figure 11.1 Steps in formation of recombinant DNA by action of restriction endonuclease Do you know what palindromes are? These are groups of letters that form the same words when read both forward and backward,e.g., "MALAYALAM". As against a word-palindrome where the sameword is read in both directions, the palindrome in DNA is a sequenceof base pairs that reads same on the two strands when orientation of BIOTECHNOLOGY : PRINCIPLES AND PROCESSES
reading is kept the same. For example, the following sequences readsthe same on the two strands in 5' à 3' direction. This is also true ifread in the 3' à 5' direction.
5' —— GAATTC —— 3' 3' —— CTTAAG —— 5' Restriction enzymes cut the strand of DNA a little away from the centre of the palindrome sites, but between the same two bases on the oppositestrands. This leaves single stranded portions at the ends. There areoverhanging stretches called sticky ends on each strand (Figure 11.1).
These are named so because they form hydrogen bonds with theircomplementary cut counterparts. This stickiness of the ends facilitatesthe action of the enzyme DNA ligase.
Restriction endonucleases are used in genetic engineering to form ‘recombinant' molecules of DNA, which are composed of DNA fromdifferent sources/genomes.
When cut by the same restriction enzyme, the resultant DNA fragments have the same kind of ‘sticky-ends' and, these can be joined together(end-to-end) using DNA ligases (Figure 11.2).
Figure 11.2 Diagrammatic representation of recombinant DNA technology You may have realised that normally, unless one cuts the vector and the source DNA with the same restriction enzyme, the recombinant vectormolecule cannot be created.
Separation and isolation of DNA fragments : The cutting of DNA byrestriction endonucleases results in the fragments of DNA. These fragmentscan be separated by a technique known as gel electrophoresis. SinceDNA fragments are negatively charged molecules they can be separatedby forcing them to move towards the anode under an electric field througha medium/matrix. Nowadays the most commonly used matrix is agarosewhich is a natural polymer extracted from sea weeds. The DNA fragmentsseparate (resolve) according to their size through sieving effect providedby the agarose gel. Hence, the smaller the fragment size, the farther itmoves. Look at the Figure 11.3 and guess at which end of the gel thesample was loaded. The separated DNA fragments can bevisualised only after staining the DNAwith a compound known as ethidiumbromide followed by exposure to UVradiation (you cannot see pure DNAfragments in the visible light andwithout staining). You can see brightorange coloured bands of DNA in aethidium bromide stained gelexposed to UV light (Figure 11.3). Theseparated bands of DNA are cut outfrom the agarose gel and extracted Figure 11.3 A typical agarose gel electrophoresis showing from the gel piece. This step is known migration of undigested as elution. The DNA fragments (lane 1) and digested set of purified in this way are used in DNA fragments (lane 2 to 4) constructing recombinant DNA byjoining them with cloning vectors.
11.2.2 Cloning Vectors You know that plasmids and bacteriophages have the ability to replicatewithin bacterial cells independent of the control of chromosomal DNA.
Bacteriophages because of their high number per cell, have very highcopy numbers of their genome within the bacterial cells. Some plasmidsmay have only one or two copies per cell whereas others may have 15-100 copies per cell. Their numbers can go even higher. If we are ableto link an alien piece of DNA with bacteriophage or plasmid DNA, we canmultiply its numbers equal to the copy number of the plasmid orbacteriophage. Vectors used at present, are engineered in such a waythat they help easy linking of foreign DNA and selection of recombinantsfrom non-recombinants.
BIOTECHNOLOGY : PRINCIPLES AND PROCESSES
The following are the features that are required to facilitate cloning into a vector.
(i) Origin of replication (ori) : This is a sequence from where replication starts and any piece of DNA when linked to this sequencecan be made to replicate within the host cells. This sequence is alsoresponsible for controlling the copy number of the linked DNA. So,if one wants to recover many copies of the target DNA it should becloned in a vector whose origin support high copy number.
(ii) Selectable marker : In addition to ‘ori', the vector requires a selectable marker, which helps in identifying and eliminating non-transformants and selectively permitting the growth of thetransformants. Transformation is a procedure through which apiece of DNA is introduced in a host bacterium (you will study theprocess in subsequent section). Normally, the genes encodingresistance to antibiotics such as ampicillin, chloramphenicol,tetracycline or kanamycin, etc., are considered useful selectablemarkers for E. coli. The normal E. coli cells do not carry resistanceagainst any of these antibiotics.
