Transcription

Transcription is the process by which the genetic instructions in a specific gene are transcribed or "rewritten" into an RNA molecule. Transcription takes place in the nucleus of eukaryotic cells and in the DNA-containing region in the cytoplasm of prokaryotic cells.

Steps of Transcription

Transcription occurs in three steps, as shown in Figure 10-15. In step O, RNA polymerase, an enzyme that catalyzes the formation of RNA on a DNA template, binds to a promoter. A promoter is a specific nucleotide sequence of DNA where RNA polymerase binds and initiates transcription. After RNA polymerase binds to the promoter, the DNA strands unwind and separate.

In step ©, RNA polymerase adds free RNA nucleotides that are complementary to the nucleotides on one of the DNA strands. The resulting chain is an RNA molecule. As in DNA replication, complementary base pairing determines the nucleotide sequence in the newly made RNA. For example, if the bases on the DNA strand was ATCGAC, the bases on the RNA strand would be UAGCUG. Unlike DNA replication, transcription uses only a specific region (a gene) on one of the two DNA strands to serve as the template. As RNA polymerase moves past, the separated DNA strands rewind.

During step ©, RNA polymerase reaches a termination signal, a specific sequence of nucleotides that marks the end of a gene. Upon reaching this "stop" signal, RNA polymerase releases both the DNA and the newly formed RNA. The RNA made during transcription can be one of many types including mRNA, tRNA, or rRNA. The newly made RNA can now perform its job in the cell, and the RNA polymerase can transcribe another gene.

^^ RNA polymerase binds to the gene's promoter. The two DNA strands unwind and separate.

Complementary RNA nucleotides are added and then joined.

^^ When RNA polymerase reaches a termination signal in the DNA, the DNA and new RNA are released by the polymerase.

^^ RNA polymerase binds to the gene's promoter. The two DNA strands unwind and separate.

Complementary RNA nucleotides are added and then joined.

^^ When RNA polymerase reaches a termination signal in the DNA, the DNA and new RNA are released by the polymerase.

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