![]() ![]() RNA polymerase III is also located in the nucleus. RNA polymerase II is located in the nucleus and synthesizes all protein-coding nuclear pre-mRNAs. ![]() RNA polymerase I is located in the nucleolus, a specialized nuclear substructure in which ribosomal RNA (rRNA) is transcribed, processed, and assembled into ribosomes. Several additional transcription factors and RNA polymerase combine around the TATA box to form the pre-initiation complex.Īs mentioned earlier, there are three RNA polymerases in eukaryotes. The TATA box is the binding site for a transcription factor called TATA-binding protein (TBP). The most well-studied promoter element in eukaryotes is a short DNA sequence known as a TATA box, found 25-30 base pairs upstream from the start site of transcription. The completed assembly of transcription factors and RNA polymerase bind to the promoter, forming a transcription pre-initiation complex (PIC). Unlike the prokaryotic RNA polymerase that can bind to a DNA template on its own, eukaryotes require several other proteins, called transcription factors, to first bind to the promoter region and then recruit the appropriate polymerase. At the -10 and -35 regions upstream of the initiation site (that is, 10 and 35 nucleotides upstream of the site), there are two promoter consensus sequences. Although promoters vary among prokaryotic genomes, a few elements are conserved. ![]() The specific sequence of a promoter is very important because it determines whether the corresponding gene is transcribed all the time, some of the time, or infrequently. Prokaryotic transcription initiation begins when the transcription machinery binds to the promoter region of a DNA sequence. You can get a sense of where the promoter is located relative to the gene downstream of it in the image below: Each gene (or, in bacteria, each group of genes transcribed together) has its own promoter. To begin transcribing a gene, RNA polymerase binds to the DNA of the gene at a region called the promoter, which points out on the DNA where the polymerase should begin transcribing. Take a look at a diagram of an RNA polymerase in action below: Each one specializes in transcribing certain classes of genes. Humans and other eukaryotes have three different kinds of RNA polymerase: I, II, and III. RNA polymerases are large enzymes with multiple subunits, even in simple organisms like bacteria. ![]() For instance, if there is a G nucleotide in the DNA template, RNA polymerase will add a C nucleotide to the growing RNA strand. Using a DNA template, RNA polymerase builds a new RNA molecule through base pairing. Transcription is performed by enzymes called RNA polymerases. Nucleotides that come after the initiation site are marked with positive numbers and are referred to as downstream. Nucleotides that come before the initiation site are given negative numbers and said to be upstream. The site on the DNA from which the first RNA nucleotide is transcribed is called the +1 site, or the initiation site. However, there is one important difference: in the newly made RNA, all of the T (thymine) nucleotides are replaced with U (uracil) nucleotides. The RNA product is complementary to the template strand and is almost identical to the other DNA strand, called the nontemplate strand. Transcription uses one of the two exposed DNA strands as a template this strand is called the template strand. The region of opened-up DNA is called a transcription bubble. Before transcription can take place, the DNA double helix must unwind near the gene that is being transcribed. Both types of transcription involve initiation, elongation, and termination. In eukaryotic cells it takes place in the nucleus and in prokaryotic cells it takes place in the cytoplasm. Both in pro and eukaryotic cells the process takes place in different places. Prokaryotic transcription and eukaryotic transcription differ in a number of ways, which will be discussed below. ![]()
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