Again, notice the base-pairing between the template strand of DNA and the newly forming RNA. Figure 3: During elongation, RNA polymerase tracks along the DNA template, synthesizes mRNA in the 5′ to 3′ direction, and unwinds then rewinds the DNA as it is read. This means that the base-pairing rules between a DNA molecule and an RNA molecule are: DNAĪn enzyme called RNA polymerase proceeds along the DNA template adding nucleotides by base pairing with the DNA template in a manner similar to DNA replication. The mRNA product is complementary to the template strand and is almost identical to the other DNA strand, called the non-template strand, with the exception that RNA contains a uracil (U) in place of the thymine (T) found in DNA. Transcription always proceeds from one of the two DNA strands, which is called the template strand. Note the base-pairing between the RNA transcript and the template strand of DNA. Enzymes and other proteins involved in transcription bind at the promoter. Figure 2: The initiation of transcription begins when DNA is unwound, forming a transcription bubble. The specific sequence of a promoter is very important because it determines whether the corresponding gene is transcribed all of the time, some of the time, or hardly at all. In most cases, promoters exist upstream of the genes they regulate. The DNA sequence onto which the proteins and enzymes involved in transcription bind to initiate the process is called a promoter. Transcription requires the DNA double helix to partially unwind in the region of mRNA synthesis. The prokaryotes, which include bacteria and archaea, lack membrane-bound nuclei and other organelles, and transcription occurs in the cytoplasm of the cell. With the genes bound in the nucleus, transcription occurs in the nucleus of the cell and the mRNA transcript must be transported to the cytoplasm. Both prokaryotes and eukaryotes perform fundamentally the same process of transcription, with the important difference of the membrane-bound nucleus in eukaryotes.
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