Steps of Transcription: Videos & Practice Problems

Transcription involves three key steps: initiation, elongation, and termination. During initiation, RNA polymerase binds to the promoter region of DNA, with transcription factors aiding in eukaryotes. Elongation sees RNA polymerase synthesize RNA by pairing free RNA nucleotides with the DNA template, moving in the 5' to 3' direction. Termination occurs when RNA polymerase reaches a terminator sequence, releasing the RNA molecule. In eukaryotes, this RNA is a premature mRNA that requires further processing before translation, unlike in prokaryotes where the RNA is ready for translation immediately.

1) Initiation of Transcription

Transcription is a crucial biological process that involves three main steps: initiation, elongation, and termination. Understanding these steps is essential for grasping how genetic information is converted into RNA, which ultimately leads to protein synthesis.

The first step, initiation, marks the beginning of transcription. This process involves the enzyme RNA polymerase binding to a specific region of DNA known as the promoter. In prokaryotes, RNA polymerase can attach directly to the promoter without any additional factors. However, in eukaryotes, transcription factors are necessary for RNA polymerase to bind effectively to the promoter region. These transcription factors assist in recruiting RNA polymerase, ensuring that it can initiate the transcription process.

Once RNA polymerase is bound to the promoter, it unwinds the DNA strands, exposing the template strand. This unwinding is critical as it allows RNA polymerase to use the DNA template to synthesize a complementary RNA strand. The RNA is built by adding ribonucleotides that are complementary to the DNA template, following the base pairing rules where adenine pairs with uracil (in RNA) and cytosine pairs with guanine.

In summary, the initiation of transcription is characterized by the binding of RNA polymerase to the promoter, facilitated by transcription factors in eukaryotes, and the unwinding of DNA to expose the template strand. This sets the stage for the next phase of transcription, elongation, where the RNA strand is synthesized. Understanding these mechanisms is fundamental to molecular biology and genetics, as they play a vital role in gene expression and regulation.

1) Elongation of Transcription

Elongation in transcription is a crucial step where the RNA polymerase enzyme synthesizes an RNA molecule by adding nucleotides that pair with the DNA template. This process involves the enzyme moving along the DNA strand, unwinding it, and constructing the RNA in the 5' to 3' direction. As the RNA polymerase progresses, it incorporates free RNA nucleotides, which are abundant in the cell, into the growing RNA strand. Each nucleotide pairs with the corresponding base on the DNA template, ensuring that the RNA carries the genetic information encoded by the DNA.

During elongation, the RNA molecule, specifically mRNA, is formed and extends from its 5' end to its 3' end. The RNA polymerase continues this process until it encounters a terminator sequence, signaling the end of transcription. It is important to note that in eukaryotic cells, the initial RNA produced is a precursor known as pre-mRNA, which undergoes further processing before becoming mature mRNA.

Additionally, multiple RNA polymerases can transcribe the same gene simultaneously, allowing for the production of several RNA molecules at once. This feature enhances the efficiency of gene expression, particularly when high levels of a specific protein are required. Understanding the elongation phase of transcription is essential for grasping how genetic information is expressed in living organisms.

3) Termination of Transcription

Termination of transcription is the final step in the process of synthesizing RNA from a DNA template. This process marks the end of transcription, resulting in the formation of an RNA molecule. It's important to note that prokaryotes and eukaryotes have distinct mechanisms for terminating transcription.

In eukaryotic cells, the termination process produces a premature RNA molecule, known as pre-mRNA. This pre-mRNA is not the final product and requires further modifications through RNA processing before it can be translated into a protein. In contrast, prokaryotic cells produce a fully functional RNA molecule that is ready for translation immediately after transcription, without the need for additional processing.

The termination of transcription is initiated when RNA polymerase encounters a specific sequence on the DNA known as the terminator sequence. Upon reaching this sequence, RNA polymerase releases both the newly synthesized RNA molecule and itself from the DNA template. This step is crucial as it ensures that the transcription process is completed efficiently.

In summary, while both prokaryotes and eukaryotes undergo transcription, the key difference lies in the nature of the RNA produced at the end of the process. Eukaryotic organisms produce a pre-mRNA that requires processing, whereas prokaryotic organisms generate a ready-to-translate RNA molecule. Understanding these differences is essential for grasping the complexities of gene expression in different types of organisms.

During which stage of transcription do the following processes take place? 
1. RNA polymerase binds to the promoter. 
2. The RNA transcript is released. 
3. The RNA transcript extended.