(iii) Cloning sites: In order to link the alien DNA, the vector needs to havevery few, preferably single,recognition sites for the commonlyused restriction enzymes. Presence ofmore than one recognition sites withinthe vector will generate severalfragments, which will complicate thegene cloning (Figure 11.4). Theligation of alien DNA is carried out ata restriction site present in one of thetwo antibiotic resistance genes. Forexample, you can ligate a foreign DNA Figure 11.4 E. coli cloning vector pBR322 at the BamH I site of tetracycline showing restriction sites resistance gene in the vector pBR322.
(Hind III, EcoR I, BamH I, Sal I, The recombinant plasmids will lose Pvu II, Pst I, Cla I), ori and tetracycline resistance due to insertion antibiotic resistance genes of foreign DNA but can still be selected (ampR and tetR). rop codes forthe proteins involved in the out from non-recombinant ones by replication of the plasmid.
plating the transformants on ampicillincontaining medium. The transformants growing on ampicillin containing medium are then transferred on a medium containingtetracycline. The recombinants will grow in ampicillin containingmedium but not on that containing tetracycline. But, non-recombinants will grow on the medium containing both theantibiotics. In this case, one antibiotic resistance gene helps inselecting the transformants, whereas the other antibiotic resistance gene gets ‘inactivated due to insertion' of alien DNA, and helps inselection of recombinants.
Selection of recombinants due to inactivation of antibiotics is a cumbersome procedure because it requires simultaneous platingon two plates having different antibiotics. Therefore, alternativeselectable markers have been developed which differentiaterecombinants from non-recombinants on the basis of their abilityto produce colour in the presence of a chromogenic substrate. Inthis, a recombinant DNA is inserted within the coding sequence ofan enzyme, β-galactosidase. This results into inactivation of theenzyme, which is referred to as insertional inactivation. Thepresence of a chromogenic substrate gives blue coloured colonies ifthe plasmid in the bacteria does not have an insert. Presence ofinsert results into insertional inactivation of the â-galactosidase andthe colonies do not produce any colour, these are identified asrecombinant colonies.
(iv) Vectors for cloning genes in plants and animals : You may be surprised to know that we have learnt the lesson of transferring genesinto plants and animals from bacteria and viruses which have knownthis for ages – how to deliver genes to transform eukaryotic cells andforce them to do what the bacteria or viruses want. For example,Agrobacterium tumifaciens, a pathogen of several dicot plants is ableto deliver a piece of DNA known as ‘ T-DNA' to transform normalplant cells into a tumor and direct these tumor cells to produce thechemicals required by the pathogen. Similarly, retroviruses in animalshave the ability to transform normal cells into cancerous cells. Abetter understanding of the art of delivering genes by pathogens intheir eukaryotic hosts has generated knowledge to transform thesetools of pathogens into useful vectors for delivering genes of interestto humans. The tumor inducing (Ti) plasmid of Agrobacteriumtumifaciens has now been modified into a cloning vector which is nomore pathogenic to the plants but is still able to use the mechanismsto deliver genes of our interest into a variety of plants. Similarly,retroviruses have also been disarmed and are now used to deliverdesirable genes into animal cells. So, once a gene or a DNA fragmenthas been ligated into a suitable vector it is transferred into a bacterial,plant or animal host (where it multiplies).
11.2.3 Competent Host (For Transformation with Since DNA is a hydrophilic molecule, it cannot pass through cellmembranes. Why? In order to force bacteria to take up the plasmid, thebacterial cells must first be made ‘competent' to take up DNA. This isdone by treating them with a specific concentration of a divalent cation,such as calcium, which increases the efficiency with which DNA enters BIOTECHNOLOGY : PRINCIPLES AND PROCESSES
the bacterium through pores in its cell wall. Recombinant DNA can thenbe forced into such cells by incubating the cells with recombinant DNAon ice, followed by placing them briefly at 420C (heat shock), and thenputting them back on ice. This enables the bacteria to take up therecombinant DNA.
This is not the only way to introduce alien DNA into host cells. In a method known as micro-injection, recombinant DNA is directly injectedinto the nucleus of an animal cell. In another method, suitable for plants,cells are bombarded with high velocity micro-particles of gold or tungstencoated with DNA in a method known as biolistics or gene gun. And thelast method uses ‘disarmed pathogen' vectors, which when allowed toinfect the cell, transfer the recombinant DNA into the host.
Now that we have learnt about the tools for constructing recombinant DNA, let us discuss the processes facilitating recombinant DNA technology.
