In RNA thymine from DNA is replaced by what?

In RNA, thymine from DNA is replaced by uracil. During transcription, RNA polymerase builds an RNA strand complementary to a DNA template, using uracil (U) wherever the corresponding DNA pairing calls for thymine (T). Uracil then pairs with adenine through hydrogen bonds.

Uracil and thymine are closely related pyrimidine bases. Thymine is essentially uracil with an added methyl group, and DNA uses thymine partly because it helps cells detect damage caused when cytosine spontaneously changes into uracil. DNA-repair enzymes can recognize an unexpected uracil and remove it; using uracil normally in DNA would make that error harder to spot.

RNA generally uses uracil because it is chemically sufficient for short-lived transcripts and avoids the extra methyl group. A common mix-up is saying that RNA contains no thymine at all: standard RNA uses uracil, but unusual modified RNAs can contain thymine-like bases. Uracil is also part of uridine, the nucleoside formed when it attaches to ribose.