• A. 1-termination; 2-initiation; 3-elongation.
• B. 1-initiation; 2-elongation; 3-termination.
• C. 1-elongation; 2-termination; 3- initiation.
• D. 1-initiation; 2-termination; 3-elongation.
• E. 1-termination; 2-elongation; 3-initiation.

Answer: (D)

Transcription in eukaryotes requires which of the following molecules in addition to RNA polymerase?

• A. Anticodons.
• B. Ribosomes and tRNA.
• C. Transcription factors.
• D. Aminoacyl-tRNA synthetase.

Answer: (C)

Which of the following is the correct transcript of mRNA for the following DNA template?  
DNA Template: 3'-ATGAAGCCGAGTCAT-5'.

• A. 3'-TACTTCGGCTCAGTA-5'.
• B. 3'-AUGACUCGGCUUCAU-5'.
• C. 3'-UACUUCGGCUCAGUA-5'.
• D. 3'-ATGACTCGGCTTCAT-5'.

Answer: (B)

During transcription of DNA to RNA:

• A. The RNA polymerase moves along the DNA in the 5' to the 3' direction.
• B. The 3' end of the RNA molecule is produced first.
• C. The RNA polymerase must first bind to a promoter sequence.
• D. Transcription is always initiated at a "start codon".

Answer: (C)

What happens during the initiation step of DNA transcription?

• A. The mRNA detaches from the RNA polymerase as the RNA polymerase leaves the DNA strand.
• B. RNA polymerase attaches to the DNA at the promoter sequence of the gene.
• C. A ribosome attaches to the initiation codon of a completed mRNA strand.
• D. RNA polymerase moves along the template strand of the DNA creating an mRNA strand.

Answer: (B)

Which of the following properties is NOT shared by RNA polymerase and DNA polymerase?

• A. Can initiate strand synthesis.
• B. Dependent on a DNA sequence template.
• C. Catalyzes phosphodiester bond formation.
• D. Cannot proofread using a 3'-to-5' exonuclease activity.
• E. Polymerizes nucleotides in a 5'-to-3' direction.

Answer: (D)

Here’s what students ask on this topic:

What are the three main steps of transcription?

The three main steps of transcription are initiation, elongation, and termination. During initiation, RNA polymerase binds to the promoter region of the DNA, with transcription factors aiding in eukaryotes. In elongation, RNA polymerase synthesizes RNA by pairing free RNA nucleotides with the DNA template, moving in the 5' to 3' direction. Finally, termination occurs when RNA polymerase reaches a terminator sequence, releasing the RNA molecule. In eukaryotes, this RNA is a premature mRNA that requires further processing before translation, unlike in prokaryotes where the RNA is ready for translation immediately.

How does transcription initiation differ between prokaryotes and eukaryotes?

In prokaryotes, RNA polymerase binds directly to the promoter region of the DNA to initiate transcription. In eukaryotes, however, transcription initiation is more complex and requires transcription factor proteins. These transcription factors help RNA polymerase bind to the promoter region. Once bound, RNA polymerase unwinds the DNA, exposing the template strand for RNA synthesis. This difference highlights the additional regulatory mechanisms present in eukaryotic cells.

What happens during the elongation step of transcription?

During elongation, RNA polymerase synthesizes the RNA molecule by adding RNA nucleotides that are complementary to the DNA template strand. The enzyme moves along the DNA in the 5' to 3' direction, unwinding the DNA as it goes and elongating the RNA strand. The RNA polymerase continues this process until it reaches a terminator sequence. In eukaryotes, the initial RNA product is a premature mRNA that requires further processing, while in prokaryotes, the RNA is ready for translation immediately.

What is the role of the terminator sequence in transcription?

The terminator sequence in transcription signals the end of RNA synthesis. When RNA polymerase reaches this sequence, it triggers the termination process, causing the enzyme to release the newly synthesized RNA molecule and detach from the DNA template. In prokaryotes, the resulting RNA is ready for translation. In eukaryotes, the RNA is a premature mRNA that requires additional processing, such as splicing, capping, and polyadenylation, before it can be translated into a protein.

Why is RNA processing necessary in eukaryotes but not in prokaryotes?

RNA processing is necessary in eukaryotes because the initial RNA transcript, known as pre-mRNA, contains non-coding regions (introns) that need to be removed. Additionally, eukaryotic mRNA requires modifications such as the addition of a 5' cap and a poly-A tail to protect the RNA and facilitate its export from the nucleus. These processing steps ensure that the mRNA is mature and functional for translation. In contrast, prokaryotic RNA does not contain introns and is immediately ready for translation after transcription, eliminating the need for extensive processing.