11.3 PROCESSES OF RECOMBINANT DNA TECHNOLOGY Recombinant DNA technology involves several steps in specificsequence such as isolation of DNA, fragmentation of DNA byrestriction endonucleases, isolation of a desired DNA fragment,ligation of the DNA fragment into a vector, transferring therecombinant DNA into the host, culturing the host cells in amedium at large scale and extraction of the desired product.
Let us examine each of these steps in some details.
11.3.1 Isolation of the Genetic Material (DNA) Recall that nucleic acid is the genetic material of all organisms without exception. In majority of organisms this is deoxyribonucleic acid or DNA. In order to cut the DNA with restriction enzymes, it needs to be in pure form, free from other macro-molecules. Since the DNA is enclosed within the membranes, we have to break the cell open to release DNA along with other macromolecules such as RNA, proteins, polysaccharides and also lipids. This can be achieved by treating the bacterial cells/plant or animal tissue with enzymes such as Figure 11.5 DNA that separates out can be lysozyme (bacteria), cellulase (plant cells), chitinase (fungus).
removed by spooling You know that genes are located on long molecules of DNA interwined with proteins such as histones. The RNA can be removed by treatment with ribonuclease whereas proteins can be removed by treatment with protease. Other molecules can be removed by appropriate treatments and purified DNA ultimately precipitates out after the addition of chilled ethanol. This can be seen as collection of fine threads in the suspension (Figure 11.5).
11.3.2 Cutting of DNA at Specific Locations Restriction enzyme digestions are performed by incubating purified DNAmolecules with the restriction enzyme, at the optimal conditions for thatspecific enzyme. Agarose gel electrophoresis is employed to check theprogression of a restriction enzyme digestion. DNA is a negatively chargedmolecule, hence it moves towards the positive electrode (anode)(Figure 11.3). The process is repeated with the vector DNA also.
The joining of DNA involves several processes. After having cut the source DNA as well as the vector DNA with a specific restriction enzyme,the cut out ‘gene of interest' from the source DNA and the cut vector withspace are mixed and ligase is added. This results in the preparation ofrecombinant DNA.
11.3.3 Amplification of Gene of Interest using PCR PCR stands for Polymerase Chain Reaction. In this reaction, multiplecopies of the gene (or DNA) of interest is synthesised in vitro using two Polymerase chain reaction (PCR) : Each cycle has three steps: (i) Denaturation;(ii) Primer annealing; and (iii) Extension of primers BIOTECHNOLOGY : PRINCIPLES AND PROCESSES
sets of primers (small chemically synthesised oligonucleotides that arecomplementary to the regions of DNA) and the enzyme DNA polymerase.
The enzyme extends the primers using the nucleotides provided in thereaction and the genomic DNA as template. If the process of replicationof DNA is repeated many times, the segment of DNA can be amplifiedto approximately billion times, i.e., 1 billion copies are made. Suchrepeated amplification is achieved by the use of a thermostable DNApolymerase (isolated from a bacterium, Thermus aquaticus), whichremain active during the high temperature induced denaturation ofdouble stranded DNA. The amplified fragment if desired can now beused to ligate with a vector for further cloning (Figure11.6).
11.3.4 Insertion of Recombinant DNA into the Host There are several methods of introducing the ligated DNA into recipientcells. Recipient cells after making them ‘competent' to receive, take upDNA present in its surrounding. So, if a recombinant DNA bearing genefor resistance to an antibiotic (e.g., ampicillin) is transferred into E. colicells, the host cells become transformed into ampicillin-resistant cells. Ifwe spread the transformed cells on agar plates containing ampicillin, onlytransformants will grow, untransformed recipient cells will die. Since, dueto ampicillin resistance gene, one is able to select a transformed cell in thepresence of ampicillin. The ampicillin resistance gene in this case is calleda selectable marker.
11.3.5 Obtaining the Foreign Gene Product When you insert a piece of alien DNA into a cloning vector and transfer itinto a bacterial, plant or animal cell, the alien DNA gets multiplied. Inalmost all recombinant technologies, the ultimate aim is to produce adesirable protein. Hence, there is a need for the recombinant DNA to beexpressed. The foreign gene gets expressed under appropriate conditions.
The expression of foreign genes in host cells involve understanding manytechnical details.
After having cloned the gene of interest and having optimised the conditions to induce the expression of the target protein, one has toconsider producing it on a large scale. Can you think of any reasonwhy there is a need for large-scale production? If any protein encodinggene is expressed in a heterologous host, it is called a recombinantprotein. The cells harbouring cloned genes of interest may be grown on a small scale in the laboratory. The cultures may be used forextracting the desired protein and then purifying it by using differentseparation techniques.
