Summary

1. DNA is transcribed into RNA by the enzyme RNA polymerase. The three types of RNA are ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA). Uracil, adenine, cytosine, and guanine are the four bases in RNA.

2. In bacteria, RNA polymerase binds to the promoter sequence just upstream of the start site of transcription. The enzyme moves down the DNA template and synthesizes an RNA molecule. RNA synthesis stops once the enzyme has transcribed a terminator sequence.

3. Bacterial genes encoding proteins for the same metabolic pathway are often clustered into operons. Some operons are induced in the presence of the substrate of their pathway, for example, the lac operon. Others are repressed in the presence of the product of the pathway, for example, the trp operon.

4. Eukaryotic mRNAs are modified by the addition of a 7-methyl-guanosine cap at their 5' end. A poly-A tail is added to their 3' end. Intron sequences are removed, and the exon sequences joined together by the process known as splicing. The fully processed mRNA is then ready for transport to the cytoplasm and protein synthesis.

5. In eukaryotes, there are three RNA polymerases—RNA polymerases I, II, and III. RNA polymerase II needs the help of the TATA-binding protein and other transcription factors to become bound to a promoter. This group of proteins is called the transcription preinitiation complex, and this is sufficient to make a small number of RNA molecules. However, to make a lot of RNA in response to a signal, such as a hormone, other proteins bind to sequences called enhancers. These proteins interact with the initiation complex and increase the rate of RNA synthesis.

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