Isolation of DNA

Mucilaginous tissues can have high polysaccharides. These can interfere with DNA purification and PCR.

White blood cells contain nuclei, so their nuclear DNA can be isolated from a blood sample. Mature mammalian red blood cells lack nuclei, while platelets are cell fragments and are not a suitable source of genomic DNA. Salts in plasma are not cells. Exam tip: For genomic DNA isolation from blood, choose white blood cells.

Mammalian mature RBCs lack nucleus so they are not a genomic DNA source. WBCs are better source.

A cheek swab collects buccal epithelial cells from the inner lining of the cheek. These cells contain nuclei and therefore genomic DNA, making them the source material for DNA isolation. The watery part of saliva is mainly water; DNA is obtained chiefly from the cells present in the sample. Exam tip: For DNA isolation, first identify whether the sample contains nucleated cells.

Cells attached to the hair root may be living or recently living and can contain nuclear DNA. In contrast, the hair shaft is mainly made of keratinised dead cells, so it may contain very little or degraded nuclear DNA. Therefore, a hair with its root attached is more useful for DNA isolation. Exam tip: In forensic DNA questions, remember that the hair root can be a source of nuclear DNA.

Too much sample can make lysis and purification inefficient. Keep sample amount according to protocol.

If the starting sample is too small, the total amount of DNA obtained after extraction may also be low. Therefore, option A is correct. A low sample amount does not convert DNA into protein or produce ethanol automatically. Exam tip: In DNA isolation, a smaller starting sample generally affects DNA yield first.

The direct answer is A: incubation time helps lysis and enzyme action become sufficiently complete. During DNA extraction, a lysis buffer opens cells, and enzymes such as proteinase may digest proteins. These processes need suitable time, temperature, and mixing. If incubation is too short, cells may not break fully and proteins may remain, causing poor DNA yield or purity. Option A is correct because controlled incubation supports efficient cell disruption and protein digestion. Option B is wrong: incubation does not convert DNA into sugar; DNA is a nucleic acid and is not changed into sugar in this step. Option C is wrong: ethanol does not become agar; ethanol is used in some protocols to precipitate DNA, while agar or agarose is a separate gel material. Option D is wrong: colour is not the purpose of incubation, although a reagent may have colour incidentally. Excessive time or unsuitable temperature can also damage DNA or reduce quality, so the protocol must be followed. Memory cue: enough time helps “break cells and digest proteins,” but more time is not always better.

Direct answer: Option A, the risk of DNA degradation can increase. Lysis breaks open cells so that DNA is released. During a carefully controlled incubation, chemicals and enzymes help this process. However, if incubation continues for too long, especially at the wrong temperature, pH, or chemical concentration, DNA may be damaged or broken. The reasoning is: cells are opened; DNA becomes exposed; longer exposure to harsh conditions gives damaging enzymes or chemicals more time to act; therefore DNA quality may decrease. Option A is correct because it states this possible risk. Option B is wrong because longer treatment does not always improve DNA; it can reduce purity or integrity. Option C is wrong because lysis incubation does not automatically produce ethanol. Option D is wrong because proteins do not always disappear; many are removed or denatured by later steps. Follow the validated protocol time and conditions. Memory cue: too long is not always better; in DNA isolation, controlled timing protects DNA.

Some steps need cold conditions and some enzyme steps need specific temperature. Temperature affects DNA quality.

Keeping a sample on ice lowers the temperature and can slow the activity of nucleases. Since nucleases may break down DNA, cold conditions help reduce DNA degradation during isolation. DNA precipitation mainly depends on alcohol and salts, not simply on keeping the sample on ice. Exam tip: In DNA isolation, low temperature is used chiefly to protect DNA from enzyme-mediated degradation.

Boiling can release crude DNA but impurities may remain. Purification is needed for high-quality work.

Crude extract can contain DNA along with proteins salts and inhibitors. Cleanup may be needed before downstream use.

Pure DNA is better for downstream reactions like PCR and cloning. Purity ratios and gel check help.

