Are there any instances where DNA evidence should not be allowed?

In modern times, people can modify their DNA so therefore it should not be allowed. If the accused has not modified their DNA, then DNA evidence sould always be allowed.

How reliable is DNA evidence in court or what is the level of “certainty” does DNA evidence provide?

In courts, DNA is very reliable because no two humans have the same DNA. With this being said, if one set of DNA is found, then only that person with that matching DNA was there making the 'level of certainty' very high.

How is DNA obtained, studied, and used in courts?

DNA samples are found at the crime scene. At the crime scene, detectives and DNA analysts look for hair, saliva, blood, semen, skin, sweat, mucus or earwax which contains enough DNA to prosecute someone. Then, from these samples, cells are taken and studied in high technology labs. For court purposes, the DNA is matched to the suspects and if there is a match, then it is known that they were at the crime scene.