When cutting DNA, scientist uses enzymes called restriction enzymes. These enzymes are naturally produced in bacteria as a defense against phages. So, enzymes cut DNA of the phages and thus inactivate it. Also, these enzymes do not cat DNA randomly, but on specific sites. This characteristic of restriction enzymes is used for some molecular biology techniques. All species' DNA contains target sites of restriction enzymes, but most organisms do not have restriction enzymes. Therefore, restriction enzymes are suitable for different techniques.
Horses have 64 chromosomes and donkeys 62, so when the two breed their mule offspring ends up with 63. Because this is an odd number, it's impossible for them to divide evenly into two. That means the mule cannot produce sperm and egg cells that carry exactly half the animal's chromosomes, as should happen.Aug 28, 2018