The cells can also be multiplied in a continuous culture system wherein the used medium is drained out from one side while fresh medium isadded from the other to maintain the cells in their physiologically most active log/exponential phase. This type of culturing method produces alarger biomass leading to higher yields of desired protein.
Small volume cultures cannot yield appreciable quantities of products.
To produce in large quantities, the development of bioreactors, wherelarge volumes (100-1000 litres) of culture can be processed, was required.
Thus, bioreactors can be thought of as vessels in which raw materials arebiologically converted into specific products, individual enzymes, etc.,using microbial plant, animal or human cells. A bioreactor provides theoptimal conditions for achieving the desired product by providingoptimum growth conditions (temperature, pH, substrate, salts, vitamins,oxygen).
The most commonly used bioreactors are of stirring type, which are shown in Figure 11.7.
(a) Simple stirred-tank bioreactor; (b) Sparged stirred-tank bioreactor through whichsterile air bubbles are sparged A stirred-tank reactor is usually cylindrical or with a curved base to facilitate the mixing of the reactor contents. The stirrer facilitates evenmixing and oxygen availability throughout the bioreactor. Alternativelyair can be bubbled through the reactor. If you look at the figure closelyyou will see that the bioreactor has an agitator system, an oxygen deliverysystem and a foam control system, a temperature control system, pH control system and sampling ports so that small volumes of the culturecan be withdrawn periodically.
11.3.6 Downstream Processing After completion of the biosynthetic stage, the product has to be subjectedthrough a series of processes before it is ready for marketing as a finished BIOTECHNOLOGY : PRINCIPLES AND PROCESSES
product. The processes include separation and purification, which arecollectively referred to as downstream processing. The product has to beformulated with suitable preservatives. Such formulation has to undergothorough clinical trials as in case of drugs. Strict quality control testingfor each product is also required. The downstream processing and qualitycontrol testing vary from product to product.
Biotechnology deals with large scale production and marketing ofproducts and processes using live organisms, cells or enzymes.
Modern biotechnology using genetically modified organisms wasmade possible only when man learnt to alter the chemistry of DNAand construct recombinant DNA. This key process is calledrecombinant DNA technology or genetic engineering. This processinvolves the use of restriction endonucleases, DNA ligase,appropriate plasmid or viral vectors to isolate and ferry the foreignDNA into host organisms, expression of the foreign gene, purificationof the gene product, i.e., the functional protein and finally making asuitable formulation for marketing. Large scale production involvesuse of bioreactors.
Can you list 10 recombinant proteins which are used in medical practice? Find out where they are used as therapeutics (use the internet).
Make a chart (with diagrammatic representation) showing a restrictionenzyme, the substrate DNA on which it acts, the site at which it cutsDNA and the product it produces.
From what you have learnt, can you tell whether enzymes are bigger orDNA is bigger in molecular size? How did you know? What would be the molar concentration of human DNA in a humancell? Consult your teacher.
Do eukaryotic cells have restriction endonucleases? Justify your answer.
Besides better aeration and mixing properties, what other advantagesdo stirred tank bioreactors have over shake flasks? Collect 5 examples of palindromic DNA sequences by consulting your teacher.
Better try to create a palindromic sequence by following base-pair rules.
Can you recall meiosis and indicate at what stage a recombinant DNAis made? Can you think and answer how a reporter enzyme can be used to monitortransformation of host cells by foreign DNA in addition to a selectablemarker? 10. Describe briefly the following: (a) Origin of replication(b) Bioreactors(c) Downstream processing 11. Explain briefly (a) PCR(b) Restriction enzymes and DNA(c) Chitinase 12. Discuss with your teacher and find out how to distinguish between (a) Plasmid DNA and Chromosomal DNA(b) RNA and DNA(c) Exonuclease and Endonuclease
Lessons from Private Equity How to increase the value of private companies in B.C. Vancouver Private Company Services2011 Edition "Someone's sitting in the shade today because someone planted a tree a long time ago." Private Equity background Lessons from BC's top private equity firms Concluding remarks Appendix A – Participant companies Despite a slow economic recovery over the last couple
Chapter 14 veterinary aspects
COMMISSIONED PAPER (UK) This paper was commissioned by FECAVA for the Special issue of EJCAP, Genetic/Hereditary Disease and Breeding. Must not be copied without permission © 2014 Chiari–like malformation and syringomyelia Clare Rusbridge Introduction Syringomyelia is a condition characterised by fluid filed cavities (syrinxes or syringes) within the central spinal