In DNA isolation, yield means the quantity of DNA recovered from the original biological sample after cell lysis, removal of proteins and other contaminants, and purification. It is commonly estimated by measuring DNA concentration and multiplying it by the final elution volume: yield = concentration × volume. For example, 50 ng/µL in 40 µL gives 2,000 ng, or 2 µg, of DNA. Yield is not the same as purity or integrity. A sample may have high yield but contain contaminants, or have pure DNA that is fragmented. Therefore, the amount obtained, represented by option D, is the correct meaning; colour, smell, and agar thickness do not measure DNA recovery.

Purity tells how low impurities like protein salts phenol or RNA are in the sample. It is important for downstream reactions.

DNA integrity means the physical completeness and quality of a DNA sample. DNA with good integrity is largely intact, with long molecules and little fragmentation. DNA with poor integrity is broken into many smaller pieces because of physical shearing, nucleases, heat, or unsuitable handling. Therefore, option B correctly describes integrity as how intact or broken the DNA is. Taste, ethanol quantity, and tube colour do not define the condition of the DNA molecule.

Integrity can be examined by methods such as gel electrophoresis. A mostly intact genomic DNA sample commonly appears as a high-molecular-weight band or compact region, whereas degraded DNA produces a smear toward smaller sizes. The test does not measure the taste of DNA or the colour of a tube. It gives information about molecular fragmentation and preservation. Thus the supplied answer B follows directly from the biological meaning of DNA integrity and is correctly explained.

The direct answer is option C: DNA may be unusable for some downstream uses. DNA quality has more than one part: yield means how much DNA is present, purity means how free it is from contaminants, and integrity means whether the long DNA molecules are intact. A sample can have a large measured amount but still be broken into many small fragments. Step by step, degradation breaks the DNA backbone; fragments may then be too short for restriction mapping, long-range PCR, library preparation, cloning or other applications needing intact molecules. Option A is wrong because degraded DNA is not always best; many experiments need long, complete DNA. Option B is wrong because yield alone cannot describe quality. Option C is correct because fragmentation can make the sample unsuitable even when its concentration is high. Option D is wrong because DNA degradation does not mean that protein will disappear; protein contamination and DNA integrity are separate issues. Exam cue: remember “quantity is how much, integrity is how whole.” Good DNA needs both sufficient amount and suitable intactness.

The direct answer is option D: use aseptic handling and fresh tips. Contamination means unwanted DNA, microbes, chemicals or material from another sample entering the experiment. Prevention is better than trying to remove contamination later. Step by step: clean the work area; label tubes before starting; use sterile or clean disposable tips; change the tip whenever moving to another reagent or sample; keep tubes closed except when adding something; and avoid touching the tip to surfaces. Option A is wrong because leaving tubes open allows dust, droplets and microbes to enter. Option B is wrong because reusing a tip can carry material from one tube into another and cause cross-contamination. Option C is wrong because removing labels creates identification errors and can mix up samples; it does not prevent contamination. Option D is correct because aseptic technique and fresh tips reduce both environmental contamination and transfer between samples, giving more trustworthy DNA results. Remember: closed tubes, clean workspace, fresh tip, correct label. These simple habits protect the whole experiment.

Open tubes can bring airborne contaminants and nucleases. Short careful handling keeps samples safe.

Keeping pre-PCR and post-PCR areas separate can reduce contamination risk. This is useful in sensitive DNA work.

A DNA extraction blank control contains no sample DNA. Therefore, a positive result indicates that DNA has entered through contaminated reagents, equipment, or handling. A high DNA concentration in the sample cannot make the blank positive. Exam tip: a positive blank control usually points to contamination.

A positive extraction control contains a sample known to have DNA, so it should produce DNA after extraction. If this control fails, the likely problem is with the extraction procedure, such as an incorrect step, degraded reagents, or faulty reagents, rather than with the test sample. Option B is incorrect because DNA is already known to be present in the positive control. Exam tip: failure of a positive control indicates that the procedure should be checked before interpreting sample results.

An internal standard can indicate process performance. This makes comparison and troubleshooting easier.

Proteins, phenol, salts, and other contaminants in a DNA sample can inhibit enzyme-based reactions such as PCR and restriction digestion. Removing these inhibitors produces purer DNA and makes downstream tests more reliable. Inhibitor removal does not convert DNA into protein. Exam tip: DNA purity is essential before downstream steps such as PCR, sequencing, or restriction digestion.