The BEST explanation of the role of DNA and RNA in making proteins is:
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DNA unwinds into two separate strands, and a complementary copy of each is produced during replication. During transcription, a DNA strand is used to produce a complementary strand of mRNA, with uracil taking the place of thymine. During translation, each set of three bases in the mRNA results in an amino acid being added to a chain, and a protein is formed.
This option accurately describes the processes of replication, transcription, and translation, and correctly states that during transcription uracil replaces thymine. It also correctly notes that during translation, sets of three bases (codons) in the mRNA code for specific amino acids.