Problems with Uracil
Uracil is not perfect, there are some problems that Uracil does endeavor. One problem that Uracil faces is that it is very similar to thymine. When DNA is being synthesized, some of the enzymes that are being used to catalyze the reaction, can not tell apart thymine and Uracil. That is because, thymine only has one bond different than Uracil, making their structure look very much similar. The enzymes can not tell weather the molecule is thymine or Uracil, so they can not "fix" the problem if Uracil is bonding with adenine, because the enzymes see this as a regular bond. If this problem does occur, copious amounts of Uracil can enter the DNA, but the DNA will not be able to repaired because of the large amounts of Uracil. This could possible cause the DNA to destroy itself and start the process of synthesis all over again, this time being careful of the differences in thymine and Uracil
Another problem with Uracil is called spontaneous deamination. Spontaneous deamination is when cytosine (one of the four main bases in DNA) will randomly and quick turn into the base Uracil. This change can cause great damage to the DNA because it causes a mutation where guanine bonds to adenine. This problem occurs in the DNA's replication stage, because now with the mutation, the DNA code is being misread, and another message is being given out.