In this tutorial, you will learn about the process of DNA transcription and how it is important to cell function.
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What is DNA Transcription?
Transcription (RNA synthesis) in biology is the first stage of gene expression. Gene expression is when gene information is used to create a functional product like a protein. The purpose of transcription is to create messenger RNA (mRNA) from an existing DNA strand. For example, in a protein-coding gene, the RNA copy (transcript) carries the information needed to build a polypeptide. DNA transcription in eukaryotes needs to go through the same processing steps before translation into proteins.
The major enzyme used in DNA transcription is RNA polymerase. In eukaryotes, three types of RNA polymerase are used (I, II, and II). However, in prokaryotes, only one type of RNA polymerase is used. RNA polymerase uses a single-stranded DNA template to synthesize a complementary strand of RNA. Specifically, it builds an RNA strand in the 5′ to 3′ direction.
The major functions of RNA polymerase include:
- Formation of the initiator complex which is responsible for unwinding the double helix structure of DNA.
- Synthesis and elongation of the RNA transcript by adding nucleotide bases (adenine, cytosine, guanine, and uracil).
- Formation of the termination sequence that stops transcription.
Process of DNA Transcription
There are three stages that transcription undergoes: initiation, elongation, and termination. However, the processes that occur during these stages differ from prokaryotes to eukaryotes.
Initiation is the first step of transcription. The RNA polymerase binds to the promoter which is a region of DNA. The promoter is responsible for telling the RNA polymerase where to begin transcribing on the strand.
Each gene in prokaryotes and eukaryotes has a promoter that contains DNA sequences that allow RNA polymerase to attach. This binding causes a transcription bubble to form which starts transcribing.
Promoters in Prokaryotes
An example of a prokaryote would be bacteria. The promoter found in bacteria has two important DNA sequences the -10 and -35 elements. When RNA polymerase binds to these promoters, the DNA opens at both sites. This can occur because the bonds between adenine and thymine are fairly weak, making them easy to separate. The reason they have this name is that they come 35 and 10 nucleotides before the initiation site.
Promoters in Eukaryotes
In comparison to prokaryotes, the RNA polymerase in eukaryotes does not directly attach to the promoters. Instead, a basal transcription factor (helper protein) binds to the promoter first, helping the RNA polymerase attach to the DNA. The most common eukaryotic promoter is the TATA box.
After the RNA polymerase to bonded to the DNA strand, elongation takes place. Elongation is when the RNA strand gets longer through the addition of new nucleotides. The RNA polymerase travels across the template strand in the 3′ to 5′ direction. For each nucleotide, RNA polymerase adds a matching RNA nucleotide to the 3′ end according to Watson-Crick base pairing.
The RNA transcript will be identical to the non-template strand, which is the strand not involved in transcription. However, it is important to note that uracil will be used in place of thymine.
Termination during DNA Transcription
The final step of transcription is termination. Transcription stops when RNA polymerase transcribes a sequence of DNA called the terminator. The end result is an RNA transcript that will be used in the next process: translation. The RNA transcript is also known as messenger RNA (mRNA).
Termination in Bacteria
There are two strategies that bacteria undergo to initiate termination: Rho-dependent and Rho-independent.
In Rho-dependent termination, the RNA strand created has a binding site for the Rho factor (protein). The Rho factor binds to the RNA sequence and travels up to the RNA polymerase.
When the Rho factor reaches the RNA polymerase, it pulls it away from the template strand of DNA.
In Rho-independent termination, a specific sequence of DNA causes termination. When RNA polymerase approaches the end of the gene, it enters a region rich in cytosine and guanine. This causes the transcribed RNA to fold on itself to create a hairpin structure. This stalls the polymerase causing the enzyme to fall off and release the RNA transcript.