Q.What is the significance of adding proteases at the time of isolation of genetic material (DNA).

When you're isolating DNA from a cell, you're not just dealing with DNA — the cell is packed with proteins, including histones that DNA is tightly wound around, and various enzymes that could degrade your sample. Proteases are added specifically to break down these contaminating proteins.

The key reason is that proteases digest the histone proteins and other DNA-binding proteins, freeing the DNA from its protein coat. Without this step, the DNA would remain bound to proteins and would not be pure. Additionally, proteases inactivate any nucleases (enzymes that would chop up DNA) by digesting them, thereby protecting the integrity of the genetic material you're trying to isolate. …

When you set out to isolate DNA from a cell, you are not simply scooping out a clean, naked molecule. Inside the nucleus, DNA is wound around proteins called histones — these form the nucleosome structure, and further scaffolding proteins help fold the DNA into chromosomes. If you try to extract DNA without dealing with these proteins, you end up with a sticky, viscous mess where the DNA is still entangled with protein debris. The DNA will not dissolve properly, it will not precipitate cleanly, and it will be useless for downstream applications like restriction digestion or PCR.

This is where proteases enter the picture. Proteases are enzymes that break down proteins by cleaving the peptide bonds between amino acids. In the context of DNA isolation, you add a protease — most commonly proteinase K — to the cell lysate. This enzyme chews up the histones and other chromosomal proteins into small peptides and amino acids. Once the proteins are degraded, the DNA is released from its protein cage and floats freely in the solution.

The practical significance is twofold. First, removing proteins prevents them from interfering with the subsequent purification steps — for example, during phenol-chloroform extraction, proteins get denatured and partition into the organic phase, but if they are still bound to DNA, they can drag DNA into that phase as well, causing loss of material. Second, and more critically, pure DNA is essential for accurate quantification and for enzymatic reactions. If you try to cut DNA with a restriction enzyme while histones are still attached, the enzyme cannot access its recognition sequence — the protein coat blocks the way. Similarly, PCR polymerases will stall or give nonspecific products if protein contamination is high. …

Method 2 -- trace what fails downstream if the protease step is skipped

Rather than describing what proteases do, work through the isolation protocol and ask what would go wrong at each later step if the protease treatment were left out:

1. Ethanol precipitation: leftover protein co-precipitates with the DNA, giving a sticky, impure pellet instead of clean DNA.
2. Restriction digestion: bound protein can physically block a restriction enzyme's access to its recognition site, so the enzyme fails to cut, or cuts inefficiently/at the wrong places.
3. PCR / quantification: residual protein interferes with polymerase activity and with accurate measurement of DNA concentration (for example, on a spectrophotometer or gel). …