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Dna Extraction

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Submitted By shaayini
Words 961
Pages 4
Date: 22/08/2013

Title: Extraction of genomic DNA

Introduction:
Isolation of genomic DNA is one of the most commonly used practices in several areas of science. Purified, high molecular weight DNA is used for many applications such as: * To prepare genomic DNA libraries * To isolate genes to be cloned * To carry out southern blotting and hybridization
Many different methods are available for isolation of genomic DNA, and they all involve the following basic stages. * Disruption and lysis of starting material: * Cell walls and cell membranes are broken to release DNA and other cellular components. * Removal of proteins and other contaminants: * Protein contaminations are removed by digestion with proteinase K followed by salting out and denaturing some proteins by SDS. * Recovery of DNA: * DNA is precipitated using ethanol or isopropanol in the presence of monovalent cations
All DNA isolation protocols have a common goal in isolating high molecular weight DNA, which meet the following three criteria: 1) Purity should be high enough for further downstream applications such as restriction digestion. 2) DNA should be intact to give accurate and reproducible migration patterns on gel electrophoresis. 3) Yield should be high enough to obtain a required quantity of DNA from a reasonable amount of tissue.

Materials and Methods:

Materials
Bench centrifuge, incubator, Falcon tubes, eppendorf tubes, micropipettes, micropipette tips gloves. DNA extraction
Frozen blood samples were thawn

30ml of lysis buffer was added, mixed and kept on crushed ice for 15mins

Centrifuged at 2000rpm for 10mins and supernatant decanted without disturbing the pellet

Pellet was washed with 10ml SE buffer, vortexing briefly and centrifuged as above.

Pellet washed for several times until clear whit pellet obtained

Pellet re-suspended in 5.0ml cold SE buffer

500µl 10% SDS added and mixed briefly

25µl Proteinase K (20mg/ml) added and mixed gently, incubate at 37°C for 3-4 hours.

2.0ml saturated NaCl added, vigorous shaking and centrifuged at 2500rpm for 15mins

Decant supernatant very carefully into sterile tube and 2 volumes absolute ethanol added,

Carefully inverted several times and DNA precipitates.

Dissolving DNA
Fluffy DNA precipitate picked out with sterile eppendorf pipette tip

Several washings done with DNA placed on open lid of a 1.5ml eppendorf tube with 70% ethanol in the tube

After washing, DNA dropped into 500µl of TE buffer in eppendorf and allowed to stand overnight at room temperature.

Quantification of DNA
Aliquots of DNA diluted with TE at 1:50 (30µl DNA + 1470µl TE) and then from 1:50 solution 1:100 (500µl DNA + 500µl TE) ratios.
Concentration, yield and purity of DNA were determined by measuring OD at 260nm, 280nm and 320nm.

Assessing quality of DNA

5µg of DNA run on a 0.8% agarose gel

Results: wavelengths | 260nm | 280nm | 320nm | Absorbance (1:50) | 0.277 | 0.151 | 0.007 | Absorbance (1:100) | 0.098 | 0.056 | 0.005 |

Sample loaded onto well no: 1

Calculations:
Expected amount of DNA from 10 cm3 blood sample
= (10000X cells/ mm3 X 10cm3X 3.3X109 bp/ cells X 1X10-9 pg/bp X 2)
= 660 µg
According to 1:50 dilution:
A260 = (0.277-0.007) A280 = (0.151-0.007) A260/ A280= 0.270/ 0.144 = 0.270 = 0.144 = 1.875
Original concentration of the sample = 0.270 X 50µg/ ml X 50 = 675 µg/ ml
Needed volume to load the gel with 5 µg DNA = (1000/ 675) X 5 µl = 7.4 µl

Total amount of DNA extracted in 500µl = 675 µg/ml X 0.5 ml = 337.5 µg
Yield = 337.5 µg
Discussion:
Some of the lanes on the gel (3,4 & 6) show very thick bands as they contained large quantity of DNA and other lanes show less quantity of DNA. A smear appearing in front of DNA bands indicate that DNA has been digested to fragments, due to physical or enzymatic factors. Wells 1,2 and 5 contain relatively intact DNA that has been extracted efficiently but in less quantity.
If the extracted DNA is pure, the OD260/OD280 value should be between 1.8–2.0. In our sample, 1.875 was obtained. So there are no protein contaminations. The OD320 value gives an idea about the background absorption of the solution. Hence, it should be deduced from the OD260 value, when calculating DNA concentration.
Expected yield of DNA for 10ml of blood is 660µg. But in this experiment, 337.5 µg was obtained. The yield might have reduced due to various experimental errors.
Blood consists of several cell types. Since, RBC and platelets are devoid of nuclei, WBC is the only source of DNA in blood. Blood is collected and mixed with anticoagulants such as EDTA, heparin sulfate and acid citrate dextrose (ACD) to prevent blood clotting.
Cells contain DNAses, which can cause degradation of DNA. Activity of DNAses can be ceased by, treating with EDTA which chelates Mg2+ ions that act as cofactors for DNAses, keeping cell suspension on ice or by addition of SDS to denature DNAses.
Protein contaminations are removed by treating with proteinase K, which is an endopeptidase. This enzyme is active even in the presence of SDS, EDTA and at elevated temperatures.
Since RNA is very unstable, it is not necessary to remove RNA contaminations because they can be degraded automatically. Denatured and digested proteins are removed by “salting out” in the presence of monovalent cations such as Na+.
References:
Genomic-DNA-Isolation-and-Applications. Retrieved on August 27, 2013.
Web site: http://www.scribd.com/doc/13890135/Genomic-DNA-Isolation-and-Applications

Absorbance-dna-quantitation. Retrieved on August 27, 2013.
Web site: http://www.bmglabtech.com/application-notes/absorbance/absorbance-dna-quantitation-168.cfm
DNA concentration purity. Retrieved on August 27, 2013.
Web site: http://bitesizebio.com/articles/dna-concentration-purity/

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